Messages in this thread | | | Date | Mon, 7 Mar 2022 07:16:36 -0800 | Subject | Re: [PATCH v3 2/2] lkdtm: Add Shadow Call Stack tests | From | Dan Li <> |
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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.
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