Subject: [PATCH 17/17] tun: add mutex_unlock() call and napi.skb clearing in tun_get_user()

From: Eric Dumazet <[email-masked]>
Date: Wed, 22 Jan 2020 09:07:35 -0800

[ Upstream commit 1efba987c48629c0c64703bb4ea76ca1a3771d17 ]

If both IFF_NAPI_FRAGS mode and XDP are enabled, and the XDP program
consumes the skb, we need to clear the napi.skb (or risk
a use-after-free) and release the mutex (or risk a deadlock)

WARNING: lock held when returning to user space!
5.5.0-rc6-syzkaller #0 Not tainted
------------------------------------------------
syz-executor.0/455 is leaving the kernel with locks still held!
1 lock held by syz-executor.0/455:
 #0: ffff888098f6e748 (&tfile->napi_mutex){+.+.}, at: tun_get_user+0x1604/0x3fc0 drivers/net/tun.c:1835

Fixes: 90e33d459407 ("tun: enable napi_gro_frags() for TUN/TAP driver")
Signed-off-by: Eric Dumazet <[email-masked]>
Reported-by: syzbot <[email-masked]>
Cc: Petar Penkov <[email-masked]>
Cc: Willem de Bruijn <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 drivers/net/tun.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index bbd92221c6ca..09c444d3b496 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1900,6 +1900,10 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 			if (ret != XDP_PASS) {
 				rcu_read_unlock();
 				local_bh_enable();
+				if (frags) {
+					tfile->napi.skb = NULL;
+					mutex_unlock(&tfile->napi_mutex);
+				}
 				return total_len;
 			}
 		}
-- 
2.21.1