[PATCH net 2/3] tun: initialize napi_mutex unconditionally

[Eric Dumazet]

This is the first part to fix following syzbot report :

console output: https://syzkaller.appspot.com/x/log.txt?x=145378e6400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=443816db871edd66
dashboard link: https://syzkaller.appspot.com/bug?extid=e662df0ac1d753b57e80

Following patch is fixing the race condition, but it seems safer
to initialize this mutex at tfile creation anyway.

Fixes: 90e33d459407 ("tun: enable napi_gro_frags() for TUN/TAP driver")
Signed-off-by: Eric Dumazet <eduma...@google.com>
Reported-by: syzbot+e662df0ac1d753b57...@syzkaller.appspotmail.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 
71d10fb59849bff091ee64b6f7e9cc8ae2e0cf6f..729686babbf3b7d2f76ce64a5ebf7676e45eb681
 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -320,7 +320,6 @@ static void tun_napi_init(struct tun_struct *tun, struct 
tun_file *tfile,
                netif_napi_add(tun->dev, &tfile->napi, tun_napi_poll,
                               NAPI_POLL_WEIGHT);
                napi_enable(&tfile->napi);
-               mutex_init(&tfile->napi_mutex);
        }
 }
 
@@ -3199,6 +3198,7 @@ static int tun_chr_open(struct inode *inode, struct file 
* file)
                return -ENOMEM;
        }
 
+       mutex_init(&tfile->napi_mutex);
        RCU_INIT_POINTER(tfile->tun, NULL);
        tfile->flags = 0;
        tfile->ifindex = 0;
-- 
2.19.0.605.g01d371f741-goog