[SRU][B][PATCH 5/5] x86/mm/fault: Allow stack access below %rsp

Andrea Righi andrea.righi at canonical.com
Thu Jul 4 15:17:45 UTC 2019

From: Waiman Long <longman at redhat.com>

BugLink: https://bugs.launchpad.net/bugs/1827884

The current x86 page fault handler allows stack access below the stack
pointer if it is no more than 64k+256 bytes. Any access beyond the 64k+
limit will cause a segmentation fault.

The gcc -fstack-check option generates code to probe the stack for
large stack allocation to see if the stack is accessible. The newer gcc
does that while updating the %rsp simultaneously. Older gcc's like gcc4
doesn't do that. As a result, an application compiled with an old gcc
and the -fstack-check option may fail to start at all:

  $ cat test.c
  int main() {
	char tmp[1024*128];
	printf("### ok\n");
	return 0;

  $ gcc -fstack-check -g -o test test.c

  $ ./test
  Segmentation fault

The old binary was working in older kernels where expand_stack() was
somehow called before the check. But it is not working in newer kernels.
Besides, the 64k+ limit check is kind of crude and will not catch a
lot of mistakes that userspace applications may be misbehaving anyway.
I think the kernel isn't the right place for this kind of tests. We
should leave it to userspace instrumentation tools to perform them.

The 64k+ limit check is now removed to just let expand_stack() decide
if a segmentation fault should happen, when the RLIMIT_STACK limit is
exceeded, for example.

Signed-off-by: Waiman Long <longman at redhat.com>
Cc: Andy Lutomirski <luto at kernel.org>
Cc: Borislav Petkov <bp at alien8.de>
Cc: Brian Gerst <brgerst at gmail.com>
Cc: Dave Hansen <dave.hansen at linux.intel.com>
Cc: Denys Vlasenko <dvlasenk at redhat.com>
Cc: H. Peter Anvin <hpa at zytor.com>
Cc: Linus Torvalds <torvalds at linux-foundation.org>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Rik van Riel <riel at surriel.com>
Cc: Thomas Gleixner <tglx at linutronix.de>
Link: http://lkml.kernel.org/r/1541535149-31963-1-git-send-email-longman@redhat.com
Signed-off-by: Ingo Molnar <mingo at kernel.org>
(cherry picked from commit 1d8ca3be86ebc6a38dad8236f45c7a9c61681e78)
Signed-off-by: Andrea Righi <andrea.righi at canonical.com>
 arch/x86/mm/fault.c | 12 ------------
 1 file changed, 12 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index fca3c8def63a..6b6c92bbea28 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1411,18 +1411,6 @@ void do_user_addr_fault(struct pt_regs *regs,
 		bad_area(regs, sw_error_code, address);
-	if (sw_error_code & X86_PF_USER) {
-		/*
-		 * Accessing the stack below %sp is always a bug.
-		 * The large cushion allows instructions like enter
-		 * and pusha to work. ("enter $65535, $31" pushes
-		 * 32 pointers and then decrements %sp by 65535.)
-		 */
-		if (unlikely(address + 65536 + 32 * sizeof(unsigned long) < regs->sp)) {
-			bad_area(regs, sw_error_code, address);
-			return;
-		}
-	}
 	if (unlikely(expand_stack(vma, address))) {
 		bad_area(regs, sw_error_code, address);

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