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>

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 6a5c0ec5ee0e5a55d04f971730b7a9239bd05c2b..5abd9a535a2437b94951786402ce2a43943ded4e 100644 (file)
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -291,9 +291,9 @@ struct tss_struct {
        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;
 

diff --git a/arch/x86/kernel/asm-offsets_32.c b/arch/x86/kernel/asm-offsets_32.c
index 3b3b9d33ac1d2f0809db3b16230c840e98774b70..47703aed74cfb7d013b2d577488d68c7280bf8c6 100644 (file)
--- a/arch/x86/kernel/asm-offsets_32.c
+++ b/arch/x86/kernel/asm-offsets_32.c
@@ -68,7 +68,7 @@ void foo(void)
 
        /* Offset from the sysenter stack to 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)
        BLANK();

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 76348334b934722d353003c53782a0832fcd1bed..7a3dfb1db78d240c4acc20e6209ea708cbb0135e 100644 (file)
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -987,7 +987,7 @@ void enable_sep_cpu(void)
        }
 
        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_ESP, tss->x86_tss.sp1, 0);
        wrmsr(MSR_IA32_SYSENTER_EIP, (unsigned long) ia32_sysenter_target, 0);