Re: [PATCH v3 2/2] lkdtm: Add Shadow Call Stack tests

Dan Li
2022-03-04T14:54:34Z

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;
}