tcp: must unclone packets before mangling them

TCP stack should make sure it owns skbs before mangling them.

We had various crashes using bnx2x, and it turned out gso_size
was cleared right before bnx2x driver was populating TC descriptor
of the _previous_ packet send. TCP stack can sometime retransmit
packets that are still in Qdisc.

Of course we could make bnx2x driver more robust (using
ACCESS_ONCE(shinfo->gso_size) for example), but the bug is TCP stack.

We have identified two points where skb_unclone() was needed.

This patch adds a WARN_ON_ONCE() to warn us if we missed another
fix of this kind.

Kudos to Neal for finding the root cause of this bug. Its visible
using small MSS.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 3 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index c6f01f2cdb32..8fad1c155688 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -986,6 +986,9 @@ static void tcp_queue_skb(struct sock *sk, struct sk_buff *skb)
 static void tcp_set_skb_tso_segs(const struct sock *sk, struct sk_buff *skb,
                                  unsigned int mss_now)
 {
+        /* Make sure we own this skb before messing gso_size/gso_segs */
+        WARN_ON_ONCE(skb_cloned(skb));
+
         if (skb->len <= mss_now || !sk_can_gso(sk) ||
             skb->ip_summed == CHECKSUM_NONE) {
                 /* Avoid the costly divide in the normal
@@ -1067,9 +1070,7 @@ int tcp_fragment(struct sock *sk, struct sk_buff *skb, u32 len,
         if (nsize < 0)
                 nsize = 0;
 
-        if (skb_cloned(skb) &&
-            skb_is_nonlinear(skb) &&
-            pskb_expand_head(skb, 0, 0, GFP_ATOMIC))
+        if (skb_unclone(skb, GFP_ATOMIC))
                 return -ENOMEM;
 
         /* Get a new skb... force flag on. */
@@ -2344,6 +2345,8 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb)
                 int oldpcount = tcp_skb_pcount(skb);
 
                 if (unlikely(oldpcount > 1)) {
+                        if (skb_unclone(skb, GFP_ATOMIC))
+                                return -ENOMEM;
                         tcp_init_tso_segs(sk, skb, cur_mss);
                         tcp_adjust_pcount(sk, skb, oldpcount - tcp_skb_pcount(skb));
                 }