From 1d8ca3be86ebc6a38dad8236f45c7a9c61681e78 Mon Sep 17 00:00:00 2001 From: Waiman Long Date: Tue, 6 Nov 2018 15:12:29 -0500 Subject: 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 Cc: Andy Lutomirski Cc: Borislav Petkov Cc: Brian Gerst Cc: Dave Hansen Cc: Denys Vlasenko Cc: H. Peter Anvin Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Rik van Riel Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/1541535149-31963-1-git-send-email-longman@redhat.com Signed-off-by: Ingo Molnar --- arch/x86/mm/fault.c | 12 ------------ 1 file changed, 12 deletions(-) (limited to 'arch/x86/mm/fault.c') 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; -- cgit v1.2.3