net: take care of cloned skbs in tcp_try_coalesce()

Before stealing fragments or skb head, we must make sure skbs are not
cloned.

Alexander was worried about destination skb being cloned : In bridge
setups, a driver could be fooled if skb->data_len would not match skb
nr_frags.

If source skb is cloned, we must take references on pages instead.

Bug happened using tcpdump (if not using mmap())

Introduce kfree_skb_partial() helper to cleanup code.

Reported-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 28 insertions, 14 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 7096790e06b..a8829370f71 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4532,6 +4532,7 @@ static inline int tcp_try_rmem_schedule(struct sock *sk, unsigned int size)
  * @sk: socket
  * @to: prior buffer
  * @from: buffer to add in queue
+ * @fragstolen: pointer to boolean
  *
  * Before queueing skb @from after @to, try to merge them
  * to reduce overall memory use and queue lengths, if cost is small.
@@ -4544,10 +4545,10 @@ static bool tcp_try_coalesce(struct sock *sk,
                              struct sk_buff *from,
                              bool *fragstolen)
 {
-        int delta, len = from->len;
+        int i, delta, len = from->len;
 
         *fragstolen = false;
-        if (tcp_hdr(from)->fin)
+        if (tcp_hdr(from)->fin || skb_cloned(to))
                 return false;
         if (len <= skb_tailroom(to)) {
                 BUG_ON(skb_copy_bits(from, 0, skb_put(to, len), len));
@@ -4574,7 +4575,13 @@ copyfrags:
                        skb_shinfo(from)->frags,
                        skb_shinfo(from)->nr_frags * sizeof(skb_frag_t));
                 skb_shinfo(to)->nr_frags += skb_shinfo(from)->nr_frags;
-                skb_shinfo(from)->nr_frags = 0;
+
+                if (skb_cloned(from))
+                        for (i = 0; i < skb_shinfo(from)->nr_frags; i++)
+                                skb_frag_ref(from, i);
+                else
+                        skb_shinfo(from)->nr_frags = 0;
+
                 to->truesize += delta;
                 atomic_add(delta, &sk->sk_rmem_alloc);
                 sk_mem_charge(sk, delta);
@@ -4592,13 +4599,26 @@ copyfrags:
                 offset = from->data - (unsigned char *)page_address(page);
                 skb_fill_page_desc(to, skb_shinfo(to)->nr_frags,
                                    page, offset, skb_headlen(from));
-                *fragstolen = true;
+
+                if (skb_cloned(from))
+                        get_page(page);
+                else
+                        *fragstolen = true;
+
                 delta = len; /* we dont know real truesize... */
                 goto copyfrags;
         }
         return false;
 }
 
+static void kfree_skb_partial(struct sk_buff *skb, bool head_stolen)
+{
+        if (head_stolen)
+                kmem_cache_free(skbuff_head_cache, skb);
+        else
+                __kfree_skb(skb);
+}
+
 static void tcp_data_queue_ofo(struct sock *sk, struct sk_buff *skb)
 {
         struct tcp_sock *tp = tcp_sk(sk);
@@ -4642,10 +4662,7 @@ static void tcp_data_queue_ofo(struct sock *sk, struct sk_buff *skb)
                 if (!tcp_try_coalesce(sk, skb1, skb, &fragstolen)) {
                         __skb_queue_after(&tp->out_of_order_queue, skb1, skb);
                 } else {
-                        if (fragstolen)
-                                kmem_cache_free(skbuff_head_cache, skb);
-                        else
-                                __kfree_skb(skb);
+                        kfree_skb_partial(skb, fragstolen);
                         skb = NULL;
                 }
 
@@ -4804,12 +4821,9 @@ queue_and_out:
 
                 tcp_fast_path_check(sk);
 
-                if (eaten > 0) {
-                        if (fragstolen)
-                                kmem_cache_free(skbuff_head_cache, skb);
-                        else
-                                __kfree_skb(skb);
-                } else if (!sock_flag(sk, SOCK_DEAD))
+                if (eaten > 0)
+                        kfree_skb_partial(skb, fragstolen);
+                else if (!sock_flag(sk, SOCK_DEAD))
                         sk->sk_data_ready(sk, 0);
                 return;
         }