netdev - [PATCH net] tun: add mutex_unlock() call and napi.skb clearing in tun_get

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Message-Id: <20200122170735.4126-1-edumazet@google.com>
Date:   Wed, 22 Jan 2020 09:07:35 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     "David S . Miller" <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        syzbot <syzkaller@...glegroups.com>,
        Petar Penkov <ppenkov@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: [PATCH net] tun: add mutex_unlock() call and napi.skb clearing in tun_get_user()

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 <edumazet@...gle.com>
Reported-by: syzbot <syzkaller@...glegroups.com>
Cc: Petar Penkov <ppenkov@...gle.com>
Cc: Willem de Bruijn <willemb@...gle.com>
---
 drivers/net/tun.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 683d371e6e82063bec7ade102a3b40aa38b1b6af..35e884a8242d95a4e866ad740a234face2fa0ac0 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1936,6 +1936,10 @@ 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;
 			}
 		}
-- 
2.25.0.341.g760bfbb309-goog