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/platform | |
| parent | Linux 5.12-rc2 (diff) | |
| download | linux-dev-3fb0fdb3bbe7aed495109b3296b06c2409734023.tar.xz linux-dev-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/platform')
| -rw-r--r-- | arch/x86/platform/pvh/head.S | 14 |
1 files changed, 0 insertions, 14 deletions
diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S index d2ccadc247e6..b0490701da2a 100644 --- a/arch/x86/platform/pvh/head.S +++ b/arch/x86/platform/pvh/head.S @@ -46,10 +46,8 @@ #define PVH_GDT_ENTRY_CS 1 #define PVH_GDT_ENTRY_DS 2 -#define PVH_GDT_ENTRY_CANARY 3 #define PVH_CS_SEL (PVH_GDT_ENTRY_CS * 8) #define PVH_DS_SEL (PVH_GDT_ENTRY_DS * 8) -#define PVH_CANARY_SEL (PVH_GDT_ENTRY_CANARY * 8) SYM_CODE_START_LOCAL(pvh_start_xen) cld @@ -111,17 +109,6 @@ SYM_CODE_START_LOCAL(pvh_start_xen) #else /* CONFIG_X86_64 */ - /* Set base address in stack canary descriptor. */ - movl $_pa(gdt_start),%eax - movl $_pa(canary),%ecx - movw %cx, (PVH_GDT_ENTRY_CANARY * 8) + 2(%eax) - shrl $16, %ecx - movb %cl, (PVH_GDT_ENTRY_CANARY * 8) + 4(%eax) - movb %ch, (PVH_GDT_ENTRY_CANARY * 8) + 7(%eax) - - mov $PVH_CANARY_SEL,%eax - mov %eax,%gs - call mk_early_pgtbl_32 mov $_pa(initial_page_table), %eax @@ -165,7 +152,6 @@ SYM_DATA_START_LOCAL(gdt_start) .quad GDT_ENTRY(0xc09a, 0, 0xfffff) /* PVH_CS_SEL */ #endif .quad GDT_ENTRY(0xc092, 0, 0xfffff) /* PVH_DS_SEL */ - .quad GDT_ENTRY(0x4090, 0, 0x18) /* PVH_CANARY_SEL */ SYM_DATA_END_LABEL(gdt_start, SYM_L_LOCAL, gdt_end) .balign 16 |