ipv6: protect mtu calculation of wrap-around and infinite loop by rounding issues

Raw sockets with hdrincl enabled can insert ipv6 extension headers
right into the data stream. In case we need to fragment those packets,
we reparse the options header to find the place where we can insert
the fragment header. If the extension headers exceed the link's MTU we
actually cannot make progress in such a case.

Instead of ending up in broken arithmetic or rounding towards 0 and
entering an endless loop in ip6_fragment, just prevent those cases by
aborting early and signal -EMSGSIZE to user space.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index d03d6da772f3bc394aaee0aa7e6694bed6cf56ac..8dddb45c433e53ad35d30cd4263a5a5080cd0da9 100644 (file)
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -28,6 +28,7 @@
 
 #include <linux/errno.h>
 #include <linux/kernel.h>
+#include <linux/overflow-arith.h>
 #include <linux/string.h>
 #include <linux/socket.h>
 #include <linux/net.h>
@@ -584,7 +585,10 @@ int ip6_fragment(struct sock *sk, struct sk_buff *skb,
                if (np->frag_size)
                        mtu = np->frag_size;
        }
-       mtu -= hlen + sizeof(struct frag_hdr);
+
+       if (overflow_usub(mtu, hlen + sizeof(struct frag_hdr), &mtu) ||
+           mtu <= 7)
+               goto fail_toobig;
 
        frag_id = ipv6_select_ident(net, &ipv6_hdr(skb)->daddr,
                                    &ipv6_hdr(skb)->saddr);