https://bugs.llvm.org/show_bug.cgi?id=35449
Bug ID: 35449
Summary: Windows stack guard needs to XOR the stack pointer
with __stack_cookie
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: Common Code Generator Code
Assignee: unassignedb...@nondot.org
Reporter: r...@google.com
CC: llvm-bugs@lists.llvm.org
Consider:
void g(int*);
void f() {
int x[16];
g(&x[0]);
}
Compiled like so, here is a comparison of the relevant code:
$ cl -O2 -GS t2.c -Fat.s -c && grep -B1 -A2 security t.s
...
sub rsp, 120 ; 00000078H
mov rax, QWORD PTR __security_cookie
xor rax, rsp
mov QWORD PTR __$ArrayPad$[rsp], rax
--
xor rcx, rsp
call __security_check_cookie
add rsp, 120 ; 00000078H
ret 0
$ clang-cl -O2 -GS t2.c -Fat.s -c && grep -B1 -A2 security t.s
.seh_endprologue
movq __security_cookie(%rip), %rax
movq %rax, 96(%rsp)
leaq 32(%rsp), %rcx
--
movq 96(%rsp), %rcx
callq __security_check_cookie
nop
addq $104, %rsp
The same is done with EBP on 32-bit. XORing the stack pointer (or frame pointer
when appropriate) into the canary improves security by making it harder to a
valid cookie from one location to another to bypass the guard.
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