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:
> > 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;
}