x86: be more careful when walking back the frame pointer chain

When showing the stack backtrace, make sure that we never accept not
only an unchanging frame pointer, but also a frame pointer that moves
back down the stack frame.  It must always grow up (toward older stack
frames).

I doubt this has triggered, but a subtly corrupt stack with extremely
unlucky contents could cause us to loop forever on a bogus endless frame
pointer chain.

This review was triggered by much worse problems happening in some of
the other stack unwinding code.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 7 insertions, 3 deletions

diff --git a/arch/i386/kernel/traps.c b/arch/i386/kernel/traps.c
index 00489b706d27..fe9c5e8e7e6f 100644
--- a/arch/i386/kernel/traps.c
+++ b/arch/i386/kernel/traps.c
@@ -129,15 +129,19 @@ static inline unsigned long print_context_stack(struct thread_info *tinfo,
 
 #ifdef  CONFIG_FRAME_POINTER
         while (valid_stack_ptr(tinfo, (void *)ebp)) {
+                unsigned long new_ebp;
                 addr = *(unsigned long *)(ebp + 4);
                 ops->address(data, addr);
                 /*
                  * break out of recursive entries (such as
-                 * end_of_stack_stop_unwind_function):
+                 * end_of_stack_stop_unwind_function). Also,
+                 * we can never allow a frame pointer to
+                 * move downwards!
                  */
-                if (ebp == *(unsigned long *)ebp)
+                new_ebp = *(unsigned long *)ebp;
+                if (new_ebp <= ebp)
                         break;
-                ebp = *(unsigned long *)ebp;
+                ebp = new_ebp;
         }
 #else
         while (valid_stack_ptr(tinfo, stack)) {