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

[Dan Li]

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.