[alsa-devel] [PATCH] ctl: fix to handle several elements added by one operation for userspace element

[Takashi Sakamoto]

Iwai-san,

On Apr 12 2015 14:51, Takashi Iwai wrote:
> At Sun, 12 Apr 2015 10:12:25 +0900,
> Takashi Sakamoto wrote:
>>
>> An element instance can have several elements with the same feature.
>> Some userspace applications can add such an element instance by add
>> operation with the number of elements. Then, the element instance
>> gets a memory object to keep states of these elements.
>>
>> But the element instance has just one memory object for the elements.
>> This causes the same result to each read/write operations to the
>> different elements.
>>
>> This commit fixes this bug by allocating enough memory objects to the
>> element instance for each of elements.
>>
>> Signed-off-by: Takashi Sakamoto <o-takashi at sakamocchi.jp>
>> ---
>>  sound/core/control.c | 16 +++++++++++-----
>>  1 file changed, 11 insertions(+), 5 deletions(-)
>>
>> diff --git a/sound/core/control.c b/sound/core/control.c
>> index ccb1ca2..5b2c352 100644
>> --- a/sound/core/control.c
>> +++ b/sound/core/control.c
>> @@ -1029,7 +1029,7 @@ static int snd_ctl_elem_unlock(struct snd_ctl_file *file,
>>  struct user_element {
>>  	struct snd_ctl_elem_info info;
>>  	struct snd_card *card;
>> -	void *elem_data;		/* element data */
>> +	char *elem_data;		/* element data */
>>  	unsigned long elem_data_size;	/* size of element data in bytes */
>>  	void *tlv_data;			/* TLV data */
>>  	unsigned long tlv_data_size;	/* TLV data size */
>> @@ -1078,9 +1078,12 @@ static int snd_ctl_elem_user_get(struct snd_kcontrol *kcontrol,
>>  				 struct snd_ctl_elem_value *ucontrol)
>>  {
>>  	struct user_element *ue = kcontrol->private_data;
>> +	unsigned int size = ue->elem_data_size;
>> +	char *src = ue->elem_data +
>> +			snd_ctl_get_ioff(kcontrol, &ucontrol->id) * size;
> 
> You still need to check the validity of snd_ctl_get_ioff() value.
> The user-space can give any value here, and accessing without the
> check will expose the kernel memory to user-space at get or even
> corrupt at put.

I don't agree this indication.

The 'ucontrol' has ID information re-calculated by the caller
(snd_ctl_elem_read() / snd_ctl_elem_write()), thus it's not directly
passed from userspace. If the information includes something wrong, the
caller returns error and don't execute callee.

The snd_ctl_elem_add() has already allocated enough memory, so there're
no wrong reference as long as the re-calculated ID includes larger numid
than (kctl->numid + kctl->count), or larger index than kctl->count.

Are there any cases that the re-calculated ID includes such wrong
information after passing these functions below?
 * snd_ctl_find_id()    -> find kctl with given ID info
 * snd_ctl_get_ioff()   -> calculate offset with kctl and given ID
 * snd_ctl_build_ioff() -> re-calculate ID with kctl and the offset

If it still includes wrong information, it's the caller's fault and
should be fixed.


Thanks

Takashi Sakamoto

> thanks,
> 
> Takashi
> 
>>  
>>  	mutex_lock(&ue->card->user_ctl_lock);
>> -	memcpy(&ucontrol->value, ue->elem_data, ue->elem_data_size);
>> +	memcpy(&ucontrol->value, src, size);
>>  	mutex_unlock(&ue->card->user_ctl_lock);
>>  	return 0;
>>  }
>> @@ -1090,11 +1093,14 @@ static int snd_ctl_elem_user_put(struct snd_kcontrol *kcontrol,
>>  {
>>  	int change;
>>  	struct user_element *ue = kcontrol->private_data;
>> +	unsigned int size = ue->elem_data_size;
>> +	char *dst = ue->elem_data +
>> +			snd_ctl_get_ioff(kcontrol, &ucontrol->id) * size;
>>  
>>  	mutex_lock(&ue->card->user_ctl_lock);
>> -	change = memcmp(&ucontrol->value, ue->elem_data, ue->elem_data_size) != 0;
>> +	change = memcmp(&ucontrol->value, dst, size) != 0;
>>  	if (change)
>> -		memcpy(ue->elem_data, &ucontrol->value, ue->elem_data_size);
>> +		memcpy(dst, &ucontrol->value, size);
>>  	mutex_unlock(&ue->card->user_ctl_lock);
>>  	return change;
>>  }
>> @@ -1278,7 +1284,7 @@ static int snd_ctl_elem_add(struct snd_ctl_file *file,
>>  	if (err < 0)
>>  		return err;
>>  	memcpy(&kctl->id, &info->id, sizeof(kctl->id));
>> -	kctl->private_data = kzalloc(sizeof(struct user_element) + private_size,
>> +	kctl->private_data = kzalloc(sizeof(struct user_element) + private_size * count,
>>  				     GFP_KERNEL);
>>  	if (kctl->private_data == NULL) {
>>  		kfree(kctl);
>> -- 
>> 2.1.0