Re: [PATCH v3 2/2] lkdtm: Add Shadow Call Stack tests

[Kees Cook]

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!

> 
> Signed-off-by: Dan Li <ashimida@xxxxxxxxxxxxxxxxx>
> ---
>  drivers/misc/lkdtm/Makefile             |  1 +
>  drivers/misc/lkdtm/core.c               |  2 +
>  drivers/misc/lkdtm/lkdtm.h              |  4 ++
>  drivers/misc/lkdtm/scs.c                | 67 +++++++++++++++++++++++++
>  tools/testing/selftests/lkdtm/tests.txt |  2 +
>  5 files changed, 76 insertions(+)
>  create mode 100644 drivers/misc/lkdtm/scs.c
> 
> diff --git a/drivers/misc/lkdtm/Makefile b/drivers/misc/lkdtm/Makefile
> index 2e0aa74ac185..e2fb17868af2 100644
> --- a/drivers/misc/lkdtm/Makefile
> +++ b/drivers/misc/lkdtm/Makefile
> @@ -10,6 +10,7 @@ lkdtm-$(CONFIG_LKDTM)		+= rodata_objcopy.o
>  lkdtm-$(CONFIG_LKDTM)		+= usercopy.o
>  lkdtm-$(CONFIG_LKDTM)		+= stackleak.o
>  lkdtm-$(CONFIG_LKDTM)		+= cfi.o
> +lkdtm-$(CONFIG_LKDTM)		+= scs.o

I'd expect these to be in cfi.c, rather than making a new source file.

>  lkdtm-$(CONFIG_LKDTM)		+= fortify.o
>  lkdtm-$(CONFIG_PPC_64S_HASH_MMU)	+= powerpc.o
>  
> diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
> index f69b964b9952..d0ce0bec117c 100644
> --- a/drivers/misc/lkdtm/core.c
> +++ b/drivers/misc/lkdtm/core.c
> @@ -178,6 +178,8 @@ static const struct crashtype crashtypes[] = {
>  	CRASHTYPE(USERCOPY_KERNEL),
>  	CRASHTYPE(STACKLEAK_ERASING),
>  	CRASHTYPE(CFI_FORWARD_PROTO),
> +	CRASHTYPE(CFI_BACKWARD_SHADOW),
> +	CRASHTYPE(CFI_BACKWARD_SHADOW_WITH_NOSCS),
>  	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..a23d32dfc10b 100644
> --- a/drivers/misc/lkdtm/lkdtm.h
> +++ b/drivers/misc/lkdtm/lkdtm.h
> @@ -158,6 +158,10 @@ void lkdtm_STACKLEAK_ERASING(void);
>  /* cfi.c */
>  void lkdtm_CFI_FORWARD_PROTO(void);
>  
> +/* scs.c */
> +void lkdtm_CFI_BACKWARD_SHADOW(void);
> +void lkdtm_CFI_BACKWARD_SHADOW_WITH_NOSCS(void);
> +
>  /* fortify.c */
>  void lkdtm_FORTIFIED_OBJECT(void);
>  void lkdtm_FORTIFIED_SUBOBJECT(void);
> diff --git a/drivers/misc/lkdtm/scs.c b/drivers/misc/lkdtm/scs.c
> new file mode 100644
> index 000000000000..5922a55a8844
> --- /dev/null
> +++ b/drivers/misc/lkdtm/scs.c
> @@ -0,0 +1,67 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * This is for all the tests relating directly to Shadow Call Stack.
> + */
> +#include "lkdtm.h"
> +
> +#ifdef CONFIG_ARM64
> +/* Function clears its return address. */
> +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;

> +}
> +
> +/* Function with __noscs attribute clears its return address. */
> +static noinline void __noscs lkdtm_noscs_clear_lr(void)
> +{
> +	unsigned long *lr = (unsigned long *)__builtin_frame_address(0) + 1;
> +
> +	asm volatile("str xzr, [%0]\n\t" : : "r"(lr) : "x30");
> +}
> +#endif
> +
> +/*
> + * 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.

> +{
> +#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.

> +}
> +
> +/*
> + * 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
}

And we should, actually, be able to make the "set_lr" functions be
arch-specific, leaving the test itself arch-agnostic....

-- 
Kees Cook