On 3/3/22 11:09, Kees Cook wrote:
On Thu, Mar 03, 2022 at 10:42:45AM -0800, Kees Cook wrote:
Though, having the IS_ENABLED in there makes me wonder if this test should instead be made _survivable_ on failure. Something like this, completely untested:
And we should, actually, be able to make the "set_lr" functions be arch-specific, leaving the test itself arch-agnostic....
Yeah, as a tested example, this works for x86_64, and based on what you had, I'd expect it to work on arm64 too:
#include <stdio.h>
static __attribute__((noinline)) void set_return_addr(unsigned long *expected, unsigned long *addr) { /* Use of volatile is to make sure final write isn't seen as a dead store. */ unsigned long * volatile *ret_addr = (unsigned long **)__builtin_frame_address(0) + 1;
/* Make sure we've found the right place on the stack before writing it. */ if (*ret_addr == expected) *ret_addr = addr;}
volatile int force_label; int main(void) { do { /* Keep labels in scope. */ if (force_label) goto normal; if (force_label) goto redirected;
set_return_addr(&&normal, &&redirected);normal: printf("I should be skipped\n"); break;
From the assembly code, it seems that "&&normal" does't always equal to the address of label "normal" when we use clang with -O2.
redirected: printf("Redirected\n"); } while (0);
The address of "&&redirected" may appear in the middle of the assembly instructions of the printf. If we unconditionally jump to "&&normal", it may crash directly because x0 is not set correctly.
return 0;}
It does _not_ work under Clang, though, which I'm still looking at.
AFAICT, maybe we could specify -O0 optimization to bypass this.
BTW: Occasionally found, the following code works correctly, but i think it doesn't solve the issue :)
#include <stdio.h>
static __attribute__((noinline)) void set_return_addr(unsigned long *expected, unsigned long *addr) { /* Use of volatile is to make sure final write isn't seen as a dead store. */ unsigned long * volatile *ret_addr = (unsigned long **)__builtin_frame_address(0) + 1;
/* Make sure we've found the right place on the stack before writing it. */ // if (*ret_addr == expected) *ret_addr = addr; } volatile int force_label; int main(void) { do { /* Keep labels in scope. */ if (force_label) goto normal; if (force_label) goto redirected;
set_return_addr(&&normal, &&redirected); normal: printf("I should be skipped\n"); break;
redirected: printf("Redirected\n"); printf("\n"); //add a new printf } while (0);
return 0; }