On 3/4/22 06:54, Dan Li wrote:
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.
Sorry, it should be:
The address of "&&redirected" may appear in the middle of the assembly
instructions of the printf. If we unconditionally jump to "&&redirected",
it may crash directly because x0 of printf is not set correctly.
Thanks,
Dan.
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;
}
