linux-kernel - Re: [PATCH] selftests/resctrl: Fix null pointer dereference on open failed

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Date:   Mon, 25 Apr 2022 17:06:19 +0100
From:   "Colin King (gmail)" <colin.i.king@...il.com>
To:     Shuah Khan <skhan@...uxfoundation.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Shuah Khan <shuah@...nel.org>, Babu Moger <babu.moger@....com>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
        linux-kselftest@...r.kernel.org
Cc:     kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] selftests/resctrl: Fix null pointer dereference on open
 failed

On 25/04/2022 16:51, Shuah Khan wrote:
> On 4/24/22 3:15 PM, Colin Ian King wrote:
>> Currently if opening /dev/null fails to open then file pointer fp
>> is null and further access to fp via fprintf will cause a null
>> pointer dereference. Fix this by returning a negative error value
>> when a null fp is detected.
>>
> 
> How did you find this problem and how can it be reproduced? Is there
> a case where test fails to open "/dev/null"?

Found with static analysis, cppcheck. Open on /dev/null is unlikely to 
fail, but it's good to fail reliably rather than have a SIGSEGV :-)

Colin

> 
>> Fixes: a2561b12fe39 ("selftests/resctrl: Add built in benchmark")
>> Signed-off-by: Colin Ian King <colin.i.king@...il.com>
>> ---
>>   tools/testing/selftests/resctrl/fill_buf.c | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/tools/testing/selftests/resctrl/fill_buf.c 
>> b/tools/testing/selftests/resctrl/fill_buf.c
>> index 51e5cf22632f..56ccbeae0638 100644
>> --- a/tools/testing/selftests/resctrl/fill_buf.c
>> +++ b/tools/testing/selftests/resctrl/fill_buf.c
>> @@ -121,8 +121,10 @@ static int fill_cache_read(unsigned char 
>> *start_ptr, unsigned char *end_ptr,
>>       /* Consume read result so that reading memory is not optimized 
>> out. */
>>       fp = fopen("/dev/null", "w");
>> -    if (!fp)
>> +    if (!fp) {
>>           perror("Unable to write to /dev/null");
>> +        return -1;
>> +    }
>>       fprintf(fp, "Sum: %d ", ret);
>>       fclose(fp);
>>
> 
> thanks,
> -- Shuah