diff options
| 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/head_32.S | |
| parent | a38fd8748464831584a19438cbb3082b5a2dab15 (diff) | |
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/head_32.S')
| -rw-r--r-- | arch/x86/kernel/head_32.S | 18 |
1 files changed, 2 insertions, 16 deletions
diff --git a/arch/x86/kernel/head_32.S b/arch/x86/kernel/head_32.S index 7ed84c282233..67f590425d90 100644 --- a/arch/x86/kernel/head_32.S +++ b/arch/x86/kernel/head_32.S @@ -318,8 +318,8 @@ SYM_FUNC_START(startup_32_smp) movl $(__KERNEL_PERCPU), %eax movl %eax,%fs # set this cpu's percpu - movl $(__KERNEL_STACK_CANARY),%eax - movl %eax,%gs + xorl %eax,%eax + movl %eax,%gs # clear possible garbage in %gs xorl %eax,%eax # Clear LDT lldt %ax @@ -339,20 +339,6 @@ SYM_FUNC_END(startup_32_smp) */ __INIT setup_once: -#ifdef CONFIG_STACKPROTECTOR - /* - * Configure the stack canary. The linker can't handle this by - * relocation. Manually set base address in stack canary - * segment descriptor. - */ - movl $gdt_page,%eax - movl $stack_canary,%ecx - movw %cx, 8 * GDT_ENTRY_STACK_CANARY + 2(%eax) - shrl $16, %ecx - movb %cl, 8 * GDT_ENTRY_STACK_CANARY + 4(%eax) - movb %ch, 8 * GDT_ENTRY_STACK_CANARY + 7(%eax) -#endif - andl $0,setup_once_ref /* Once is enough, thanks */ ret |