tcp: fix skb_availroom()

Chrome OS team reported a crash on a Pixel ChromeBook in TCP stack :

https://code.google.com/p/chromium/issues/detail?id=182056

commit a21d45726acac (tcp: avoid order-1 allocations on wifi and tx
path) did a poor choice adding an 'avail_size' field to skb, while
what we really needed was a 'reserved_tailroom' one.

It would have avoided commit 22b4a4f22da (tcp: fix retransmit of
partially acked frames) and this commit.

Crash occurs because skb_split() is not aware of the 'avail_size'
management (and should not be aware)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Mukesh Agrawal <quiche@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 6 insertions, 4 deletions

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 821c7f45d2a7..6f2bb860e051 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -500,7 +500,7 @@ struct sk_buff {
         union {
                 __u32           mark;
                 __u32           dropcount;
-                __u32           avail_size;
+                __u32           reserved_tailroom;
         };
 
         sk_buff_data_t          inner_transport_header;
@@ -1447,7 +1447,10 @@ static inline int skb_tailroom(const struct sk_buff *skb)
  */
 static inline int skb_availroom(const struct sk_buff *skb)
 {
-        return skb_is_nonlinear(skb) ? 0 : skb->avail_size - skb->len;
+        if (skb_is_nonlinear(skb))
+                return 0;
+
+        return skb->end - skb->tail - skb->reserved_tailroom;
 }
 
 /**
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 47e854fcae24..e22020790709 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -775,7 +775,7 @@ struct sk_buff *sk_stream_alloc_skb(struct sock *sk, int size, gfp_t gfp)
                          * Make sure that we have exactly size bytes
                          * available to the caller, no more, no less.
                          */
-                        skb->avail_size = size;
+                        skb->reserved_tailroom = skb->end - skb->tail - size;
                         return skb;
                 }
                 __kfree_skb(skb);
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index e2b4461074da..817fbb396bc8 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1298,7 +1298,6 @@ static void __pskb_trim_head(struct sk_buff *skb, int len)
         eat = min_t(int, len, skb_headlen(skb));
         if (eat) {
                 __skb_pull(skb, eat);
-                skb->avail_size -= eat;
                 len -= eat;
                 if (!len)
                         return;