Re: net/packet: use-after-free in packet_rcv_fanout

On Fri, Feb 10, 2017 at 9:59 AM, Eric Dumazet [off-list ref] wrote:

On Fri, 2017-02-10 at 09:49 -0800, Cong Wang wrote:

quoted

On Thu, Feb 9, 2017 at 7:23 PM, Eric Dumazet [off-list ref] wrote:

quoted

On Thu, 2017-02-09 at 19:19 -0800, Eric Dumazet wrote:

quoted

More likely the bug is in fanout_add(), with a buggy sequence in error
case, and not correct locking.

kfree(po->rollover);
po->rollover = NULL;

Two cpus entering fanout_add() (using the same af_packet socket,
syzkaller courtesy...) might both see po->fanout being NULL.

Then they grab the mutex.  Too late...

Patch could be :

For me, clearly the data structure that use-after-free'd is struct sock
rather than struct packet_rollover.

Fine. But your patch makes absolutely no sense.

I don't have to give a 100% correct patch to prove my explanation
of the crash. At least it makes more sense than yours...