Commit-ID: 1d8ca3be86ebc6a38dad8236f45c7a9c61681e78
Gitweb: https://git.kernel.org/tip/1d8ca3be86ebc6a38dad8236f45c7a9c61681e78
Author: Waiman Long <[email protected]>
AuthorDate: Tue, 6 Nov 2018 15:12:29 -0500
Committer: Ingo Molnar <[email protected]>
CommitDate: Mon, 12 Nov 2018 11:06:19 +0100
x86/mm/fault: Allow stack access below %rsp
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 <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link:
http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>
---
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 71d4b9d4d43f..29525cf21100 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1380,18 +1380,6 @@ retry:
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;
