From: Waiman Long <longman@redhat.com>
To: Dave Hansen <dave.hansen@linux.intel.com>,
Andy Lutomirski <luto@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>,
x86@kernel.org, linux-kernel@vger.kernel.org,
Waiman Long <longman@redhat.com>
Subject: [RFC PATCH] x86/mm/fault: Allow stack access below %rsp
Date: Fri, 2 Nov 2018 15:40:10 -0400 [thread overview]
Message-ID: <1541187610-5322-1-git-send-email-longman@redhat.com> (raw)
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 64k+ limit check is kind of arbitrary. So the check is now removed
to just let expand_stack() decide if a segmentation fault should happen.
Signed-off-by: Waiman Long <longman@redhat.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 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
next reply other threads:[~2018-11-02 19:40 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-02 19:40 Waiman Long [this message]
2018-11-02 19:44 ` [RFC PATCH] x86/mm/fault: Allow stack access below %rsp Dave Hansen
2018-11-02 19:50 ` Waiman Long
2018-11-02 20:11 ` Andy Lutomirski
2018-11-02 20:34 ` Waiman Long
2018-11-02 22:28 ` Dave Hansen
2018-11-05 5:11 ` Andy Lutomirski
2018-11-05 5:14 ` Andy Lutomirski
2018-11-05 17:20 ` Dave Hansen
2018-11-05 19:21 ` Andy Lutomirski
2018-11-05 16:27 ` Waiman Long
2018-11-05 17:51 ` Dave Hansen
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