[PATCH net-next] net: take care of cloned skbs in tcp_try_coalesce()

[Eric Dumazet <eric.dumazet@gmail.com>]

From: Eric Dumazet <edumazet@google.com>

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

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

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

Reported-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/ipv4/tcp_input.c |   17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 96a631d..7686d7f 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4467,7 +4467,7 @@ 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)
@@ -4497,7 +4497,13 @@ 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);
@@ -4515,7 +4521,12 @@ 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;
 	}