Re: BUG: KASAN: use-after-free Read in tun_net_xmit

[Jason Wang <jasowang@redhat.com>]

On 2019/4/23 下午2:00, Cong Wang wrote:
> On Mon, Apr 22, 2019 at 2:41 AM Jason Wang <jasowang@redhat.com> wrote:
>>
>> On 2019/4/22 上午11:57, YueHaibing wrote:
>>> We get a KASAN report as below, but don't have any reproducer.
>>>
>>> Any comments are appreciated.
>>>
>>> ==================================================================
>>> BUG: KASAN: use-after-free in tun_net_xmit+0x1670/0x1750 drivers/net/tun.c:1104
>>> Read of size 8 at addr ffff88836cc26a70 by task swapper/3/0
>>
>> Which kernel version did you use? The calltrace points out the a use
>> after free for tun_file structure which should be synchronized through
>> RCU + RTNL lock.
> The tfile socket has to be marked with SOCK_RCU_FREE in order
> to fully respect the RCU grace period.
>
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index e9ca1c088d0b..31c3210288cb 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -3431,6 +3431,7 @@ static int tun_chr_open(struct inode *inode,
> struct file * file)
>          file->private_data = tfile;
>          INIT_LIST_HEAD(&tfile->next);
>
> +       sock_set_flag(&tfile->sk, SOCK_RCU_FREE);
>          sock_set_flag(&tfile->sk, SOCK_ZEROCOPY);
>
>          return 0;


We did a synchronize_net() when socket is detached from netdevice in 
__tun_detach() so it looks to me this is unnecessary.

But if it solves the use-after-free, it means we have bugs somewhere.

Thanks