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Date:   Tue, 5 Apr 2022 18:48:09 -0700
From:   Dan Li <ashimida@...ux.alibaba.com>
To:     catalin.marinas@....com, will@...nel.org, keescook@...omium.org,
        arnd@...db.de, gregkh@...uxfoundation.org, nathan@...nel.org,
        ndesaulniers@...gle.com, shuah@...nel.org, mark.rutland@....com,
        ojeda@...nel.org, akpm@...ux-foundation.org, elver@...gle.com,
        luc.vanoostenryck@...il.com, samitolvanen@...gle.com
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, linux-kselftest@...r.kernel.org,
        linux-hardening@...r.kernel.org,
        Dan Li <ashimida@...ux.alibaba.com>
Subject: Re: [PATCH v4 2/2] lkdtm: Add Shadow Call Stack tests

Hi Kees,

Gentile ping for this :).

I also saw the discussion on llvm-project, use address of labels as a
parameter doesn't seem to be stable.

Do we need to split it into two cases here?

Link: https://github.com/llvm/llvm-project/issues/54328

Thanks,
Dan


On 3/14/22 06:53, Dan Li wrote:
> Add tests for SCS (Shadow Call Stack) based backward CFI.
> 
> Signed-off-by: Dan Li <ashimida@...ux.alibaba.com>
> ---
>   arch/arm64/include/asm/compiler.h       | 18 ++++++
>   drivers/misc/lkdtm/cfi.c                | 84 +++++++++++++++++++++++++
>   drivers/misc/lkdtm/core.c               |  1 +
>   drivers/misc/lkdtm/lkdtm.h              |  1 +
>   include/linux/compiler-clang.h          |  1 +
>   include/linux/compiler-gcc.h            |  2 +
>   include/linux/compiler_types.h          |  4 ++
>   tools/testing/selftests/lkdtm/tests.txt |  1 +
>   8 files changed, 112 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/compiler.h b/arch/arm64/include/asm/compiler.h
> index dc3ea4080e2e..96590fb4a8de 100644
> --- a/arch/arm64/include/asm/compiler.h
> +++ b/arch/arm64/include/asm/compiler.h
> @@ -8,6 +8,24 @@
>   #define ARM64_ASM_PREAMBLE
>   #endif
>   
> +#ifndef __ASSEMBLY__
> +#ifdef __KERNEL__
> +
> +#ifdef CONFIG_ARM64_PTR_AUTH_KERNEL
> +
> +#ifdef CONFIG_ARM64_BTI_KERNEL
> +# define __no_ptrauth __attribute__((target("branch-protection=bti")))
> +#elif defined(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET)
> +# define __no_ptrauth __attribute__((target("branch-protection=none")))
> +#elif defined(CONFIG_CC_HAS_SIGN_RETURN_ADDRESS)
> +# define __no_ptrauth __attribute__((target("sign-return-address=none")))
> +#endif
> +
> +#endif /* CONFIG_ARM64_PTR_AUTH_KERNEL */
> +
> +#endif /* __KERNEL__ */
> +#endif /* __ASSEMBLY__ */
> +
>   /*
>    * The EL0/EL1 pointer bits used by a pointer authentication code.
>    * This is dependent on TBI0/TBI1 being enabled, or bits 63:56 would also apply.
> diff --git a/drivers/misc/lkdtm/cfi.c b/drivers/misc/lkdtm/cfi.c
> index c9aeddef1044..468ba2f26f74 100644
> --- a/drivers/misc/lkdtm/cfi.c
> +++ b/drivers/misc/lkdtm/cfi.c
> @@ -41,3 +41,87 @@ void lkdtm_CFI_FORWARD_PROTO(void)
>   	pr_err("FAIL: survived mismatched prototype function call!\n");
>   	pr_expected_config(CONFIG_CFI_CLANG);
>   }
> +
> +#ifdef CONFIG_ARM64
> +/*
> + * This function is used to modify its return address. The PAC needs to be turned
> + * off here to ensure that the modification of the return address will not be blocked.
> + */
> +static noinline __no_ptrauth
> +void lkdtm_scs_set_lr(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;
> +}
> +
> +/* Function with __noscs attribute attempts to modify its return address. */
> +static noinline __no_ptrauth __noscs
> +void lkdtm_noscs_set_lr(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;
> +}
> +#else
> +static inline void lkdtm_noscs_set_lr(unsigned long *expected, unsigned long *addr) { }
> +static inline void lkdtm_scs_set_lr(unsigned long *expected, unsigned long *addr) { }
> +#endif
> +
> +static volatile unsigned int force_label;
> +
> +/*
> + * This first checks whether a function with the __noscs attribute under
> + * the current platform can directly modify its return address, and if so,
> + * checks whether scs takes effect.
> + */
> +void __no_optimize lkdtm_CFI_BACKWARD_SHADOW(void)
> +{
> +	void *array[] = {&&unexpected, &&expected, &&good_scs, &&bad_scs};
> +
> +	if (force_label && (force_label < sizeof(array))) {
> +		/*
> +		 * Call them with "NULL" first to avoid
> +		 * arguments being treated as constants in -02.
> +		 */
> +		lkdtm_noscs_set_lr(NULL, NULL);
> +		lkdtm_scs_set_lr(NULL, NULL);
> +		goto *array[force_label];
> +	}
> +
> +	/* Keep labels in scope to avoid compiler warnings. */
> +	do {
> +		/* Verify the "normal" condition of LR corruption working. */
> +		pr_info("Trying to corrupt lr in a function without scs protection ...\n");
> +		lkdtm_noscs_set_lr(&&unexpected, &&expected);
> +
> +unexpected:
> +		/*
> +		 * If lr cannot be modified, the following check is meaningless,
> +		 * returns directly.
> +		 */
> +		pr_err("XPASS: Unexpectedly survived lr corruption without scs?!\n");
> +		break;
> +
> +expected:
> +		pr_info("ok: lr corruption redirected without scs.\n");
> +
> +		/* Verify that SCS is in effect. */
> +		pr_info("Trying to corrupt lr in a function with scs protection ...\n");
> +		lkdtm_scs_set_lr(&&good_scs, &&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);
> +}
> diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
> index f69b964b9952..7af7268b82e4 100644
> --- a/drivers/misc/lkdtm/core.c
> +++ b/drivers/misc/lkdtm/core.c
> @@ -178,6 +178,7 @@ static const struct crashtype crashtypes[] = {
>   	CRASHTYPE(USERCOPY_KERNEL),
>   	CRASHTYPE(STACKLEAK_ERASING),
>   	CRASHTYPE(CFI_FORWARD_PROTO),
> +	CRASHTYPE(CFI_BACKWARD_SHADOW),
>   	CRASHTYPE(FORTIFIED_OBJECT),
>   	CRASHTYPE(FORTIFIED_SUBOBJECT),
>   	CRASHTYPE(FORTIFIED_STRSCPY),
> diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
> index d6137c70ebbe..a66fba949ab5 100644
> --- a/drivers/misc/lkdtm/lkdtm.h
> +++ b/drivers/misc/lkdtm/lkdtm.h
> @@ -157,6 +157,7 @@ void lkdtm_STACKLEAK_ERASING(void);
>   
>   /* cfi.c */
>   void lkdtm_CFI_FORWARD_PROTO(void);
> +void lkdtm_CFI_BACKWARD_SHADOW(void);
>   
>   /* fortify.c */
>   void lkdtm_FORTIFIED_OBJECT(void);
> diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
> index 3c4de9b6c6e3..2db37db36651 100644
> --- a/include/linux/compiler-clang.h
> +++ b/include/linux/compiler-clang.h
> @@ -68,3 +68,4 @@
>   
>   #define __nocfi		__attribute__((__no_sanitize__("cfi")))
>   #define __cficanonical	__attribute__((__cfi_canonical_jump_table__))
> +#define __no_optimize	__attribute__((optnone))
> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> index deff5b308470..28d1b0ec6656 100644
> --- a/include/linux/compiler-gcc.h
> +++ b/include/linux/compiler-gcc.h
> @@ -162,3 +162,5 @@
>   #if GCC_VERSION < 90100
>   #undef __alloc_size__
>   #endif
> +
> +#define __no_optimize	__attribute__((optimize("-O0")))
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 3c1795fdb568..f5ad83f7ea2f 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -257,6 +257,10 @@ struct ftrace_likely_data {
>   # define __nocfi
>   #endif
>   
> +#ifndef __no_ptrauth
> +# define __no_ptrauth
> +#endif
> +
>   #ifndef __cficanonical
>   # define __cficanonical
>   #endif
> diff --git a/tools/testing/selftests/lkdtm/tests.txt b/tools/testing/selftests/lkdtm/tests.txt
> index 6b36b7f5dcf9..12df67a3b419 100644
> --- a/tools/testing/selftests/lkdtm/tests.txt
> +++ b/tools/testing/selftests/lkdtm/tests.txt
> @@ -73,6 +73,7 @@ USERCOPY_STACK_BEYOND
>   USERCOPY_KERNEL
>   STACKLEAK_ERASING OK: the rest of the thread stack is properly erased
>   CFI_FORWARD_PROTO
> +CFI_BACKWARD_SHADOW ok: scs takes effect
>   FORTIFIED_STRSCPY
>   FORTIFIED_OBJECT
>   FORTIFIED_SUBOBJECT

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