Re: [PATCH] tap: double-free in error path in tap_open()

[Girish Moodalbail <girish.moodalbail@oracle.com>]

On 10/24/17 6:55 PM, Jason Wang wrote:
> 
> 
> On 2017年10月25日 05:41, Girish Moodalbail wrote:
>> Double free of skb_array in tap module is causing kernel panic. When
>> tap_set_queue() fails we free skb_array right away by calling
>> skb_array_cleanup(). However, later on skb_array_cleanup() is called
>> again by tap_sock_destruct through sock_put(). This patch fixes that
>> issue.
>>
>> Fixes: 362899b8725b35e3 (macvtap: switch to use skb array)
>> Signed-off-by: Girish Moodalbail <girish.moodalbail@oracle.com>
>> ---
>>   drivers/net/tap.c | 12 ++++++------
>>   1 file changed, 6 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/net/tap.c b/drivers/net/tap.c
>> index 21b71ae..878520b 100644
>> --- a/drivers/net/tap.c
>> +++ b/drivers/net/tap.c
>> @@ -542,20 +542,20 @@ static int tap_open(struct inode *inode, struct file *file)
>>       err = -ENOMEM;
>>       if (skb_array_init(&q->skb_array, tap->dev->tx_queue_len, GFP_KERNEL))
>> -        goto err_array;
>> +        goto err_put;
> 
> Looks like this will cause skb_array_cleanup() to be called when skb array was 
> uninitialized?

Thanks Jason.

This is an existing issue outside of my code. I will send a v2 of the patch that 
incorporates the fix for this issue as well. I am considering moving 
skb_array_init() to the beginning of tap_open() (near to where we allocate 
memory for the tap_queue itself).

Thanks again.

regards,
~Girish


> 
> Thanks
> 
>>       err = tap_set_queue(tap, file, q);
>> -    if (err)
>> -        goto err_queue;
>> +    if (err) {
>> +        /* tap_sock_destruct() will take care of freeing skb_array */
>> +        goto err_put;
>> +    }
>>       dev_put(tap->dev);
>>       rtnl_unlock();
>>       return err;
>> -err_queue:
>> -    skb_array_cleanup(&q->skb_array);
>> -err_array:
>> +err_put:
>>       sock_put(&q->sk);
>>   err:
>>       if (tap)
>