ipv4: keep skb->dst around in presence of IP options

Andrey Konovalov got crashes in __ip_options_echo() when a NULL skb->dst
is accessed.

ipv4_pktinfo_prepare() should not drop the dst if (evil) IP options
are present.

We could refine the test to the presence of ts_needtime or srr,
but IP options are not often used, so let's be conservative.

Thanks to syzkaller team for finding this bug.

Fixes: d826eb14ecef ("ipv4: PKTINFO doesnt need dst reference")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 34b2cef20f19c87999fff3da4071e66937db9644)

Orabug: 25543766
CVE: CVE-2017-5970
Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>
Reviewed-by: Jack Vogel <jack.vogel@oracle.com>

diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index 7cfb0893f2636bcc87537da3014643362f72b10f..1ac9ecfcb03c2dba9c07e647c7c352da5c645c80 100644 (file)
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -1175,7 +1175,14 @@ void ipv4_pktinfo_prepare(const struct sock *sk, struct sk_buff *skb)
                pktinfo->ipi_ifindex = 0;
                pktinfo->ipi_spec_dst.s_addr = 0;
        }
-       skb_dst_drop(skb);
+       /* We need to keep the dst for __ip_options_echo()
+        * We could restrict the test to opt.ts_needtime || opt.srr,
+        * but the following is good enough as IP options are not often used.
+        */
+       if (unlikely(IPCB(skb)->opt.optlen))
+               skb_dst_force(skb);
+       else
+               skb_dst_drop(skb);
 }
 
 int ip_setsockopt(struct sock *sk, int level,