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authorAndy Lutomirski <luto@kernel.org>2021-02-13 22:19:44 +0300
committerBorislav Petkov <bp@suse.de>2021-03-08 15:19:05 +0300
commit3fb0fdb3bbe7aed495109b3296b06c2409734023 (patch)
tree72e8821e31ab11816ab4bf26f7d41e915cb022cd /arch/x86/power
parenta38fd8748464831584a19438cbb3082b5a2dab15 (diff)
downloadlinux-3fb0fdb3bbe7aed495109b3296b06c2409734023.tar.xz
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/power')
-rw-r--r--arch/x86/power/cpu.c6
1 files changed, 1 insertions, 5 deletions
diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c
index db1378c6ff26..ef4329d67a5f 100644
--- a/arch/x86/power/cpu.c
+++ b/arch/x86/power/cpu.c
@@ -99,11 +99,8 @@ static void __save_processor_state(struct saved_context *ctxt)
/*
* segment registers
*/
-#ifdef CONFIG_X86_32_LAZY_GS
savesegment(gs, ctxt->gs);
-#endif
#ifdef CONFIG_X86_64
- savesegment(gs, ctxt->gs);
savesegment(fs, ctxt->fs);
savesegment(ds, ctxt->ds);
savesegment(es, ctxt->es);
@@ -232,7 +229,6 @@ static void notrace __restore_processor_state(struct saved_context *ctxt)
wrmsrl(MSR_GS_BASE, ctxt->kernelmode_gs_base);
#else
loadsegment(fs, __KERNEL_PERCPU);
- loadsegment(gs, __KERNEL_STACK_CANARY);
#endif
/* Restore the TSS, RO GDT, LDT, and usermode-relevant MSRs. */
@@ -255,7 +251,7 @@ static void notrace __restore_processor_state(struct saved_context *ctxt)
*/
wrmsrl(MSR_FS_BASE, ctxt->fs_base);
wrmsrl(MSR_KERNEL_GS_BASE, ctxt->usermode_gs_base);
-#elif defined(CONFIG_X86_32_LAZY_GS)
+#else
loadsegment(gs, ctxt->gs);
#endif