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

[Eric Dumazet]

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

> 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 :

 net/packet/af_packet.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index d56ee46b11fc9524e457e5fe8adf10c105a66ab6..11725a350f6953d077f754c10e9f52e48924d780 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1657,7 +1657,6 @@ static int fanout_add(struct sock *sk, u16 id, u16 type_flags)
 		atomic_long_set(&po->rollover->num_failed, 0);
 	}
 
-	mutex_lock(&fanout_mutex);
 	match = NULL;
 	list_for_each_entry(f, &fanout_list, list) {
 		if (f->id == id &&
@@ -1704,7 +1703,6 @@ static int fanout_add(struct sock *sk, u16 id, u16 type_flags)
 		}
 	}
 out:
-	mutex_unlock(&fanout_mutex);
 	if (err) {
 		kfree(po->rollover);
 		po->rollover = NULL;
@@ -3698,7 +3696,10 @@ packet_setsockopt(struct socket *sock, int level, int optname, char __user *optv
 		if (copy_from_user(&val, optval, sizeof(val)))
 			return -EFAULT;
 
-		return fanout_add(sk, val & 0xffff, val >> 16);
+		mutex_lock(&fanout_mutex);
+		ret = fanout_add(sk, val & 0xffff, val >> 16);
+		mutex_unlock(&fanout_mutex);
+		return ret;
 	}
 	case PACKET_FANOUT_DATA:
 	{