net: drop dst before queueing fragments

Commit 4a94445c9a5c (net: Use ip_route_input_noref() in input path)
added a bug in IP defragmentation handling, as non refcounted
dst could escape an RCU protected section.

Commit 64f3b9e203bd068 (net: ip_expire() must revalidate route) fixed
the case of timeouts, but not the general problem.

Tom Parkin noticed crashes in UDP stack and provided a patch,
but further analysis permitted us to pinpoint the root cause.

Before queueing a packet into a frag list, we must drop its dst,
as this dst has limited lifetime (RCU protected)

When/if a packet is finally reassembled, we use the dst of the very
last skb, still protected by RCU and valid, as the dst of the
reassembled packet.

Use same logic in IPv6, as there is no need to hold dst references.

Reported-by: Tom Parkin <tparkin@katalix.com>
Tested-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
index a6445b843ef40774f2bf5014e6800e1f4fd86b3e..52c273ea05c34c902e07c00609b4f881c392c37b 100644 (file)
--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c
@@ -248,8 +248,7 @@ static void ip_expire(unsigned long arg)
                if (!head->dev)
                        goto out_rcu_unlock;
 
-               /* skb dst is stale, drop it, and perform route lookup again */
-               skb_dst_drop(head);
+               /* skb has no dst, perform route lookup again */
                iph = ip_hdr(head);
                err = ip_route_input_noref(head, iph->daddr, iph->saddr,
                                           iph->tos, head->dev);
@@ -523,9 +522,16 @@ found:
                qp->q.max_size = skb->len + ihl;
 
        if (qp->q.last_in == (INET_FRAG_FIRST_IN | INET_FRAG_LAST_IN) &&
-           qp->q.meat == qp->q.len)
-               return ip_frag_reasm(qp, prev, dev);
+           qp->q.meat == qp->q.len) {
+               unsigned long orefdst = skb->_skb_refdst;
 
+               skb->_skb_refdst = 0UL;
+               err = ip_frag_reasm(qp, prev, dev);
+               skb->_skb_refdst = orefdst;
+               return err;
+       }
+
+       skb_dst_drop(skb);
        inet_frag_lru_move(&qp->q);
        return -EINPROGRESS;
 

diff --git a/net/ipv6/reassembly.c b/net/ipv6/reassembly.c
index 196ab9347ad1df2bab5a9c98720a51e81773985c..0ba10e53a6298f07cb64193681ccbfb419d40f36 100644 (file)
--- a/net/ipv6/reassembly.c
+++ b/net/ipv6/reassembly.c
@@ -330,9 +330,17 @@ found:
        }
 
        if (fq->q.last_in == (INET_FRAG_FIRST_IN | INET_FRAG_LAST_IN) &&
-           fq->q.meat == fq->q.len)
-               return ip6_frag_reasm(fq, prev, dev);
+           fq->q.meat == fq->q.len) {
+               int res;
+               unsigned long orefdst = skb->_skb_refdst;
+
+               skb->_skb_refdst = 0UL;
+               res = ip6_frag_reasm(fq, prev, dev);
+               skb->_skb_refdst = orefdst;
+               return res;
+       }
 
+       skb_dst_drop(skb);
        inet_frag_lru_move(&fq->q);
        return -1;