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| author | Andy Lutomirski <luto@kernel.org> | 2021-02-13 11:19:44 -0800 |
|---|---|---|
| committer | Borislav Petkov <bp@suse.de> | 2021-03-08 13:19:05 +0100 |
| commit | 3fb0fdb3bbe7aed495109b3296b06c2409734023 (patch) | |
| tree | 72e8821e31ab11816ab4bf26f7d41e915cb022cd /arch/x86/kernel/tls.c | |
| parent | a38fd8748464831584a19438cbb3082b5a2dab15 (diff) | |
| download | linux-stable-3fb0fdb3bbe7aed495109b3296b06c2409734023.tar.gz linux-stable-3fb0fdb3bbe7aed495109b3296b06c2409734023.tar.bz2 linux-stable-3fb0fdb3bbe7aed495109b3296b06c2409734023.zip | |
x86/stackprotector/32: Make the canary into a regular percpu variable
On 32-bit kernels, the stackprotector canary is quite nasty -- it is
stored at %gs:(20), which is nasty because 32-bit kernels use %fs for
percpu storage. It's even nastier because it means that whether %gs
contains userspace state or kernel state while running kernel code
depends on whether stackprotector is enabled (this is
CONFIG_X86_32_LAZY_GS), and this setting radically changes the way
that segment selectors work. Supporting both variants is a
maintenance and testing mess.
Merely rearranging so that percpu and the stack canary
share the same segment would be messy as the 32-bit percpu address
layout isn't currently compatible with putting a variable at a fixed
offset.
Fortunately, GCC 8.1 added options that allow the stack canary to be
accessed as %fs:__stack_chk_guard, effectively turning it into an ordinary
percpu variable. This lets us get rid of all of the code to manage the
stack canary GDT descriptor and the CONFIG_X86_32_LAZY_GS mess.
(That name is special. We could use any symbol we want for the
%fs-relative mode, but for CONFIG_SMP=n, gcc refuses to let us use any
name other than __stack_chk_guard.)
Forcibly disable stackprotector on older compilers that don't support
the new options and turn the stack canary into a percpu variable. The
"lazy GS" approach is now used for all 32-bit configurations.
Also makes load_gs_index() work on 32-bit kernels. On 64-bit kernels,
it loads the GS selector and updates the user GSBASE accordingly. (This
is unchanged.) On 32-bit kernels, it loads the GS selector and updates
GSBASE, which is now always the user base. This means that the overall
effect is the same on 32-bit and 64-bit, which avoids some ifdeffery.
[ bp: Massage commit message. ]
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/c0ff7dba14041c7e5d1cae5d4df052f03759bef3.1613243844.git.luto@kernel.org
Diffstat (limited to 'arch/x86/kernel/tls.c')
| -rw-r--r-- | arch/x86/kernel/tls.c | 8 |
1 files changed, 1 insertions, 7 deletions
diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c index 64a496a0687f..3c883e064242 100644 --- a/arch/x86/kernel/tls.c +++ b/arch/x86/kernel/tls.c @@ -164,17 +164,11 @@ int do_set_thread_area(struct task_struct *p, int idx, savesegment(fs, sel); if (sel == modified_sel) loadsegment(fs, sel); - - savesegment(gs, sel); - if (sel == modified_sel) - load_gs_index(sel); #endif -#ifdef CONFIG_X86_32_LAZY_GS savesegment(gs, sel); if (sel == modified_sel) - loadsegment(gs, sel); -#endif + load_gs_index(sel); } else { #ifdef CONFIG_X86_64 if (p->thread.fsindex == modified_sel) |