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authorAndy Lutomirski <luto@kernel.org>2021-02-13 11:19:44 -0800
committerBorislav Petkov <bp@suse.de>2021-03-08 13:19:05 +0100
commit3fb0fdb3bbe7aed495109b3296b06c2409734023 (patch)
tree72e8821e31ab11816ab4bf26f7d41e915cb022cd /arch/x86/platform
parenta38fd8748464831584a19438cbb3082b5a2dab15 (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/platform')
-rw-r--r--arch/x86/platform/pvh/head.S14
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