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:
#ifdef CONFIG_ARM64 static noinline void lkdtm_scs_set_lr(unsigned long *addr) { unsigned long **lr = (unsigned long **)__builtin_frame_address(0) + 1; *lr = addr; }
/* Function with __noscs attribute clears its return address. */ static noinline void __noscs lkdtm_noscs_set_lr(unsigned long *addr) { unsigned long **lr = (unsigned long **)__builtin_frame_address(0) + 1; *lr = addr; } #endif
void lkdtm_CFI_BACKWARD_SHADOW(void) { #ifdef CONFIG_ARM64
/* Verify the "normal" condition of LR corruption working. */ do { /* Keep label in scope to avoid compiler warning. */ if ((volatile int)0) goto unexpected;
pr_info("Trying to corrupt lr in a function without scs protection ...\n"); lkdtm_noscs_set_lr(&&expected);unexpected: pr_err("XPASS: Unexpectedly survived lr corruption without scs?!\n"); break;
expected: pr_err("ok: lr corruption redirected without scs.\n"); } while (0);
do { /* Keep labe in scope to avoid compiler warning. */ if ((volatile int)0) goto good_scs;
pr_info("Trying to corrupt lr in a function with scs protection ...\n"); lkdtm_scs_set_lr(&&bad_scs);good_scs: pr_info("ok: scs takes effect.\n"); break;
bad_scs: pr_err("FAIL: return address rewritten!\n"); pr_expected_config(CONFIG_SHADOW_CALL_STACK); } while (0); #else pr_err("XFAIL: this test is arm64-only\n"); #endif }
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.