Re: [PATCH v13 16/16] selftests/x86/fsgsbase: Test ptracer-induced GS base write with FSGSBASE

[Shuah Khan]

On 5/28/20 2:14 PM, Sasha Levin wrote:
> From: "Chang S. Bae" <chang.seok.bae@xxxxxxxxx>
>
> This validates that GS selector and base are independently preserved in
> ptrace commands.
>
> Suggested-by: Andy Lutomirski <luto@xxxxxxxxxx>
> Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
> Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Cc: Tony Luck <tony.luck@xxxxxxxxx>
> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> ---
>   tools/testing/selftests/x86/fsgsbase.c | 11 +++++++++--
>   1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/tools/testing/selftests/x86/fsgsbase.c b/tools/testing/selftests/x86/fsgsbase.c
> index 950a48b2e366..9a4349813a30 100644
> --- a/tools/testing/selftests/x86/fsgsbase.c
> +++ b/tools/testing/selftests/x86/fsgsbase.c
> @@ -465,7 +465,7 @@ static void test_ptrace_write_gsbase(void)
>   	wait(&status);
>   
>   	if (WSTOPSIG(status) == SIGTRAP) {
> -		unsigned long gs;
> +		unsigned long gs, base;
>   		unsigned long gs_offset = USER_REGS_OFFSET(gs);
>   		unsigned long base_offset = USER_REGS_OFFSET(gs_base);
>   
> @@ -481,6 +481,7 @@ static void test_ptrace_write_gsbase(void)
>   			err(1, "PTRACE_POKEUSER");
>   
>   		gs = ptrace(PTRACE_PEEKUSER, child, gs_offset, NULL);
> +		base = ptrace(PTRACE_PEEKUSER, child, base_offset, NULL);
>   
>   		/*
>   		 * In a non-FSGSBASE system, the nonzero selector will load
> @@ -501,8 +502,14 @@ static void test_ptrace_write_gsbase(void)
>   			 */
>   			if (gs == 0)
>   				printf("\tNote: this is expected behavior on older kernels.\n");

I know this hasn't changed. Please clarify what is "this" in this
message.

> +		} else if (have_fsgsbase && (base != 0xFF)) {
> +			nerrs++;
> +			printf("[FAIL]\tGSBASE changed to %lx\n", base);
>   		} else {
> -			printf("[OK]\tGS remained 0x%hx\n", *shared_scratch);
> +			printf("[OK]\tGS remained 0x%hx", *shared_scratch);
> +			if (have_fsgsbase)
> +				printf(" and GSBASE changed to 0xFF");
> +			printf("\n");
>   		}
>   	}
>   

thanks,
-- Shuah