[PATCH v2] x86/mm/fault: Allow stack access below %rsp

[Waiman Long]

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@xxxxxxxxxx>
---
 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 71d4b9d..29525cf 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1380,18 +1380,6 @@ void do_user_addr_fault(struct pt_regs *regs,
 		bad_area(regs, sw_error_code, address);
 		return;
 	}
-	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);
 		return;
-- 
1.8.3.1