Date:   Fri, 7 Apr 2023 10:24:24 +0200
From:   Mirsad Goran Todorovac <mirsad.todorovac@....unizg.hr>
To:     Dan Carpenter <error27@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Russ Weight <russell.h.weight@...el.com>,
        linux-kernel@...r.kernel.org, Luis Chamberlain <mcgrof@...nel.org>,
        Tianfei zhang <tianfei.zhang@...el.com>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Zhengchao Shao <shaozhengchao@...wei.com>,
        Colin Ian King <colin.i.king@...il.com>,
        Takashi Iwai <tiwai@...e.de>
Subject: Re: [PATCH v3 1/2] test_firmware: Fix some racing conditions in
 test_fw_config locking.

On 6.4.2023. 16:04, Dan Carpenter wrote:
> On Thu, Apr 06, 2023 at 03:53:17AM +0200, Mirsad Goran Todorovac wrote:
>> Some functions were called both from locked and unlocked context, so the lock
>> was dropped prematurely, introducing a race condition when deadlock was avoided.
>>
>> Having two locks wouldn't assure a race-proof mutual exclusion.
>>
>> test_dev_config_update_bool_unlocked(), test_dev_config_update_u8_unlocked()
>> and test_dev_config_update_size_t_unlocked() versions of the functions were
>> introduced to be called from the locked contexts as a workaround without
>> releasing the main driver's lock and causing a race condition, much like putc()
>> and putc_unlocked() in stdio glibc library.
>>
>> This should guarantee mutual exclusion and prevent any race conditions.
>>
> 
> Thanks for figuring this out!  It seems like a good approach to me.
> However, I feel like PATCH 1/1 needs some style changes.
> 
> The question you seem to be dealing with is how consistent to be and how
> much infrastructure to create.  Don't think about that.  Just fix the
> bug in the most minimal way possible and don't worry about being
> consistent.
> 
> (Probably the best way to make this consistent is to change the
>   test_dev_config_update_XXX functions into a single macro that calls the
>   correct kstroXXX function.  Then create a second macro that takes the
>   lock and calls the first macro.  But that is a clean up patch and
>   unrelated to this bug.)
> 
>> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>> Cc: Luis Chamberlain <mcgrof@...nel.org>
>> Cc: Russ Weight <russell.h.weight@...el.com>
>> Cc: Tianfei zhang <tianfei.zhang@...el.com>
>> Cc: Mirsad Goran Todorovac <mirsad.todorovac@....unizg.hr>
>> Cc: Christophe JAILLET <christophe.jaillet@...adoo.fr>
>> Cc: Zhengchao Shao <shaozhengchao@...wei.com>
>> Cc: Colin Ian King <colin.i.king@...il.com>
>> Cc: linux-kernel@...r.kernel.org
>> Cc: Takashi Iwai <tiwai@...e.de>
>> Suggested-by: Dan Carpenter <error27@...il.com>
>> Signed-off-by: Mirsad Goran Todorovac <mirsad.todorovac@....unizg.hr>
>> ---
>>   lib/test_firmware.c | 52 +++++++++++++++++++++++++++++++++------------
>>   1 file changed, 38 insertions(+), 14 deletions(-)
>>
>> diff --git a/lib/test_firmware.c b/lib/test_firmware.c
>> index 05ed84c2fc4c..272af8dc54b0 100644
>> --- a/lib/test_firmware.c
>> +++ b/lib/test_firmware.c
>> @@ -353,16 +353,26 @@ static ssize_t config_test_show_str(char *dst,
>>   	return len;
>>   }
>>   
>> -static int test_dev_config_update_bool(const char *buf, size_t size,
>> +static inline int test_dev_config_update_bool_unlocked(const char *buf, size_t size,
>>   				       bool *cfg)
>>   {
>>   	int ret;
>>   
>> -	mutex_lock(&test_fw_mutex);
>>   	if (kstrtobool(buf, cfg) < 0)
>>   		ret = -EINVAL;
>>   	else
>>   		ret = size;
>> +
>> +	return ret;
>> +}
> 
> This change can be left out completely.
> 
>> +
>> +static int test_dev_config_update_bool(const char *buf, size_t size,
>> +				       bool *cfg)
>> +{
>> +	int ret;
>> +
>> +	mutex_lock(&test_fw_mutex);
>> +	ret = test_dev_config_update_bool_unlocked(buf, size, cfg);
>>   	mutex_unlock(&test_fw_mutex);
>>   
>>   	return ret;
>> @@ -373,7 +383,8 @@ static ssize_t test_dev_config_show_bool(char *buf, bool val)
>>   	return snprintf(buf, PAGE_SIZE, "%d\n", val);
>>   }
>>   
>> -static int test_dev_config_update_size_t(const char *buf,
>> +static int test_dev_config_update_size_t_unlocked(
>> +					 const char *buf,
>>   					 size_t size,
>>   					 size_t *cfg)
>>   {
> 
> Do not rename this function.  Just add a comment that the mutext must be
> held.  Or a WARN_ONCE().
> 
> 	WARN_ON_ONCE(!mutex_is_locked(&test_fw_mutex));
> 
> 
>> @@ -384,9 +395,7 @@ static int test_dev_config_update_size_t(const char *buf,
>>   	if (ret)
>>   		return ret;
>>   
>> -	mutex_lock(&test_fw_mutex);
>>   	*(size_t *)cfg = new;
>> -	mutex_unlock(&test_fw_mutex);
>>   
>>   	/* Always return full write size even if we didn't consume all */
>>   	return size;
>> @@ -402,6 +411,21 @@ static ssize_t test_dev_config_show_int(char *buf, int val)
>>   	return snprintf(buf, PAGE_SIZE, "%d\n", val);
>>   }
>>   
>> +static int test_dev_config_update_u8_unlocked(const char *buf, size_t size, u8 *cfg)
>> +{
>> +	u8 val;
>> +	int ret;
>> +
>> +	ret = kstrtou8(buf, 10, &val);
>> +	if (ret)
>> +		return ret;
>> +
>> +	*(u8 *)cfg = val;
>> +
>> +	/* Always return full write size even if we didn't consume all */
>> +	return size;
>> +}
>> +
> 
> Just change the test_dev_config_update_u8() to not take the lock.
> Add the comment that the lock must be held.  Change both callers to take
> the lock.
> 
> 
> Otherwise we end up creating too much duplicate code.
> 
>>   static int test_dev_config_update_u8(const char *buf, size_t size, u8 *cfg)
>>   {
>>   	u8 val;
> 
> regards,
> dan carpenter

Hi Mr. Carpenter,

Thank you for your review.

I will proceed according to your guidelines and issue the next version of the
patch set.

But I cannot promise it will be before the holidays - I do not want to make
the gods angry either ;-)

I cannot promise to try smart macros or inline functions with smart function
parameters just yet.

I would consider the real success if I hunt down the remaining leak and races
in this driver. Despite being considered a less important one.

As you have previously asserted, it is not a real security issue with a CVE,
however, for completeness sake I would like to see these problems fixed.

Best regards,
Mirsad

-- 
Mirsad Todorovac
System engineer
Faculty of Graphic Arts | Academy of Fine Arts
University of Zagreb
Republic of Croatia, the European Union

Sistem inženjer
Grafički fakultet | Akademija likovnih umjetnosti
Sveučilište u Zagrebu

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