On 3/3/22 11:09, Kees Cook wrote:
On Thu, Mar 03, 2022 at 10:42:45AM -0800, Kees Cook wrote:
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; redirected: printf("Redirected\n"); } while (0);
return 0;}
It does _not_ work under Clang, though, which I'm still looking at.
The following code seems to work fine under clang/gcc, x86_64/aarch64 (also tested in lkdtm_CFI_BACKWARD_SHADOW):
#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); }
static volatile int force_label;
int main(void) { void *array[] = {0, &&normal, &&redirected};
if (force_label) { /* Call it with a NULL to avoid parameters being treated as constants in -02. */ set_return_addr(NULL, NULL); goto * array[force_label]; }
do {
set_return_addr(&&normal, &&redirected);
normal: printf("I should be skipped\n"); break;
redirected: printf("Redirected\n");
} while (0);
return 0; }
But currently it still crashes when I try to enable "-mbranch-protection=pac-ret+leaf+bti".
Because the address of "&&redirected" is not encrypted under pac, the autiasp check will fail when set_return_addr returns, and eventually cause the function to crash when it returns to "&&redirected" ("&&redirected" as a reserved label always seems to start with a bti j insn).
For lkdtm, if we're going to handle both cases in one function, maybe it would be better to turn off the -mbranch-protection=pac-ret+leaf+bti and maybe also turn off -O2 options for the function :)
Thanks, Dan.