[PATCH net 2/2] tun: always set skb->dev to tun->dev

[Stanislav Fomichev <sdf@google.com>]

While debugging previous issue I noticed that commit 90e33d459407 ("tun:
enable napi_gro_frags() for TUN/TAP driver") started conditionally
(!frags) calling eth_type_trans(skb, tun->dev) for IFF_TAP case. Since
eth_type_trans sets skb->dev, some skbs can now have NULL skb->dev.
Fix that by always setting skb->dev unconditionally.

The syzbot fails with the following trace:
WARNING: CPU: 0 PID: 11136 at net/core/flow_dissector.c:764
 skb_flow_dissect_flow_keys_basic include/linux/skbuff.h:1240 [inline]
 skb_probe_transport_header include/linux/skbuff.h:2403 [inline]
 tun_get_user+0x2d4a/0x4250 drivers/net/tun.c:1906
 tun_chr_write_iter+0xb9/0x160 drivers/net/tun.c:1993
 call_write_iter include/linux/fs.h:1808 [inline]
 new_sync_write fs/read_write.c:474 [inline]

But I don't think there is an actual issue since we exercise flow
dissector via eth_get_headlen which doesn't use skb (and hence BPF flow
dissector). But let's still properly set skb->dev so we don't have
any problems going forward.

Reported-by: syzbot <syzkaller@googlegroups.com>
Fixes: 90e33d459407 ("tun: enable napi_gro_frags() for TUN/TAP driver")

Signed-off-by: Stanislav Fomichev <sdf@google.com>
---
 drivers/net/tun.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 7875f06011f2..af34baf978f3 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1899,6 +1899,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 		return -EINVAL;
 	}
 
+	skb->dev = tun->dev;
 	switch (tun->flags & TUN_TYPE_MASK) {
 	case IFF_TUN:
 		if (tun->flags & IFF_NO_PI) {
@@ -1920,7 +1921,6 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 
 		skb_reset_mac_header(skb);
 		skb->protocol = pi.proto;
-		skb->dev = tun->dev;
 		break;
 	case IFF_TAP:
 		if (!frags)
-- 
2.20.1.97.g81188d93c3-goog