powerpc: Fix bad stack check in exception entry

In EXCEPTION_PROLOG_COMMON() we check to see if the stack pointer (r1)
is valid when coming from the kernel.  If it's not valid, we die but
with a nice oops message.

Currently we allocate a stack frame (subtract INT_FRAME_SIZE) before we
check to see if the stack pointer is negative.  Unfortunately, this
won't detect a bad stack where r1 is less than INT_FRAME_SIZE.

This patch fixes the check to compare the modified r1 with
-INT_FRAME_SIZE.  With this, bad kernel stack pointers (including NULL
pointers) are correctly detected again.

Kudos to Paulus for finding this.

Signed-off-by: Michael Neuling <mikey@neuling.org>
cc: stable@vger.kernel.org
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/arch/powerpc/include/asm/exception-64s.h b/arch/powerpc/include/asm/exception-64s.h
index 894662a5d4d5..243ce69ad685 100644
--- a/arch/powerpc/include/asm/exception-64s.h
+++ b/arch/powerpc/include/asm/exception-64s.h
@@ -284,7 +284,7 @@ do_kvm_##n:								\
         subi    r1,r1,INT_FRAME_SIZE;   /* alloc frame on kernel stack  */ \
         beq-    1f;                                                        \
         ld      r1,PACAKSAVE(r13);      /* kernel stack to use          */ \
-1:      cmpdi   cr1,r1,0;               /* check if r1 is in userspace  */ \
+1:      cmpdi   cr1,r1,-INT_FRAME_SIZE; /* check if r1 is in userspace  */ \
         blt+    cr1,3f;                 /* abort if it is               */ \
         li      r1,(n);                 /* will be reloaded later       */ \
         sth     r1,PACA_TRAP_SAVE(r13);                                    \