[kernel-hardening] [PATCH v2 10/13] x86/dumpstack: Try harder to get a call trace on stack overflow

[Andy Lutomirski <luto@kernel.org>]

If we overflow the stack, print_context_stack will abort.  Detect
this case and rewind back into the valid part of the stack so that
we can trace it.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/kernel/dumpstack.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index d4d085e27d04..9cdf05d768cf 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -89,7 +89,7 @@ static inline int valid_stack_ptr(struct thread_info *tinfo,
 		else
 			return 0;
 	}
-	return p > t && p < t + THREAD_SIZE - size;
+	return p >= t && p < t + THREAD_SIZE - size;
 }
 
 unsigned long
@@ -100,6 +100,13 @@ print_context_stack(struct thread_info *tinfo,
 {
 	struct stack_frame *frame = (struct stack_frame *)bp;
 
+	/*
+	 * If we overflowed the stack into a guard page, jump back to the
+	 * bottom of the usable stack.
+	 */
+	if ((unsigned long)tinfo - (unsigned long)stack < PAGE_SIZE)
+		stack = (unsigned long *)tinfo;
+
 	while (valid_stack_ptr(tinfo, stack, sizeof(*stack), end)) {
 		unsigned long addr;
 
-- 
2.5.5