[PATCH net 1/2] tun: hold napi_mutex for all napi operations

[Stanislav Fomichev <sdf@google.com>]

BUG: unable to handle kernel NULL pointer dereference at 00000000000000d1
Call Trace:
 ? napi_gro_frags+0xa7/0x2c0
 tun_get_user+0xb50/0xf20
 tun_chr_write_iter+0x53/0x70
 new_sync_write+0xff/0x160
 vfs_write+0x191/0x1e0
 __x64_sys_write+0x5e/0xd0
 do_syscall_64+0x47/0xf0
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

I think there is a subtle race between sending a packet via tap and
attaching it:

CPU0:                    CPU1:
tun_chr_ioctl(TUNSETIFF)
  tun_set_iff
    tun_attach
      rcu_assign_pointer(tfile->tun, tun);
                         tun_fops->write_iter()
                           tun_chr_write_iter
                             tun_napi_alloc_frags
                               napi_get_frags
                                 napi->skb = napi_alloc_skb
      tun_napi_init
        netif_napi_add
          napi->skb = NULL
                              napi->skb is NULL here
                              napi_gro_frags
                                napi_frags_skb
				  skb = napi->skb
				  skb_reset_mac_header(skb)
				  panic()

To fix, do the following:
* Move rcu_assign_pointer(tfile->tun, tun) to be the last thing we do
  in tun_attach(); this should guarantee that when we call tun_get()
  we always get an initialized object
* As another safeguard, always grab napi_mutex whenever doing any
  napi operation; this should prevent napi state change between
  calls to napi_get_frags and napi_gro_frags

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 | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index a4fdad475594..7875f06011f2 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -323,22 +323,30 @@ static void tun_napi_init(struct tun_struct *tun, struct tun_file *tfile,
 	tfile->napi_enabled = napi_en;
 	tfile->napi_frags_enabled = napi_en && napi_frags;
 	if (napi_en) {
+		mutex_lock(&tfile->napi_mutex);
 		netif_napi_add(tun->dev, &tfile->napi, tun_napi_poll,
 			       NAPI_POLL_WEIGHT);
 		napi_enable(&tfile->napi);
+		mutex_unlock(&tfile->napi_mutex);
 	}
 }
 
 static void tun_napi_disable(struct tun_file *tfile)
 {
-	if (tfile->napi_enabled)
+	if (tfile->napi_enabled) {
+		mutex_lock(&tfile->napi_mutex);
 		napi_disable(&tfile->napi);
+		mutex_unlock(&tfile->napi_mutex);
+	}
 }
 
 static void tun_napi_del(struct tun_file *tfile)
 {
-	if (tfile->napi_enabled)
+	if (tfile->napi_enabled) {
+		mutex_lock(&tfile->napi_mutex);
 		netif_napi_del(&tfile->napi);
+		mutex_unlock(&tfile->napi_mutex);
+	}
 }
 
 static bool tun_napi_frags_enabled(const struct tun_file *tfile)
@@ -856,7 +864,6 @@ static int tun_attach(struct tun_struct *tun, struct file *file,
 		err = 0;
 	}
 
-	rcu_assign_pointer(tfile->tun, tun);
 	rcu_assign_pointer(tun->tfiles[tun->numqueues], tfile);
 	tun->numqueues++;
 
@@ -876,6 +883,11 @@ static int tun_attach(struct tun_struct *tun, struct file *file,
 	 * refcnt.
 	 */
 
+	/* All tun_fops depend on tun_get() returning non-null pointer.
+	 * Thus, assigning tun to a tfile should be the last init operation,
+	 * otherwise we risk using half-initialized object.
+	 */
+	rcu_assign_pointer(tfile->tun, tun);
 out:
 	return err;
 }
-- 
2.20.1.97.g81188d93c3-goog