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Subject: Re: [PATCH v13 15/16] selftests/x86/fsgsbase: Test GS selector on
 ptracer-induced GS base write
To:     Sasha Levin <sashal@kernel.org>, tglx@linutronix.de,
        luto@kernel.org, ak@linux.intel.com
Cc:     corbet@lwn.net, mingo@redhat.com, bp@alien8.de, x86@kernel.org,
        shuah@kernel.org, gregkh@linuxfoundation.org, tony.luck@intel.com,
        chang.seok.bae@intel.com, dave.hansen@linux.intel.com,
        peterz@infradead.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, jarkko.sakkinen@linux.intel.com,
        "H . Peter Anvin" <hpa@zytor.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Shuah Khan <skhan@linuxfoundation.org>
References: <20200528201402.1708239-1-sashal@kernel.org>
 <20200528201402.1708239-16-sashal@kernel.org>
From:   Shuah Khan <skhan@linuxfoundation.org>
Message-ID: <d51d8e60-cb5f-fa97-a00c-9c6a2e42b42b@linuxfoundation.org>
Date:   Fri, 29 May 2020 08:40:42 -0600
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On 5/28/20 2:14 PM, Sasha Levin wrote:
> From: "Chang S. Bae" <chang.seok.bae@intel.com>
> 
> The test validates that the selector is not changed when a ptracer writes
> the ptracee's GS base.
> 
> Originally-by: Andy Lutomirski <luto@kernel.org>
> Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
> Reviewed-by: Tony Luck <tony.luck@intel.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Dave Hansen <dave.hansen@intel.com>
> Cc: Tony Luck <tony.luck@intel.com>
> Cc: Andi Kleen <ak@linux.intel.com>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
>   tools/testing/selftests/x86/fsgsbase.c | 21 +++++++++++++++------
>   1 file changed, 15 insertions(+), 6 deletions(-)
> 
> diff --git a/tools/testing/selftests/x86/fsgsbase.c b/tools/testing/selftests/x86/fsgsbase.c
> index 15a329da59fa..950a48b2e366 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, base;
> +		unsigned long gs;
>   		unsigned long gs_offset = USER_REGS_OFFSET(gs);
>   		unsigned long base_offset = USER_REGS_OFFSET(gs_base);
>   
> @@ -481,7 +481,6 @@ 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
> @@ -489,11 +488,21 @@ static void test_ptrace_write_gsbase(void)
>   		 * selector value is changed or not by the GSBASE write in
>   		 * a ptracer.
>   		 */
> -		if (gs == 0 && base == 0xFF) {
> -			printf("[OK]\tGS was reset as expected\n");
> -		} else {
> +		if (gs != *shared_scratch) {
>   			nerrs++;
> -			printf("[FAIL]\tGS=0x%lx, GSBASE=0x%lx (should be 0, 0xFF)\n", gs, base);
> +			printf("[FAIL]\tGS changed to %lx\n", gs);
> +
> +			/*
> +			 * On older kernels, poking a nonzero value into the
> +			 * base would zero the selector.  On newer kernels,
> +			 * this behavior has changed -- poking the base
> +			 * changes only the base and, if FSGSBASE is not
> +			 * available, this may not effect.
> +			 */
> +			if (gs == 0)
> +				printf("\tNote: this is expected behavior on older kernels.\n");

Let's add what is "this" for context in this message.

> +		} else {
> +			printf("[OK]\tGS remained 0x%hx\n", *shared_scratch);
>   		}
>   	}
>   
> 

thanks,
-- Shuah