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Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/livepatch/callbacks.txt | 605 | ||||
| -rw-r--r-- | Documentation/livepatch/shadow-vars.txt | 192 |
2 files changed, 797 insertions, 0 deletions
diff --git a/Documentation/livepatch/callbacks.txt b/Documentation/livepatch/callbacks.txt new file mode 100644 index 000000000000..c9776f48e458 --- /dev/null +++ b/Documentation/livepatch/callbacks.txt @@ -0,0 +1,605 @@ +====================== +(Un)patching Callbacks +====================== + +Livepatch (un)patch-callbacks provide a mechanism for livepatch modules +to execute callback functions when a kernel object is (un)patched. They +can be considered a "power feature" that extends livepatching abilities +to include: + + - Safe updates to global data + + - "Patches" to init and probe functions + + - Patching otherwise unpatchable code (i.e. assembly) + +In most cases, (un)patch callbacks will need to be used in conjunction +with memory barriers and kernel synchronization primitives, like +mutexes/spinlocks, or even stop_machine(), to avoid concurrency issues. + +Callbacks differ from existing kernel facilities: + + - Module init/exit code doesn't run when disabling and re-enabling a + patch. + + - A module notifier can't stop a to-be-patched module from loading. + +Callbacks are part of the klp_object structure and their implementation +is specific to that klp_object. Other livepatch objects may or may not +be patched, irrespective of the target klp_object's current state. + +Callbacks can be registered for the following livepatch actions: + + * Pre-patch - before a klp_object is patched + + * Post-patch - after a klp_object has been patched and is active + across all tasks + + * Pre-unpatch - before a klp_object is unpatched (ie, patched code is + active), used to clean up post-patch callback + resources + + * Post-unpatch - after a klp_object has been patched, all code has + been restored and no tasks are running patched code, + used to cleanup pre-patch callback resources + +Each callback is optional, omitting one does not preclude specifying any +other. However, the livepatching core executes the handlers in +symmetry: pre-patch callbacks have a post-unpatch counterpart and +post-patch callbacks have a pre-unpatch counterpart. An unpatch +callback will only be executed if its corresponding patch callback was +executed. Typical use cases pair a patch handler that acquires and +configures resources with an unpatch handler tears down and releases +those same resources. + +A callback is only executed if its host klp_object is loaded. For +in-kernel vmlinux targets, this means that callbacks will always execute +when a livepatch is enabled/disabled. For patch target kernel modules, +callbacks will only execute if the target module is loaded. When a +module target is (un)loaded, its callbacks will execute only if the +livepatch module is enabled. + +The pre-patch callback, if specified, is expected to return a status +code (0 for success, -ERRNO on error). An error status code indicates +to the livepatching core that patching of the current klp_object is not +safe and to stop the current patching request. (When no pre-patch +callback is provided, the transition is assumed to be safe.) If a +pre-patch callback returns failure, the kernel's module loader will: + + - Refuse to load a livepatch, if the livepatch is loaded after + targeted code. + + or: + + - Refuse to load a module, if the livepatch was already successfully + loaded. + +No post-patch, pre-unpatch, or post-unpatch callbacks will be executed +for a given klp_object if the object failed to patch, due to a failed +pre_patch callback or for any other reason. + +If a patch transition is reversed, no pre-unpatch handlers will be run +(this follows the previously mentioned symmetry -- pre-unpatch callbacks +will only occur if their corresponding post-patch callback executed). + +If the object did successfully patch, but the patch transition never +started for some reason (e.g., if another object failed to patch), +only the post-unpatch callback will be called. + + +Example Use-cases +================= + +Update global data +------------------ + +A pre-patch callback can be useful to update a global variable. For +example, 75ff39ccc1bd ("tcp: make challenge acks less predictable") +changes a global sysctl, as well as patches the tcp_send_challenge_ack() +function. + +In this case, if we're being super paranoid, it might make sense to +patch the data *after* patching is complete with a post-patch callback, +so that tcp_send_challenge_ack() could first be changed to read +sysctl_tcp_challenge_ack_limit with READ_ONCE. + + +Support __init and probe function patches +----------------------------------------- + +Although __init and probe functions are not directly livepatch-able, it +may be possible to implement similar updates via pre/post-patch +callbacks. + +48900cb6af42 ("virtio-net: drop NETIF_F_FRAGLIST") change the way that +virtnet_probe() initialized its driver's net_device features. A +pre/post-patch callback could iterate over all such devices, making a +similar change to their hw_features value. (Client functions of the +value may need to be updated accordingly.) + + +Test cases +========== + +What follows is not an exhaustive test suite of every possible livepatch +pre/post-(un)patch combination, but a selection that demonstrates a few +important concepts. Each test case uses the kernel modules located in +the samples/livepatch/ and assumes that no livepatches are loaded at the +beginning of the test. + + +Test 1 +------ + +Test a combination of loading a kernel module and a livepatch that +patches a function in the first module. (Un)load the target module +before the livepatch module: + +- load target module +- load livepatch +- disable livepatch +- unload target module +- unload livepatch + +First load a target module: + + % insmod samples/livepatch/livepatch-callbacks-mod.ko + [ 34.475708] livepatch_callbacks_mod: livepatch_callbacks_mod_init + +On livepatch enable, before the livepatch transition starts, pre-patch +callbacks are executed for vmlinux and livepatch_callbacks_mod (those +klp_objects currently loaded). After klp_objects are patched according +to the klp_patch, their post-patch callbacks run and the transition +completes: + + % insmod samples/livepatch/livepatch-callbacks-demo.ko + [ 36.503719] livepatch: enabling patch 'livepatch_callbacks_demo' + [ 36.504213] livepatch: 'livepatch_callbacks_demo': initializing patching transition + [ 36.504238] livepatch_callbacks_demo: pre_patch_callback: vmlinux + [ 36.504721] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state + [ 36.505849] livepatch: 'livepatch_callbacks_demo': starting patching transition + [ 37.727133] livepatch: 'livepatch_callbacks_demo': completing patching transition + [ 37.727232] livepatch_callbacks_demo: post_patch_callback: vmlinux + [ 37.727860] livepatch_callbacks_demo: post_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state + [ 37.728792] livepatch: 'livepatch_callbacks_demo': patching complete + +Similarly, on livepatch disable, pre-patch callbacks run before the +unpatching transition starts. klp_objects are reverted, post-patch +callbacks execute and the transition completes: + + % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled + [ 38.510209] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition + [ 38.510234] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux + [ 38.510982] livepatch_callbacks_demo: pre_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state + [ 38.512209] livepatch: 'livepatch_callbacks_demo': starting unpatching transition + [ 39.711132] livepatch: 'livepatch_callbacks_demo': completing unpatching transition + [ 39.711210] livepatch_callbacks_demo: post_unpatch_callback: vmlinux + [ 39.711779] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state + [ 39.712735] livepatch: 'livepatch_callbacks_demo': unpatching complete + + % rmmod samples/livepatch/livepatch-callbacks-demo.ko + % rmmod samples/livepatch/livepatch-callbacks-mod.ko + [ 42.534183] livepatch_callbacks_mod: livepatch_callbacks_mod_exit + + +Test 2 +------ + +This test is similar to the previous test, but (un)load the livepatch +module before the target kernel module. This tests the livepatch core's +module_coming handler: + +- load livepatch +- load target module +- disable livepatch +- unload livepatch +- unload target module + + +On livepatch enable, only pre/post-patch callbacks are executed for +currently loaded klp_objects, in this case, vmlinux: + + % insmod samples/livepatch/livepatch-callbacks-demo.ko + [ 44.553328] livepatch: enabling patch 'livepatch_callbacks_demo' + [ 44.553997] livepatch: 'livepatch_callbacks_demo': initializing patching transition + [ 44.554049] livepatch_callbacks_demo: pre_patch_callback: vmlinux + [ 44.554845] livepatch: 'livepatch_callbacks_demo': starting patching transition + [ 45.727128] livepatch: 'livepatch_callbacks_demo': completing patching transition + [ 45.727212] livepatch_callbacks_demo: post_patch_callback: vmlinux + [ 45.727961] livepatch: 'livepatch_callbacks_demo': patching complete + +When a targeted module is subsequently loaded, only its pre/post-patch +callbacks are executed: + + % insmod samples/livepatch/livepatch-callbacks-mod.ko + [ 46.560845] livepatch: applying patch 'livepatch_callbacks_demo' to loading module 'livepatch_callbacks_mod' + [ 46.561988] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init + [ 46.563452] livepatch_callbacks_demo: post_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init + [ 46.565495] livepatch_callbacks_mod: livepatch_callbacks_mod_init + +On livepatch disable, all currently loaded klp_objects' (vmlinux and +livepatch_callbacks_mod) pre/post-unpatch callbacks are executed: + + % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled + [ 48.568885] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition + [ 48.568910] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux + [ 48.569441] livepatch_callbacks_demo: pre_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state + [ 48.570502] livepatch: 'livepatch_callbacks_demo': starting unpatching transition + [ 49.759091] livepatch: 'livepatch_callbacks_demo': completing unpatching transition + [ 49.759171] livepatch_callbacks_demo: post_unpatch_callback: vmlinux + [ 49.759742] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state + [ 49.760690] livepatch: 'livepatch_callbacks_demo': unpatching complete + + % rmmod samples/livepatch/livepatch-callbacks-demo.ko + % rmmod samples/livepatch/livepatch-callbacks-mod.ko + [ 52.592283] livepatch_callbacks_mod: livepatch_callbacks_mod_exit + + +Test 3 +------ + +Test loading the livepatch after a targeted kernel module, then unload +the kernel module before disabling the livepatch. This tests the +livepatch core's module_going handler: + +- load target module +- load livepatch +- unload target module +- disable livepatch +- unload livepatch + +First load a target module, then the livepatch: + + % insmod samples/livepatch/livepatch-callbacks-mod.ko + [ 54.607948] livepatch_callbacks_mod: livepatch_callbacks_mod_init + + % insmod samples/livepatch/livepatch-callbacks-demo.ko + [ 56.613919] livepatch: enabling patch 'livepatch_callbacks_demo' + [ 56.614411] livepatch: 'livepatch_callbacks_demo': initializing patching transition + [ 56.614436] livepatch_callbacks_demo: pre_patch_callback: vmlinux + [ 56.614818] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state + [ 56.615656] livepatch: 'livepatch_callbacks_demo': starting patching transition + [ 57.759070] livepatch: 'livepatch_callbacks_demo': completing patching transition + [ 57.759147] livepatch_callbacks_demo: post_patch_callback: vmlinux + [ 57.759621] livepatch_callbacks_demo: post_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_LIVE] Normal state + [ 57.760307] livepatch: 'livepatch_callbacks_demo': patching complete + +When a target module is unloaded, the livepatch is only reverted from +that klp_object (livepatch_callbacks_mod). As such, only its pre and +post-unpatch callbacks are executed when this occurs: + + % rmmod samples/livepatch/livepatch-callbacks-mod.ko + [ 58.623409] livepatch_callbacks_mod: livepatch_callbacks_mod_exit + [ 58.623903] livepatch_callbacks_demo: pre_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away + [ 58.624658] livepatch: reverting patch 'livepatch_callbacks_demo' on unloading module 'livepatch_callbacks_mod' + [ 58.625305] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away + +When the livepatch is disabled, pre and post-unpatch callbacks are run +for the remaining klp_object, vmlinux: + + % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled + [ 60.638420] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition + [ 60.638444] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux + [ 60.638996] livepatch: 'livepatch_callbacks_demo': starting unpatching transition + [ 61.727088] livepatch: 'livepatch_callbacks_demo': completing unpatching transition + [ 61.727165] livepatch_callbacks_demo: post_unpatch_callback: vmlinux + [ 61.727985] livepatch: 'livepatch_callbacks_demo': unpatching complete + + % rmmod samples/livepatch/livepatch-callbacks-demo.ko + + +Test 4 +------ + +This test is similar to the previous test, however the livepatch is +loaded first. This tests the livepatch core's module_coming and +module_going handlers: + +- load livepatch +- load target module +- unload target module +- disable livepatch +- unload livepatch + +First load the livepatch: + + % insmod samples/livepatch/livepatch-callbacks-demo.ko + [ 64.661552] livepatch: enabling patch 'livepatch_callbacks_demo' + [ 64.662147] livepatch: 'livepatch_callbacks_demo': initializing patching transition + [ 64.662175] livepatch_callbacks_demo: pre_patch_callback: vmlinux + [ 64.662850] livepatch: 'livepatch_callbacks_demo': starting patching transition + [ 65.695056] livepatch: 'livepatch_callbacks_demo': completing patching transition + [ 65.695147] livepatch_callbacks_demo: post_patch_callback: vmlinux + [ 65.695561] livepatch: 'livepatch_callbacks_demo': patching complete + +When a targeted kernel module is subsequently loaded, only its +pre/post-patch callbacks are executed: + + % insmod samples/livepatch/livepatch-callbacks-mod.ko + [ 66.669196] livepatch: applying patch 'livepatch_callbacks_demo' to loading module 'livepatch_callbacks_mod' + [ 66.669882] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init + [ 66.670744] livepatch_callbacks_demo: post_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init + [ 66.672873] livepatch_callbacks_mod: livepatch_callbacks_mod_init + +When the target module is unloaded, the livepatch is only reverted from +the livepatch_callbacks_mod klp_object. As such, only pre and +post-unpatch callbacks are executed when this occurs: + + % rmmod samples/livepatch/livepatch-callbacks-mod.ko + [ 68.680065] livepatch_callbacks_mod: livepatch_callbacks_mod_exit + [ 68.680688] livepatch_callbacks_demo: pre_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away + [ 68.681452] livepatch: reverting patch 'livepatch_callbacks_demo' on unloading module 'livepatch_callbacks_mod' + [ 68.682094] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away + + % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled + [ 70.689225] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition + [ 70.689256] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux + [ 70.689882] livepatch: 'livepatch_callbacks_demo': starting unpatching transition + [ 71.711080] livepatch: 'livepatch_callbacks_demo': completing unpatching transition + [ 71.711481] livepatch_callbacks_demo: post_unpatch_callback: vmlinux + [ 71.711988] livepatch: 'livepatch_callbacks_demo': unpatching complete + + % rmmod samples/livepatch/livepatch-callbacks-demo.ko + + +Test 5 +------ + +A simple test of loading a livepatch without one of its patch target +klp_objects ever loaded (livepatch_callbacks_mod): + +- load livepatch +- disable livepatch +- unload livepatch + +Load the livepatch: + + % insmod samples/livepatch/livepatch-callbacks-demo.ko + [ 74.711081] livepatch: enabling patch 'livepatch_callbacks_demo' + [ 74.711595] livepatch: 'livepatch_callbacks_demo': initializing patching transition + [ 74.711639] livepatch_callbacks_demo: pre_patch_callback: vmlinux + [ 74.712272] livepatch: 'livepatch_callbacks_demo': starting patching transition + [ 75.743137] livepatch: 'livepatch_callbacks_demo': completing patching transition + [ 75.743219] livepatch_callbacks_demo: post_patch_callback: vmlinux + [ 75.743867] livepatch: 'livepatch_callbacks_demo': patching complete + +As expected, only pre/post-(un)patch handlers are executed for vmlinux: + + % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled + [ 76.716254] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition + [ 76.716278] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux + [ 76.716666] livepatch: 'livepatch_callbacks_demo': starting unpatching transition + [ 77.727089] livepatch: 'livepatch_callbacks_demo': completing unpatching transition + [ 77.727194] livepatch_callbacks_demo: post_unpatch_callback: vmlinux + [ 77.727907] livepatch: 'livepatch_callbacks_demo': unpatching complete + + % rmmod samples/livepatch/livepatch-callbacks-demo.ko + + +Test 6 +------ + +Test a scenario where a vmlinux pre-patch callback returns a non-zero +status (ie, failure): + +- load target module +- load livepatch -ENODEV +- unload target module + +First load a target module: + + % insmod samples/livepatch/livepatch-callbacks-mod.ko + [ 80.740520] livepatch_callbacks_mod: livepatch_callbacks_mod_init + +Load the livepatch module, setting its 'pre_patch_ret' value to -19 +(-ENODEV). When its vmlinux pre-patch callback executed, this status +code will propagate back to the module-loading subsystem. The result is +that the insmod command refuses to load the livepatch module: + + % insmod samples/livepatch/livepatch-callbacks-demo.ko pre_patch_ret=-19 + [ 82.747326] livepatch: enabling patch 'livepatch_callbacks_demo' + [ 82.747743] livepatch: 'livepatch_callbacks_demo': initializing patching transition + [ 82.747767] livepatch_callbacks_demo: pre_patch_callback: vmlinux + [ 82.748237] livepatch: pre-patch callback failed for object 'vmlinux' + [ 82.748637] livepatch: failed to enable patch 'livepatch_callbacks_demo' + [ 82.749059] livepatch: 'livepatch_callbacks_demo': canceling transition, going to unpatch + [ 82.749060] livepatch: 'livepatch_callbacks_demo': completing unpatching transition + [ 82.749868] livepatch: 'livepatch_callbacks_demo': unpatching complete + [ 82.765809] insmod: ERROR: could not insert module samples/livepatch/livepatch-callbacks-demo.ko: No such device + + % rmmod samples/livepatch/livepatch-callbacks-mod.ko + [ 84.774238] livepatch_callbacks_mod: livepatch_callbacks_mod_exit + + +Test 7 +------ + +Similar to the previous test, setup a livepatch such that its vmlinux +pre-patch callback returns success. However, when a targeted kernel +module is later loaded, have the livepatch return a failing status code: + +- load livepatch +- setup -ENODEV +- load target module +- disable livepatch +- unload livepatch + +Load the livepatch, notice vmlinux pre-patch callback succeeds: + + % insmod samples/livepatch/livepatch-callbacks-demo.ko + [ 86.787845] livepatch: enabling patch 'livepatch_callbacks_demo' + [ 86.788325] livepatch: 'livepatch_callbacks_demo': initializing patching transition + [ 86.788427] livepatch_callbacks_demo: pre_patch_callback: vmlinux + [ 86.788821] livepatch: 'livepatch_callbacks_demo': starting patching transition + [ 87.711069] livepatch: 'livepatch_callbacks_demo': completing patching transition + [ 87.711143] livepatch_callbacks_demo: post_patch_callback: vmlinux + [ 87.711886] livepatch: 'livepatch_callbacks_demo': patching complete + +Set a trap so subsequent pre-patch callbacks to this livepatch will +return -ENODEV: + + % echo -19 > /sys/module/livepatch_callbacks_demo/parameters/pre_patch_ret + +The livepatch pre-patch callback for subsequently loaded target modules +will return failure, so the module loader refuses to load the kernel +module. Notice that no post-patch or pre/post-unpatch callbacks are +executed for this klp_object: + + % insmod samples/livepatch/livepatch-callbacks-mod.ko + [ 90.796976] livepatch: applying patch 'livepatch_callbacks_demo' to loading module 'livepatch_callbacks_mod' + [ 90.797834] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init + [ 90.798900] livepatch: pre-patch callback failed for object 'livepatch_callbacks_mod' + [ 90.799652] livepatch: patch 'livepatch_callbacks_demo' failed for module 'livepatch_callbacks_mod', refusing to load module 'livepatch_callbacks_mod' + [ 90.819737] insmod: ERROR: could not insert module samples/livepatch/livepatch-callbacks-mod.ko: No such device + +However, pre/post-unpatch callbacks run for the vmlinux klp_object: + + % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled + [ 92.823547] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition + [ 92.823573] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux + [ 92.824331] livepatch: 'livepatch_callbacks_demo': starting unpatching transition + [ 93.727128] livepatch: 'livepatch_callbacks_demo': completing unpatching transition + [ 93.727327] livepatch_callbacks_demo: post_unpatch_callback: vmlinux + [ 93.727861] livepatch: 'livepatch_callbacks_demo': unpatching complete + + % rmmod samples/livepatch/livepatch-callbacks-demo.ko + + +Test 8 +------ + +Test loading multiple targeted kernel modules. This test-case is +mainly for comparing with the next test-case. + +- load busy target module (0s sleep), +- load livepatch +- load target module +- unload target module +- disable livepatch +- unload livepatch +- unload busy target module + + +Load a target "busy" kernel module which kicks off a worker function +that immediately exits: + + % insmod samples/livepatch/livepatch-callbacks-busymod.ko sleep_secs=0 + [ 96.910107] livepatch_callbacks_busymod: livepatch_callbacks_mod_init + [ 96.910600] livepatch_callbacks_busymod: busymod_work_func, sleeping 0 seconds ... + [ 96.913024] livepatch_callbacks_busymod: busymod_work_func exit + +Proceed with loading the livepatch and another ordinary target module, +notice that the post-patch callbacks are executed and the transition +completes quickly: + + % insmod samples/livepatch/livepatch-callbacks-demo.ko + [ 98.917892] livepatch: enabling patch 'livepatch_callbacks_demo' + [ 98.918426] livepatch: 'livepatch_callbacks_demo': initializing patching transition + [ 98.918453] livepatch_callbacks_demo: pre_patch_callback: vmlinux + [ 98.918955] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_busymod -> [MODULE_STATE_LIVE] Normal state + [ 98.923835] livepatch: 'livepatch_callbacks_demo': starting patching transition + [ 99.743104] livepatch: 'livepatch_callbacks_demo': completing patching transition + [ 99.743156] livepatch_callbacks_demo: post_patch_callback: vmlinux + [ 99.743679] livepatch_callbacks_demo: post_patch_callback: livepatch_callbacks_busymod -> [MODULE_STATE_LIVE] Normal state + [ 99.744616] livepatch: 'livepatch_callbacks_demo': patching complete + + % insmod samples/livepatch/livepatch-callbacks-mod.ko + [ 100.930955] livepatch: applying patch 'livepatch_callbacks_demo' to loading module 'livepatch_callbacks_mod' + [ 100.931668] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init + [ 100.932645] livepatch_callbacks_demo: post_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init + [ 100.934125] livepatch_callbacks_mod: livepatch_callbacks_mod_init + + % rmmod samples/livepatch/livepatch-callbacks-mod.ko + [ 102.942805] livepatch_callbacks_mod: livepatch_callbacks_mod_exit + [ 102.943640] livepatch_callbacks_demo: pre_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away + [ 102.944585] livepatch: reverting patch 'livepatch_callbacks_demo' on unloading module 'livepatch_callbacks_mod' + [ 102.945455] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away + + % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled + [ 104.953815] livepatch: 'livepatch_callbacks_demo': initializing unpatching transition + [ 104.953838] livepatch_callbacks_demo: pre_unpatch_callback: vmlinux + [ 104.954431] livepatch_callbacks_demo: pre_unpatch_callback: livepatch_callbacks_busymod -> [MODULE_STATE_LIVE] Normal state + [ 104.955426] livepatch: 'livepatch_callbacks_demo': starting unpatching transition + [ 106.719073] livepatch: 'livepatch_callbacks_demo': completing unpatching transition + [ 106.722633] livepatch_callbacks_demo: post_unpatch_callback: vmlinux + [ 106.723282] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_busymod -> [MODULE_STATE_LIVE] Normal state + [ 106.724279] livepatch: 'livepatch_callbacks_demo': unpatching complete + + % rmmod samples/livepatch/livepatch-callbacks-demo.ko + % rmmod samples/livepatch/livepatch-callbacks-busymod.ko + [ 108.975660] livepatch_callbacks_busymod: livepatch_callbacks_mod_exit + + +Test 9 +------ + +A similar test as the previous one, but force the "busy" kernel module +to do longer work. + +The livepatching core will refuse to patch a task that is currently +executing a to-be-patched function -- the consistency model stalls the +current patch transition until this safety-check is met. Test a +scenario where one of a livepatch's target klp_objects sits on such a +function for a long time. Meanwhile, load and unload other target +kernel modules while the livepatch transition is in progress. + +- load busy target module (30s sleep) +- load livepatch +- load target module +- unload target module +- disable livepatch +- unload livepatch +- unload busy target module + + +Load the "busy" kernel module, this time make it do 30 seconds worth of +work: + + % insmod samples/livepatch/livepatch-callbacks-busymod.ko sleep_secs=30 + [ 110.993362] livepatch_callbacks_busymod: livepatch_callbacks_mod_init + [ 110.994059] livepatch_callbacks_busymod: busymod_work_func, sleeping 30 seconds ... + +Meanwhile, the livepatch is loaded. Notice that the patch transition +does not complete as the targeted "busy" module is sitting on a +to-be-patched function: + + % insmod samples/livepatch/livepatch-callbacks-demo.ko + [ 113.000309] livepatch: enabling patch 'livepatch_callbacks_demo' + [ 113.000764] livepatch: 'livepatch_callbacks_demo': initializing patching transition + [ 113.000791] livepatch_callbacks_demo: pre_patch_callback: vmlinux + [ 113.001289] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_busymod -> [MODULE_STATE_LIVE] Normal state + [ 113.005208] livepatch: 'livepatch_callbacks_demo': starting patching transition + +Load a second target module (this one is an ordinary idle kernel +module). Note that *no* post-patch callbacks will be executed while the +livepatch is still in transition: + + % insmod samples/livepatch/livepatch-callbacks-mod.ko + [ 115.012740] livepatch: applying patch 'livepatch_callbacks_demo' to loading module 'livepatch_callbacks_mod' + [ 115.013406] livepatch_callbacks_demo: pre_patch_callback: livepatch_callbacks_mod -> [MODULE_STATE_COMING] Full formed, running module_init + [ 115.015315] livepatch_callbacks_mod: livepatch_callbacks_mod_init + +Request an unload of the simple kernel module. The patch is still +transitioning, so its pre-unpatch callbacks are skipped: + + % rmmod samples/livepatch/livepatch-callbacks-mod.ko + [ 117.022626] livepatch_callbacks_mod: livepatch_callbacks_mod_exit + [ 117.023376] livepatch: reverting patch 'livepatch_callbacks_demo' on unloading module 'livepatch_callbacks_mod' + [ 117.024533] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_mod -> [MODULE_STATE_GOING] Going away + +Finally the livepatch is disabled. Since none of the patch's +klp_object's post-patch callbacks executed, the remaining klp_object's +pre-unpatch callbacks are skipped: + + % echo 0 > /sys/kernel/livepatch/livepatch_callbacks_demo/enabled + [ 119.035408] livepatch: 'livepatch_callbacks_demo': reversing transition from patching to unpatching + [ 119.035485] livepatch: 'livepatch_callbacks_demo': starting unpatching transition + [ 119.711166] livepatch: 'livepatch_callbacks_demo': completing unpatching transition + [ 119.714179] livepatch_callbacks_demo: post_unpatch_callback: vmlinux + [ 119.714653] livepatch_callbacks_demo: post_unpatch_callback: livepatch_callbacks_busymod -> [MODULE_STATE_LIVE] Normal state + [ 119.715437] livepatch: 'livepatch_callbacks_demo': unpatching complete + + % rmmod samples/livepatch/livepatch-callbacks-demo.ko + % rmmod samples/livepatch/livepatch-callbacks-busymod.ko + [ 141.279111] livepatch_callbacks_busymod: busymod_work_func exit + [ 141.279760] livepatch_callbacks_busymod: livepatch_callbacks_mod_exit diff --git a/Documentation/livepatch/shadow-vars.txt b/Documentation/livepatch/shadow-vars.txt new file mode 100644 index 000000000000..89c66634d600 --- /dev/null +++ b/Documentation/livepatch/shadow-vars.txt @@ -0,0 +1,192 @@ +================ +Shadow Variables +================ + +Shadow variables are a simple way for livepatch modules to associate +additional "shadow" data with existing data structures. Shadow data is +allocated separately from parent data structures, which are left +unmodified. The shadow variable API described in this document is used +to allocate/add and remove/free shadow variables to/from their parents. + +The implementation introduces a global, in-kernel hashtable that +associates pointers to parent objects and a numeric identifier of the +shadow data. The numeric identifier is a simple enumeration that may be +used to describe shadow variable version, class or type, etc. More +specifically, the parent pointer serves as the hashtable key while the +numeric id subsequently filters hashtable queries. Multiple shadow +variables may attach to the same parent object, but their numeric +identifier distinguishes between them. + + +1. Brief API summary +==================== + +(See the full API usage docbook notes in livepatch/shadow.c.) + +A hashtable references all shadow variables. These references are +stored and retrieved through a <obj, id> pair. + +* The klp_shadow variable data structure encapsulates both tracking +meta-data and shadow-data: + - meta-data + - obj - pointer to parent object + - id - data identifier + - data[] - storage for shadow data + +It is important to note that the klp_shadow_alloc() and +klp_shadow_get_or_alloc() calls, described below, store a *copy* of the +data that the functions are provided. Callers should provide whatever +mutual exclusion is required of the shadow data. + +* klp_shadow_get() - retrieve a shadow variable data pointer + - search hashtable for <obj, id> pair + +* klp_shadow_alloc() - allocate and add a new shadow variable + - search hashtable for <obj, id> pair + - if exists + - WARN and return NULL + - if <obj, id> doesn't already exist + - allocate a new shadow variable + - copy data into the new shadow variable + - add <obj, id> to the global hashtable + +* klp_shadow_get_or_alloc() - get existing or alloc a new shadow variable + - search hashtable for <obj, id> pair + - if exists + - return existing shadow variable + - if <obj, id> doesn't already exist + - allocate a new shadow variable + - copy data into the new shadow variable + - add <obj, id> pair to the global hashtable + +* klp_shadow_free() - detach and free a <obj, id> shadow variable + - find and remove a <obj, id> reference from global hashtable + - if found, free shadow variable + +* klp_shadow_free_all() - detach and free all <*, id> shadow variables + - find and remove any <*, id> references from global hashtable + - if found, free shadow variable + + +2. Use cases +============ + +(See the example shadow variable livepatch modules in samples/livepatch/ +for full working demonstrations.) + +For the following use-case examples, consider commit 1d147bfa6429 +("mac80211: fix AP powersave TX vs. wakeup race"), which added a +spinlock to net/mac80211/sta_info.h :: struct sta_info. Each use-case +example can be considered a stand-alone livepatch implementation of this +fix. + + +Matching parent's lifecycle +--------------------------- + +If parent data structures are frequently created and destroyed, it may +be easiest to align their shadow variables lifetimes to the same +allocation and release functions. In this case, the parent data +structure is typically allocated, initialized, then registered in some +manner. Shadow variable allocation and setup can then be considered +part of the parent's initialization and should be completed before the +parent "goes live" (ie, any shadow variable get-API requests are made +for this <obj, id> pair.) + +For commit 1d147bfa6429, when a parent sta_info structure is allocated, +allocate a shadow copy of the ps_lock pointer, then initialize it: + +#define PS_LOCK 1 +struct sta_info *sta_info_alloc(struct ieee80211_sub_if_data *sdata, + const u8 *addr, gfp_t gfp) +{ + struct sta_info *sta; + spinlock_t *ps_lock; + + /* Parent structure is created */ + sta = kzalloc(sizeof(*sta) + hw->sta_data_size, gfp); + + /* Attach a corresponding shadow variable, then initialize it */ + ps_lock = klp_shadow_alloc(sta, PS_LOCK, NULL, sizeof(*ps_lock), gfp); + if (!ps_lock) + goto shadow_fail; + spin_lock_init(ps_lock); + ... + +When requiring a ps_lock, query the shadow variable API to retrieve one +for a specific struct sta_info: + +void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta) +{ + spinlock_t *ps_lock; + + /* sync with ieee80211_tx_h_unicast_ps_buf */ + ps_lock = klp_shadow_get(sta, PS_LOCK); + if (ps_lock) + spin_lock(ps_lock); + ... + +When the parent sta_info structure is freed, first free the shadow +variable: + +void sta_info_free(struct ieee80211_local *local, struct sta_info *sta) +{ + klp_shadow_free(sta, PS_LOCK); + kfree(sta); + ... + + +In-flight parent objects +------------------------ + +Sometimes it may not be convenient or possible to allocate shadow +variables alongside their parent objects. Or a livepatch fix may +require shadow varibles to only a subset of parent object instances. In +these cases, the klp_shadow_get_or_alloc() call can be used to attach +shadow variables to parents already in-flight. + +For commit 1d147bfa6429, a good spot to allocate a shadow spinlock is +inside ieee80211_sta_ps_deliver_wakeup(): + +#define PS_LOCK 1 +void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta) +{ + DEFINE_SPINLOCK(ps_lock_fallback); + spinlock_t *ps_lock; + + /* sync with ieee80211_tx_h_unicast_ps_buf */ + ps_lock = klp_shadow_get_or_alloc(sta, PS_LOCK, + &ps_lock_fallback, sizeof(ps_lock_fallback), + GFP_ATOMIC); + if (ps_lock) + spin_lock(ps_lock); + ... + +This usage will create a shadow variable, only if needed, otherwise it +will use one that was already created for this <obj, id> pair. + +Like the previous use-case, the shadow spinlock needs to be cleaned up. +A shadow variable can be freed just before its parent object is freed, +or even when the shadow variable itself is no longer required. + + +Other use-cases +--------------- + +Shadow variables can also be used as a flag indicating that a data +structure was allocated by new, livepatched code. In this case, it +doesn't matter what data value the shadow variable holds, its existence +suggests how to handle the parent object. + + +3. References +============= + +* https://github.com/dynup/kpatch +The livepatch implementation is based on the kpatch version of shadow +variables. + +* http://files.mkgnu.net/files/dynamos/doc/papers/dynamos_eurosys_07.pdf +Dynamic and Adaptive Updates of Non-Quiescent Subsystems in Commodity +Operating System Kernels (Kritis Makris, Kyung Dong Ryu 2007) presented +a datatype update technique called "shadow data structures". |