On 3/3/22 10:42, Kees Cook wrote:
On Wed, Mar 02, 2022 at 11:43:39PM -0800, Dan Li wrote:
Add tests for SCS (Shadow Call Stack) based backward CFI (as implemented by Clang and GCC).
Cool; thanks for writing these!
+lkdtm-$(CONFIG_LKDTM) += scs.o
I'd expect these to be in cfi.c, rather than making a new source file.
Got it.
+static noinline void lkdtm_scs_clear_lr(void) +{
- unsigned long *lr = (unsigned long *)__builtin_frame_address(0) + 1;
- asm volatile("str xzr, [%0]\n\t" : : "r"(lr) : "x30");
Is the asm needed here? Why not:
unsigned long *lr = (unsigned long *)__builtin_frame_address(0) + 1;
*lr = 0;
Yeah, with "volatile", this one looks better.
+/*
- This tries to call a function protected by Shadow Call Stack,
- which corrupts its own return address during execution.
- Due to the protection, the corruption will not take effect
- when the function returns.
- */
+void lkdtm_CFI_BACKWARD_SHADOW(void)
I think these two tests should be collapsed into a single one.
It seems that there is currently no cross-line matching in selftests/lkdtm/run.sh, if we put these two into one function and assume we could make noscs_set_lr _survivable_ (like in your example).
Then we could only match "CFI_BACKWARD_SHADOW ok: scs takes effect." in texts.txt
But if the test result is: XPASS: Unexpectedly survived lr corruption without scs? ok: scs takes effect.
It may not be a real pass, but the xxx_set_lr function doesn't work.
+{ +#ifdef CONFIG_ARM64
- if (!IS_ENABLED(CONFIG_SHADOW_CALL_STACK)) {
pr_err("FAIL: kernel not built with CONFIG_SHADOW_CALL_STACK\n");return;- }
- pr_info("Trying to corrupt lr in a function with scs protection ...\n");
- lkdtm_scs_clear_lr();
- pr_err("ok: scs takes effect.\n");
+#else
- pr_err("XFAIL: this test is arm64-only\n");
+#endif
This is slightly surprising -- we have no detection when a function has its non-shadow-stack return address corrupted: it just _ignores_ the value stored there. That seems like a missed opportunity for warning about an unexpected state.
Yes. Actually I used to try in the plugin to add a detection before the function returns, and call a callback when a mismatch is found. But since almost every function has to be instrumented, the performance penalty is improved from <3% to ~20% (rough calculation, should still be optimized).
+}
+/*
- This tries to call a function not protected by Shadow Call Stack,
- which corrupts its own return address during execution.
- */
+void lkdtm_CFI_BACKWARD_SHADOW_WITH_NOSCS(void) +{ +#ifdef CONFIG_ARM64
- if (!IS_ENABLED(CONFIG_SHADOW_CALL_STACK)) {
pr_err("FAIL: kernel not built with CONFIG_SHADOW_CALL_STACK\n");return;Other tests try to give some hints about failures, e.g.:
pr_err("FAIL: cannot change for SCS\n"); pr_expected_config(CONFIG_SHADOW_CALL_STACK);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 }
Thanks for the example, Kees :) This code (with a little modification) works correctly with clang 12, but to make sure it's always correct, I think we might need to add the __attribute__((optnone)) attribute to it, because under -O2 the result doesn't seem to be "very stable" (as in your example in the next email).
And we should, actually, be able to make the "set_lr" functions be arch-specific, leaving the test itself arch-agnostic....
I'm not sure if my understanding is correct, do it means we should remove the "#ifdef CONFIG_ARM64" in lkdtm_CFI_BACKWARD_SHADOW?
Then we may not be able to distinguish between failures caused by platform unsupported (XFAIL) and features not enabled (or not working properly).
Thanks, Dan.