ipv6: ip6_fragment: fix headroom tests and skb leak

David Woodhouse reports skb_under_panic when we try to push ethernet
header to fragmented ipv6 skbs:

 skbuff: skb_under_panic: text:c1277f1e len:1294 put:14 head:dec98000
 data:dec97ffc tail:0xdec9850a end:0xdec98f40 dev:br-lan
[..]
ip6_finish_output2+0x196/0x4da

David further debugged this:
  [..] offending fragments were arriving here with skb_headroom(skb)==10.
  Which is reasonable, being the Solos ADSL card's header of 8 bytes
  followed by 2 bytes of PPP frame type.

The problem is that if netfilter ipv6 defragmentation is used, skb_cow()
in ip6_forward will only see reassembled skb.

Therefore, headroom is overestimated by 8 bytes (we pulled fragment
header) and we don't check the skbs in the frag_list either.

We can't do these checks in netfilter defrag since outdev isn't known yet.

Furthermore, existing tests in ip6_fragment did not consider the fragment
or ipv6 header size when checking headroom of the fraglist skbs.

While at it, also fix a skb leak on memory allocation -- ip6_fragment
must consume the skb.

I tested this e1000 driver hacked to not allocate additional headroom
(we end up in slowpath, since LL_RESERVED_SPACE is 16).

If 2 bytes of headroom are allocated, fastpath is taken (14 byte
ethernet header was pulled, so 16 byte headroom available in all
fragments).

Reported-by: David Woodhouse <dwmw2@infradead.org>
Diagnosed-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Tested-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 26ea4793074004d0af1026bb378860b53baa0ad2..92b1aa38f121507b662e2c964423952bf995b81e 100644 (file)
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -586,20 +586,22 @@ int ip6_fragment(struct sock *sk, struct sk_buff *skb,
        frag_id = ipv6_select_ident(net, &ipv6_hdr(skb)->daddr,
                                    &ipv6_hdr(skb)->saddr);
 
+       hroom = LL_RESERVED_SPACE(rt->dst.dev);
        if (skb_has_frag_list(skb)) {
                int first_len = skb_pagelen(skb);
                struct sk_buff *frag2;
 
                if (first_len - hlen > mtu ||
                    ((first_len - hlen) & 7) ||
-                   skb_cloned(skb))
+                   skb_cloned(skb) ||
+                   skb_headroom(skb) < (hroom + sizeof(struct frag_hdr)))
                        goto slow_path;
 
                skb_walk_frags(skb, frag) {
                        /* Correct geometry. */
                        if (frag->len > mtu ||
                            ((frag->len & 7) && frag->next) ||
-                           skb_headroom(frag) < hlen)
+                           skb_headroom(frag) < (hlen + hroom + sizeof(struct frag_hdr)))
                                goto slow_path_clean;
 
                        /* Partially cloned skb? */
@@ -616,8 +618,6 @@ int ip6_fragment(struct sock *sk, struct sk_buff *skb,
 
                err = 0;
                offset = 0;
-               frag = skb_shinfo(skb)->frag_list;
-               skb_frag_list_init(skb);
                /* BUILD HEADER */
 
                *prevhdr = NEXTHDR_FRAGMENT;
@@ -625,8 +625,11 @@ int ip6_fragment(struct sock *sk, struct sk_buff *skb,
                if (!tmp_hdr) {
                        IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
                                      IPSTATS_MIB_FRAGFAILS);
-                       return -ENOMEM;
+                       err = -ENOMEM;
+                       goto fail;
                }
+               frag = skb_shinfo(skb)->frag_list;
+               skb_frag_list_init(skb);
 
                __skb_pull(skb, hlen);
                fh = (struct frag_hdr *)__skb_push(skb, sizeof(struct frag_hdr));
@@ -723,7 +726,6 @@ slow_path:
         */
 
        *prevhdr = NEXTHDR_FRAGMENT;
-       hroom = LL_RESERVED_SPACE(rt->dst.dev);
        troom = rt->dst.dev->needed_tailroom;
 
        /*