x86/asm/entry: Replace this_cpu_sp0() with current_top_of_stack() and fix it on x86_32

I broke 32-bit kernels.  The implementation of sp0 was correct
as far as I can tell, but sp0 was much weirder on x86_32 than I
realized.  It has the following issues:

 - Init's sp0 is inconsistent with everything else's: non-init tasks
   are offset by 8 bytes.  (I have no idea why, and the comment is unhelpful.)

 - vm86 does crazy things to sp0.

Fix it up by replacing this_cpu_sp0() with
current_top_of_stack() and using a new percpu variable to track
the top of the stack on x86_32.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 75182b1632a8 ("x86/asm/entry: Switch all C consumers of kernel_stack to this_cpu_sp0()")
Link: http://lkml.kernel.org/r/d09dbe270883433776e0cbee3c7079433349e96d.1425692936.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 11 insertions, 4 deletions

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index f5e3ec63767d..48a61c1c626e 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -284,6 +284,10 @@ struct tss_struct {
 
 DECLARE_PER_CPU_SHARED_ALIGNED(struct tss_struct, cpu_tss);
 
+#ifdef CONFIG_X86_32
+DECLARE_PER_CPU(unsigned long, cpu_current_top_of_stack);
+#endif
+
 /*
  * Save the original ist values for checking stack pointers during debugging
  */
@@ -564,9 +568,14 @@ static inline void native_swapgs(void)
 #endif
 }
 
-static inline unsigned long this_cpu_sp0(void)
+static inline unsigned long current_top_of_stack(void)
 {
+#ifdef CONFIG_X86_64
         return this_cpu_read_stable(cpu_tss.x86_tss.sp0);
+#else
+        /* sp0 on x86_32 is special in and around vm86 mode. */
+        return this_cpu_read_stable(cpu_current_top_of_stack);
+#endif
 }
 
 #ifdef CONFIG_PARAVIRT
diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index a2fa1899494e..7740edd56fed 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -158,9 +158,7 @@ DECLARE_PER_CPU(unsigned long, kernel_stack);
 
 static inline struct thread_info *current_thread_info(void)
 {
-        struct thread_info *ti;
-        ti = (void *)(this_cpu_sp0() - THREAD_SIZE);
-        return ti;
+        return (struct thread_info *)(current_top_of_stack() - THREAD_SIZE);
 }
 
 static inline unsigned long current_stack_pointer(void)