net: tun: call napi_schedule_prep() to ensure we own a napi

A recent patch exposed another issue in napi_get_frags()
caught by syzbot [1]

Before feeding packets to GRO, and calling napi_complete()
we must first grab NAPI_STATE_SCHED.

[1]
WARNING: CPU: 0 PID: 3612 at net/core/dev.c:6076 napi_complete_done+0x45b/0x880 net/core/dev.c:6076
Modules linked in:
CPU: 0 PID: 3612 Comm: syz-executor408 Not tainted 6.1.0-rc3-syzkaller-00175-g1118b204 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/26/2022
RIP: 0010:napi_complete_done+0x45b/0x880 net/core/dev.c:6076
Code: c1 ea 03 0f b6 14 02 4c 89 f0 83 e0 07 83 c0 03 38 d0 7c 08 84 d2 0f 85 24 04 00 00 41 89 5d 1c e9 73 fc ff ff e8 b5 53 22 fa <0f> 0b e9 82 fe ff ff e8 a9 53 22 fa 48 8b 5c 24 08 31 ff 48 89 de
RSP: 0018:ffffc90003c4f920 EFLAGS: 00010293
RAX: 0000000000000000 RBX: 0000000000000030 RCX: 0000000000000000
RDX: ffff8880251c0000 RSI: ffffffff875a58db RDI: 0000000000000007
RBP: 0000000000000001 R08: 0000000000000007 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000001 R12: ffff888072d02628
R13: ffff888072d02618 R14: ffff888072d02634 R15: 0000000000000000
FS: 0000555555f13300(0000) GS:ffff8880b9a00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055c44d3892b8 CR3: 00000000172d2000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
napi_complete include/linux/netdevice.h:510 [inline]
tun_get_user+0x206d/0x3a60 drivers/net/tun.c:1980
tun_chr_write_iter+0xdb/0x200 drivers/net/tun.c:2027
call_write_iter include/linux/fs.h:2191 [inline]
do_iter_readv_writev+0x20b/0x3b0 fs/read_write.c:735
do_iter_write+0x182/0x700 fs/read_write.c:861
vfs_writev+0x1aa/0x630 fs/read_write.c:934
do_writev+0x133/0x2f0 fs/read_write.c:977
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd
RIP: 0033:0x7f37021a3c19

Fixes: 1118b204 ("net: tun: Fix memory leaks of napi_get_frags")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Wang Yufen <wangyufen@huawei.com>
Link: https://lore.kernel.org/r/20221107180011.188437-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/tun.c

@@ -1967,18 +1967,25 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 					  skb_headlen(skb));
 
 		if (unlikely(headlen > skb_headlen(skb))) {
+			WARN_ON_ONCE(1);
+			err = -ENOMEM;
 			dev_core_stats_rx_dropped_inc(tun->dev);
+napi_busy:
 			napi_free_frags(&tfile->napi);
 			rcu_read_unlock();
 			mutex_unlock(&tfile->napi_mutex);
-			WARN_ON(1);
-			return -ENOMEM;
+			return err;
 		}
 
-		local_bh_disable();
-		napi_gro_frags(&tfile->napi);
-		napi_complete(&tfile->napi);
-		local_bh_enable();
+		if (likely(napi_schedule_prep(&tfile->napi))) {
+			local_bh_disable();
+			napi_gro_frags(&tfile->napi);
+			napi_complete(&tfile->napi);
+			local_bh_enable();
+		} else {
+			err = -EBUSY;
+			goto napi_busy;
+		}
 		mutex_unlock(&tfile->napi_mutex);
 	} else if (tfile->napi_enabled) {
 		struct sk_buff_head *queue = &tfile->sk.sk_write_queue;