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x86/asm/entry/32: Document the 32-bit SYSENTER "emergency stack" better 

Before the patch, the 'tss_struct::stack' field was not referenced anywhere.

It was used only to set SYSENTER's stack to point after the last byte
of tss_struct, thus the trailing field, stack[64], was used.

But grep would not know it. You can comment it out, compile,
and kernel will even run until an unlucky NMI corrupts
io_bitmap[] (which is also not easily detectable).

This patch changes code so that the purpose and usage of this
field is not mysterious anymore, and can be easily grepped for.

This does change generated code, for a subtle reason:
since tss_struct is ____cacheline_aligned, there happens to be
5 longs of padding at the end. Old code was using the padding
too; new code will strictly use it only for SYSENTER_stack[].

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1425912738-559-2-git-send-email-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Denys Vlasenko 2015-03-09 15:52:18 +01:00 committed by Ingo Molnar
parent 3876488444
commit d828c71fba
3 changed files with 4 additions and 4 deletions
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4
arch/x86/include/asm/processor.h
View File

@ -291,9 +291,9 @@ struct tss_struct {
unsigned long io_bitmap[IO_BITMAP_LONGS + 1]; unsigned long io_bitmap[IO_BITMAP_LONGS + 1];
/* /*
* .. and then another 0x100 bytes for the emergency kernel stack: * Space for the temporary SYSENTER stack:
*/ */
unsigned long stack[64]; unsigned long SYSENTER_stack[64];
} ____cacheline_aligned; } ____cacheline_aligned;

2
arch/x86/kernel/asm-offsets_32.c
View File

@ -68,7 +68,7 @@ void foo(void)
/* Offset from the sysenter stack to tss.sp0 */ /* Offset from the sysenter stack to tss.sp0 */
DEFINE(TSS_sysenter_sp0, offsetof(struct tss_struct, x86_tss.sp0) - DEFINE(TSS_sysenter_sp0, offsetof(struct tss_struct, x86_tss.sp0) -
sizeof(struct tss_struct)); offsetofend(struct tss_struct, SYSENTER_stack));
#if defined(CONFIG_LGUEST) || defined(CONFIG_LGUEST_GUEST) || defined(CONFIG_LGUEST_MODULE) #if defined(CONFIG_LGUEST) || defined(CONFIG_LGUEST_GUEST) || defined(CONFIG_LGUEST_MODULE)
BLANK(); BLANK();

2
arch/x86/kernel/cpu/common.c
View File

@ -987,7 +987,7 @@ void enable_sep_cpu(void)
} }
tss->x86_tss.ss1 = __KERNEL_CS; tss->x86_tss.ss1 = __KERNEL_CS;
tss->x86_tss.sp1 = sizeof(struct tss_struct) + (unsigned long) tss; tss->x86_tss.sp1 = (unsigned long)tss + offsetofend(struct tss_struct, SYSENTER_stack);
wrmsr(MSR_IA32_SYSENTER_CS, __KERNEL_CS, 0); wrmsr(MSR_IA32_SYSENTER_CS, __KERNEL_CS, 0);
wrmsr(MSR_IA32_SYSENTER_ESP, tss->x86_tss.sp1, 0); wrmsr(MSR_IA32_SYSENTER_ESP, tss->x86_tss.sp1, 0);
wrmsr(MSR_IA32_SYSENTER_EIP, (unsigned long) ia32_sysenter_target, 0); wrmsr(MSR_IA32_SYSENTER_EIP, (unsigned long) ia32_sysenter_target, 0);