Re: [RFC PATCH 3/5] selftest/x86: Fix sigreturn_64 test.

[Kees Cook <keescook@chromium.org>]

On Thu, May 21, 2020 at 02:17:18PM -0700, Yu-cheng Yu wrote:
> When shadow stack is enabled, selftests/x86/sigreturn_64 triggers a fault
> when doing sigreturn to 32-bit context but the task's shadow stack pointer
> is above 32-bit address range.  Fix it by:
> 
> - Allocate a small shadow stack below 32-bit address,
> - Switch to the new shadow stack,
> - Run tests,
> - Switch back to the original 64-bit shadow stack.
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> ---
>  tools/testing/selftests/x86/sigreturn.c | 28 +++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
> 
> diff --git a/tools/testing/selftests/x86/sigreturn.c b/tools/testing/selftests/x86/sigreturn.c
> index 57c4f67f16ef..5bcd74d416ff 100644
> --- a/tools/testing/selftests/x86/sigreturn.c
> +++ b/tools/testing/selftests/x86/sigreturn.c
> @@ -45,6 +45,14 @@
>  #include <stdbool.h>
>  #include <sys/ptrace.h>
>  #include <sys/user.h>
> +#include <x86intrin.h>
> +#include <asm/prctl.h>
> +#include <sys/prctl.h>
> +
> +#ifdef __x86_64__
> +int arch_prctl(int code, unsigned long *addr);
> +#define ARCH_CET_ALLOC_SHSTK 0x3004
> +#endif
>  
>  /* Pull in AR_xyz defines. */
>  typedef unsigned int u32;
> @@ -766,6 +774,20 @@ int main()
>  	int total_nerrs = 0;
>  	unsigned short my_cs, my_ss;
>  
> +#ifdef __x86_64__
> +	/* Alloc a shadow stack within 32-bit address range */
> +	unsigned long arg, ssp_64, ssp_32;
> +	ssp_64 = _get_ssp();
> +
> +	if (ssp_64 != 0) {
> +		arg = 0x1001;
> +		arch_prctl(ARCH_CET_ALLOC_SHSTK, &arg);
> +		ssp_32 = arg + 0x1000 - 8;
> +		asm volatile("RSTORSSP (%0)\n":: "r" (ssp_32));
> +		asm volatile("SAVEPREVSSP");
> +	}
> +#endif

If the headers and code are going to be repeated, I would put that in a
shared header so they're not copy/pasted between these two tests.

-Kees

-- 
Kees Cook