From e91c2518a5d22a07642f35d85f39001ad379dae4 Mon Sep 17 00:00:00 2001 From: Petr Mladek Date: Mon, 16 Apr 2018 13:36:46 +0200 Subject: livepatch: Initialize shadow variables safely by a custom callback The existing API allows to pass a sample data to initialize the shadow data. It works well when the data are position independent. But it fails miserably when we need to set a pointer to the shadow structure itself. Unfortunately, we might need to initialize the pointer surprisingly often because of struct list_head. It is even worse because the list might be hidden in other common structures, for example, struct mutex, struct wait_queue_head. For example, this was needed to fix races in ALSA sequencer. It required to add mutex into struct snd_seq_client. See commit b3defb791b26ea06 ("ALSA: seq: Make ioctls race-free") and commit d15d662e89fc667b9 ("ALSA: seq: Fix racy pool initializations") This patch makes the API more safe. A custom constructor function and data are passed to klp_shadow_*alloc() functions instead of the sample data. Note that ctor_data are no longer a template for shadow->data. It might point to any data that might be necessary when the constructor is called. Also note that the constructor is called under klp_shadow_lock. It is an internal spin_lock that synchronizes alloc() vs. get() operations, see klp_shadow_get_or_alloc(). On one hand, this adds a risk of ABBA deadlocks. On the other hand, it allows to do some operations safely. For example, we could add the new structure into an existing list. This must be done only once when the structure is allocated. Reported-by: Nicolai Stange Signed-off-by: Petr Mladek Acked-by: Josh Poimboeuf Acked-by: Miroslav Benes Signed-off-by: Jiri Kosina --- kernel/livepatch/shadow.c | 82 ++++++++++++++++++++++++++++++----------------- 1 file changed, 53 insertions(+), 29 deletions(-) (limited to 'kernel') diff --git a/kernel/livepatch/shadow.c b/kernel/livepatch/shadow.c index fdac27588d60..b10a0bbb7f84 100644 --- a/kernel/livepatch/shadow.c +++ b/kernel/livepatch/shadow.c @@ -113,8 +113,10 @@ void *klp_shadow_get(void *obj, unsigned long id) } EXPORT_SYMBOL_GPL(klp_shadow_get); -static void *__klp_shadow_get_or_alloc(void *obj, unsigned long id, void *data, - size_t size, gfp_t gfp_flags, bool warn_on_exist) +static void *__klp_shadow_get_or_alloc(void *obj, unsigned long id, + size_t size, gfp_t gfp_flags, + klp_shadow_ctor_t ctor, void *ctor_data, + bool warn_on_exist) { struct klp_shadow *new_shadow; void *shadow_data; @@ -125,18 +127,15 @@ static void *__klp_shadow_get_or_alloc(void *obj, unsigned long id, void *data, if (shadow_data) goto exists; - /* Allocate a new shadow variable for use inside the lock below */ + /* + * Allocate a new shadow variable. Fill it with zeroes by default. + * More complex setting can be done by @ctor function. But it is + * called only when the buffer is really used (under klp_shadow_lock). + */ new_shadow = kzalloc(size + sizeof(*new_shadow), gfp_flags); if (!new_shadow) return NULL; - new_shadow->obj = obj; - new_shadow->id = id; - - /* Initialize the shadow variable if data provided */ - if (data) - memcpy(new_shadow->data, data, size); - /* Look for again under the lock */ spin_lock_irqsave(&klp_shadow_lock, flags); shadow_data = klp_shadow_get(obj, id); @@ -150,6 +149,22 @@ static void *__klp_shadow_get_or_alloc(void *obj, unsigned long id, void *data, goto exists; } + new_shadow->obj = obj; + new_shadow->id = id; + + if (ctor) { + int err; + + err = ctor(obj, new_shadow->data, ctor_data); + if (err) { + spin_unlock_irqrestore(&klp_shadow_lock, flags); + kfree(new_shadow); + pr_err("Failed to construct shadow variable <%p, %lx> (%d)\n", + obj, id, err); + return NULL; + } + } + /* No found, so attach the newly allocated one */ hash_add_rcu(klp_shadow_hash, &new_shadow->node, (unsigned long)new_shadow->obj); @@ -170,26 +185,32 @@ exists: * klp_shadow_alloc() - allocate and add a new shadow variable * @obj: pointer to parent object * @id: data identifier - * @data: pointer to data to attach to parent * @size: size of attached data * @gfp_flags: GFP mask for allocation + * @ctor: custom constructor to initialize the shadow data (optional) + * @ctor_data: pointer to any data needed by @ctor (optional) + * + * Allocates @size bytes for new shadow variable data using @gfp_flags. + * The data are zeroed by default. They are further initialized by @ctor + * function if it is not NULL. The new shadow variable is then added + * to the global hashtable. * - * Allocates @size bytes for new shadow variable data using @gfp_flags - * and copies @size bytes from @data into the new shadow variable's own - * data space. If @data is NULL, @size bytes are still allocated, but - * no copy is performed. The new shadow variable is then added to the - * global hashtable. + * If an existing shadow variable can be found, this routine will + * issue a WARN, exit early and return NULL. * - * If an existing shadow variable can be found, this routine - * will issue a WARN, exit early and return NULL. + * This function guarantees that the constructor function is called only when + * the variable did not exist before. The cost is that @ctor is called + * in atomic context under a spin lock. * * Return: the shadow variable data element, NULL on duplicate or * failure. */ -void *klp_shadow_alloc(void *obj, unsigned long id, void *data, - size_t size, gfp_t gfp_flags) +void *klp_shadow_alloc(void *obj, unsigned long id, + size_t size, gfp_t gfp_flags, + klp_shadow_ctor_t ctor, void *ctor_data) { - return __klp_shadow_get_or_alloc(obj, id, data, size, gfp_flags, true); + return __klp_shadow_get_or_alloc(obj, id, size, gfp_flags, + ctor, ctor_data, true); } EXPORT_SYMBOL_GPL(klp_shadow_alloc); @@ -197,25 +218,28 @@ EXPORT_SYMBOL_GPL(klp_shadow_alloc); * klp_shadow_get_or_alloc() - get existing or allocate a new shadow variable * @obj: pointer to parent object * @id: data identifier - * @data: pointer to data to attach to parent * @size: size of attached data * @gfp_flags: GFP mask for allocation + * @ctor: custom constructor to initialize the shadow data (optional) + * @ctor_data: pointer to any data needed by @ctor (optional) * * Returns a pointer to existing shadow data if an shadow * variable is already present. Otherwise, it creates a new shadow * variable like klp_shadow_alloc(). * - * This function guarantees that only one shadow variable exists with - * the given @id for the given @obj. It also guarantees that the shadow - * variable will be initialized by the given @data only when it did not - * exist before. + * This function guarantees that only one shadow variable exists with the given + * @id for the given @obj. It also guarantees that the constructor function + * will be called only when the variable did not exist before. The cost is + * that @ctor is called in atomic context under a spin lock. * * Return: the shadow variable data element, NULL on failure. */ -void *klp_shadow_get_or_alloc(void *obj, unsigned long id, void *data, - size_t size, gfp_t gfp_flags) +void *klp_shadow_get_or_alloc(void *obj, unsigned long id, + size_t size, gfp_t gfp_flags, + klp_shadow_ctor_t ctor, void *ctor_data) { - return __klp_shadow_get_or_alloc(obj, id, data, size, gfp_flags, false); + return __klp_shadow_get_or_alloc(obj, id, size, gfp_flags, + ctor, ctor_data, false); } EXPORT_SYMBOL_GPL(klp_shadow_get_or_alloc); -- cgit v1.2.3 From 3b2c77d000fe9f7d02e9e726e00dccf9f92b256f Mon Sep 17 00:00:00 2001 From: Petr Mladek Date: Mon, 16 Apr 2018 13:36:47 +0200 Subject: livepatch: Allow to call a custom callback when freeing shadow variables We might need to do some actions before the shadow variable is freed. For example, we might need to remove it from a list or free some data that it points to. This is already possible now. The user can get the shadow variable by klp_shadow_get(), do the necessary actions, and then call klp_shadow_free(). This patch allows to do it a more elegant way. The user could implement the needed actions in a callback that is passed to klp_shadow_free() as a parameter. The callback usually does reverse operations to the constructor callback that can be called by klp_shadow_*alloc(). It is especially useful for klp_shadow_free_all(). There we need to do these extra actions for each found shadow variable with the given ID. Note that the memory used by the shadow variable itself is still released later by rcu callback. It is needed to protect internal structures that keep all shadow variables. But the destructor is called immediately. The shadow variable must not be access anyway after klp_shadow_free() is called. The user is responsible to protect this any suitable way. Be aware that the destructor is called under klp_shadow_lock. It is the same as for the contructor in klp_shadow_alloc(). Signed-off-by: Petr Mladek Acked-by: Josh Poimboeuf Acked-by: Miroslav Benes Signed-off-by: Jiri Kosina --- kernel/livepatch/shadow.c | 26 ++++++++++++++++++-------- 1 file changed, 18 insertions(+), 8 deletions(-) (limited to 'kernel') diff --git a/kernel/livepatch/shadow.c b/kernel/livepatch/shadow.c index b10a0bbb7f84..83958c814439 100644 --- a/kernel/livepatch/shadow.c +++ b/kernel/livepatch/shadow.c @@ -243,15 +243,26 @@ void *klp_shadow_get_or_alloc(void *obj, unsigned long id, } EXPORT_SYMBOL_GPL(klp_shadow_get_or_alloc); +static void klp_shadow_free_struct(struct klp_shadow *shadow, + klp_shadow_dtor_t dtor) +{ + hash_del_rcu(&shadow->node); + if (dtor) + dtor(shadow->obj, shadow->data); + kfree_rcu(shadow, rcu_head); +} + /** * klp_shadow_free() - detach and free a shadow variable * @obj: pointer to parent object * @id: data identifier + * @dtor: custom callback that can be used to unregister the variable + * and/or free data that the shadow variable points to (optional) * * This function releases the memory for this shadow variable * instance, callers should stop referencing it accordingly. */ -void klp_shadow_free(void *obj, unsigned long id) +void klp_shadow_free(void *obj, unsigned long id, klp_shadow_dtor_t dtor) { struct klp_shadow *shadow; unsigned long flags; @@ -263,8 +274,7 @@ void klp_shadow_free(void *obj, unsigned long id) (unsigned long)obj) { if (klp_shadow_match(shadow, obj, id)) { - hash_del_rcu(&shadow->node); - kfree_rcu(shadow, rcu_head); + klp_shadow_free_struct(shadow, dtor); break; } } @@ -276,11 +286,13 @@ EXPORT_SYMBOL_GPL(klp_shadow_free); /** * klp_shadow_free_all() - detach and free all <*, id> shadow variables * @id: data identifier + * @dtor: custom callback that can be used to unregister the variable + * and/or free data that the shadow variable points to (optional) * * This function releases the memory for all <*, id> shadow variable * instances, callers should stop referencing them accordingly. */ -void klp_shadow_free_all(unsigned long id) +void klp_shadow_free_all(unsigned long id, klp_shadow_dtor_t dtor) { struct klp_shadow *shadow; unsigned long flags; @@ -290,10 +302,8 @@ void klp_shadow_free_all(unsigned long id) /* Delete all <*, id> from hash */ hash_for_each(klp_shadow_hash, i, shadow, node) { - if (klp_shadow_match(shadow, shadow->obj, id)) { - hash_del_rcu(&shadow->node); - kfree_rcu(shadow, rcu_head); - } + if (klp_shadow_match(shadow, shadow->obj, id)) + klp_shadow_free_struct(shadow, dtor); } spin_unlock_irqrestore(&klp_shadow_lock, flags); -- cgit v1.2.3