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

On Mon, Mar 07, 2022 at 07:16:36AM -0800, Dan Li wrote:

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];
    }

Hah! I like that. :) I had a weird switch statement that was working for
me; this is cleaner.

    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).

Strictly speaking, this is entirely correct. :)

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 :)

If we can apply a function attribute to turn off pac for the "does this
work without protections", that should be sufficient.

-- 
Kees Cook

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