linux-kernel - Re: [PATCH v2] selftests/sgx: Fix Q1 and Q2 calculation in sigstruct.c

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Message-ID: <be6227d1-728a-c658-f962-380c28afc926@linuxfoundation.org>
Date:   Fri, 23 Jul 2021 13:53:06 -0600
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Jarkko Sakkinen <jarkko@...nel.org>, Shuah Khan <shuah@...nel.org>
Cc:     linux-kselftest@...r.kernel.org, linux-sgx@...r.kernel.org,
        Reinette Chatre <reinette.chatre@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Tianjia Zhang <tianjia.zhang@...ux.alibaba.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH v2] selftests/sgx: Fix Q1 and Q2 calculation in
 sigstruct.c

On 7/4/21 11:09 PM, Jarkko Sakkinen wrote:
> From: Tianjia Zhang <tianjia.zhang@...ux.alibaba.com>
> 
> Q1 and Q2 are numbers with *maximum* length of 384 bytes. If the calculated
> length of Q1 and Q2 is less than 384 bytes, things will go wrong.
> 
> E.g. if Q2 is 383 bytes, then
> 
> 1. The bytes of q2 are copied to sigstruct->q2 in calc_q1q2().
> 2. The entire sigstruct->q2 is reversed, which results it being
>     256 * Q2, given that the last byte of sigstruct->q2 is added
>     to before the bytes given by calc_q1q2().
> 
> Either change in key or measurement can trigger the bug. E.g. an unmeasured
> heap could cause a devastating change in Q1 or Q2.
> 
> Reverse exactly the bytes of Q1 and Q2 in calc_q1q2() before returning to
> the caller.
> 
> Fixes: dedde2634570 ("selftests/sgx: Trigger the reclaimer in the selftests")
> Link: https://lore.kernel.org/linux-sgx/20210301051836.30738-1-tianjia.zhang@linux.alibaba.com/
> Signed-off-by: Tianjia Zhang <tianjia.zhang@...ux.alibaba.com>
> Signed-off-by: Jarkko Sakkinen <jarkko@...nel.org>
> ---
> The original patch did a bad job explaining the code change but it
> turned out making sense. I wrote a new description.
> 
> v2:
> - Added a fixes tag.
>   tools/testing/selftests/sgx/sigstruct.c | 41 +++++++++++++------------
>   1 file changed, 21 insertions(+), 20 deletions(-)
> 
> diff --git a/tools/testing/selftests/sgx/sigstruct.c b/tools/testing/selftests/sgx/sigstruct.c
> index dee7a3d6c5a5..92bbc5a15c39 100644
> --- a/tools/testing/selftests/sgx/sigstruct.c
> +++ b/tools/testing/selftests/sgx/sigstruct.c
> @@ -55,10 +55,27 @@ static bool alloc_q1q2_ctx(const uint8_t *s, const uint8_t *m,
>   	return true;
>   }
>   
> +static void reverse_bytes(void *data, int length)
> +{
> +	int i = 0;
> +	int j = length - 1;
> +	uint8_t temp;
> +	uint8_t *ptr = data;
> +
> +	while (i < j) {
> +		temp = ptr[i];
> +		ptr[i] = ptr[j];
> +		ptr[j] = temp;
> +		i++;
> +		j--;
> +	}
> +}

I was just about apply this one and noticed this reverse_bytes().
Aren't there byteswap functions you could call instead of writing
your own?

thanks,
-- Shuah