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.
