Messages in this thread | | | Date | Fri, 4 Mar 2022 06:34:30 -0800 | Subject | Re: [PATCH v3 2/2] lkdtm: Add Shadow Call Stack tests | From | Dan Li <> |
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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.
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