tcp: do not underestimate skb->truesize in tcp_trim_head()

Andrey found a way to trigger the WARN_ON_ONCE(delta < len) in
skb_try_coalesce() using syzkaller and a filter attached to a TCP
socket over loopback interface.

I believe one issue with looped skbs is that tcp_trim_head() can end up
producing skb with under estimated truesize.

It hardly matters for normal conditions, since packets sent over
loopback are never truncated.

Bytes trimmed from skb->head should not change skb truesize, since
skb->head is not reallocated.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Tested-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 12 insertions, 7 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index c3c082ed3879..a85d863c4419 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1267,7 +1267,7 @@ int tcp_fragment(struct sock *sk, struct sk_buff *skb, u32 len,
  * eventually). The difference is that pulled data not copied, but
  * immediately discarded.
  */
-static void __pskb_trim_head(struct sk_buff *skb, int len)
+static int __pskb_trim_head(struct sk_buff *skb, int len)
 {
         struct skb_shared_info *shinfo;
         int i, k, eat;
@@ -1277,7 +1277,7 @@ static void __pskb_trim_head(struct sk_buff *skb, int len)
                 __skb_pull(skb, eat);
                 len -= eat;
                 if (!len)
-                        return;
+                        return 0;
         }
         eat = len;
         k = 0;
@@ -1303,23 +1303,28 @@ static void __pskb_trim_head(struct sk_buff *skb, int len)
         skb_reset_tail_pointer(skb);
         skb->data_len -= len;
         skb->len = skb->data_len;
+        return len;
 }
 
 /* Remove acked data from a packet in the transmit queue. */
 int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
 {
+        u32 delta_truesize;
+
         if (skb_unclone(skb, GFP_ATOMIC))
                 return -ENOMEM;
 
-        __pskb_trim_head(skb, len);
+        delta_truesize = __pskb_trim_head(skb, len);
 
         TCP_SKB_CB(skb)->seq += len;
         skb->ip_summed = CHECKSUM_PARTIAL;
 
-        skb->truesize        -= len;
-        sk->sk_wmem_queued   -= len;
-        sk_mem_uncharge(sk, len);
-        sock_set_flag(sk, SOCK_QUEUE_SHRUNK);
+        if (delta_truesize) {
+                skb->truesize      -= delta_truesize;
+                sk->sk_wmem_queued -= delta_truesize;
+                sk_mem_uncharge(sk, delta_truesize);
+                sock_set_flag(sk, SOCK_QUEUE_SHRUNK);
+        }
 
         /* Any change of skb->len requires recalculation of tso factor. */
         if (tcp_skb_pcount(skb) > 1)