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

Dan Li
2022-03-04T15:01:30Z

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;
> }