ip: fix truesize mismatch in ip fragmentation

Special care should be taken when slow path is hit in ip_fragment() :

When walking through frags, we transfert truesize ownership from skb to
frags. Then if we hit a slow_path condition, we must undo this or risk
uncharging frags->truesize twice, and in the end, having negative socket
sk_wmem_alloc counter, or even freeing socket sooner than expected.

Many thanks to Nick Bowler, who provided a very clean bug report and
test program.

Thanks to Jarek for reviewing my first patch and providing a V2

While Nick bisection pointed to commit 2b85a34e911 (net: No more
expensive sock_hold()/sock_put() on each tx), underlying bug is older
(2.6.12-rc5)

A side effect is to extend work done in commit b2722b1c3a893e
(ip_fragment: also adjust skb->truesize for packets not owned by a
socket) to ipv6 as well.

Reported-and-bisected-by: Nick Bowler <nbowler@elliptictech.com>
Tested-by: Nick Bowler <nbowler@elliptictech.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Jarek Poplawski <jarkao2@gmail.com>
CC: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 26 insertions, 11 deletions

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 04b69896df5f..7649d7750075 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -488,9 +488,8 @@ int ip_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
          * we can switch to copy when see the first bad fragment.
          */
         if (skb_has_frags(skb)) {
-                struct sk_buff *frag;
+                struct sk_buff *frag, *frag2;
                 int first_len = skb_pagelen(skb);
-                int truesizes = 0;
 
                 if (first_len - hlen > mtu ||
                     ((first_len - hlen) & 7) ||
@@ -503,18 +502,18 @@ int ip_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
                         if (frag->len > mtu ||
                             ((frag->len & 7) && frag->next) ||
                             skb_headroom(frag) < hlen)
-                            goto slow_path;
+                                goto slow_path_clean;
 
                         /* Partially cloned skb? */
                         if (skb_shared(frag))
-                                goto slow_path;
+                                goto slow_path_clean;
 
                         BUG_ON(frag->sk);
                         if (skb->sk) {
                                 frag->sk = skb->sk;
                                 frag->destructor = sock_wfree;
                         }
-                        truesizes += frag->truesize;
+                        skb->truesize -= frag->truesize;
                 }
 
                 /* Everything is OK. Generate! */
@@ -524,7 +523,6 @@ int ip_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
                 frag = skb_shinfo(skb)->frag_list;
                 skb_frag_list_init(skb);
                 skb->data_len = first_len - skb_headlen(skb);
-                skb->truesize -= truesizes;
                 skb->len = first_len;
                 iph->tot_len = htons(first_len);
                 iph->frag_off = htons(IP_MF);
@@ -576,6 +574,15 @@ int ip_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
                 }
                 IP_INC_STATS(dev_net(dev), IPSTATS_MIB_FRAGFAILS);
                 return err;
+
+slow_path_clean:
+                skb_walk_frags(skb, frag2) {
+                        if (frag2 == frag)
+                                break;
+                        frag2->sk = NULL;
+                        frag2->destructor = NULL;
+                        skb->truesize += frag2->truesize;
+                }
         }
 
 slow_path:
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index d40b330c0ee6..980912ed7a38 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -639,7 +639,7 @@ static int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
 
         if (skb_has_frags(skb)) {
                 int first_len = skb_pagelen(skb);
-                int truesizes = 0;
+                struct sk_buff *frag2;
 
                 if (first_len - hlen > mtu ||
                     ((first_len - hlen) & 7) ||
@@ -651,18 +651,18 @@ static int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
                         if (frag->len > mtu ||
                             ((frag->len & 7) && frag->next) ||
                             skb_headroom(frag) < hlen)
-                            goto slow_path;
+                                goto slow_path_clean;
 
                         /* Partially cloned skb? */
                         if (skb_shared(frag))
-                                goto slow_path;
+                                goto slow_path_clean;
 
                         BUG_ON(frag->sk);
                         if (skb->sk) {
                                 frag->sk = skb->sk;
                                 frag->destructor = sock_wfree;
-                                truesizes += frag->truesize;
                         }
+                        skb->truesize -= frag->truesize;
                 }
 
                 err = 0;
@@ -693,7 +693,6 @@ static int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
 
                 first_len = skb_pagelen(skb);
                 skb->data_len = first_len - skb_headlen(skb);
-                skb->truesize -= truesizes;
                 skb->len = first_len;
                 ipv6_hdr(skb)->payload_len = htons(first_len -
                                                    sizeof(struct ipv6hdr));
@@ -756,6 +755,15 @@ static int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
                               IPSTATS_MIB_FRAGFAILS);
                 dst_release(&rt->dst);
                 return err;
+
+slow_path_clean:
+                skb_walk_frags(skb, frag2) {
+                        if (frag2 == frag)
+                                break;
+                        frag2->sk = NULL;
+                        frag2->destructor = NULL;
+                        skb->truesize += frag2->truesize;
+                }
         }
 
 slow_path: