[PATCH net] tun: properly test for IFF_UP

[Eric Dumazet]

Same reasons than the ones explained in commit 4179cb5a4c92
("vxlan: test dev->flags & IFF_UP before calling netif_rx()")

netif_rx_ni() or napi_gro_frags() must be called under a strict contract.

At device dismantle phase, core networking clears IFF_UP
and flush_all_backlogs() is called after rcu grace period
to make sure no incoming packet might be in a cpu backlog
and still referencing the device.

A similar protocol is used for gro layer.

Most drivers call netif_rx() from their interrupt handler,
and since the interrupts are disabled at device dismantle,
netif_rx() does not have to check dev->flags & IFF_UP

Virtual drivers do not have this guarantee, and must
therefore make the check themselves.

Fixes: 1bd4978a88ac ("tun: honor IFF_UP in tun_get_user()")
Signed-off-by: Eric Dumazet <eduma...@google.com>
Reported-by: syzbot <syzkal...@googlegroups.com>
---
 drivers/net/tun.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 
1d68921723dc08532b3f5321a52865076ad66336..0d343359f647ff58fee35462358827e61857c837
 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1763,9 +1763,6 @@ static ssize_t tun_get_user(struct tun_struct *tun, 
struct tun_file *tfile,
        int skb_xdp = 1;
        bool frags = tun_napi_frags_enabled(tfile);
 
-       if (!(tun->dev->flags & IFF_UP))
-               return -EIO;
-
        if (!(tun->flags & IFF_NO_PI)) {
                if (len < sizeof(pi))
                        return -EINVAL;
@@ -1867,6 +1864,8 @@ static ssize_t tun_get_user(struct tun_struct *tun, 
struct tun_file *tfile,
                        err = skb_copy_datagram_from_iter(skb, 0, from, len);
 
                if (err) {
+                       err = -EFAULT;
+drop:
                        this_cpu_inc(tun->pcpu_stats->rx_dropped);
                        kfree_skb(skb);
                        if (frags) {
@@ -1874,7 +1873,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, 
struct tun_file *tfile,
                                mutex_unlock(&tfile->napi_mutex);
                        }
 
-                       return -EFAULT;
+                       return err;
                }
        }
 
@@ -1958,6 +1957,12 @@ static ssize_t tun_get_user(struct tun_struct *tun, 
struct tun_file *tfile,
            !tfile->detached)
                rxhash = __skb_get_hash_symmetric(skb);
 
+       rcu_read_lock();
+       if (unlikely(!(tun->dev->flags & IFF_UP))) {
+               err = -EIO;
+               goto drop;
+       }
+
        if (frags) {
                /* Exercise flow dissector code path. */
                u32 headlen = eth_get_headlen(skb->data, skb_headlen(skb));
@@ -1965,6 +1970,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, 
struct tun_file *tfile,
                if (unlikely(headlen > skb_headlen(skb))) {
                        this_cpu_inc(tun->pcpu_stats->rx_dropped);
                        napi_free_frags(&tfile->napi);
+                       rcu_read_unlock();
                        mutex_unlock(&tfile->napi_mutex);
                        WARN_ON(1);
                        return -ENOMEM;
@@ -1992,6 +1998,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, 
struct tun_file *tfile,
        } else {
                netif_rx_ni(skb);
        }
+       rcu_read_unlock();
 
        stats = get_cpu_ptr(tun->pcpu_stats);
        u64_stats_update_begin(&stats->syncp);
-- 
2.21.0.360.g471c308f928-goog