[PATCH 1/1] ipv4: keep skb->dst around in presence of IP options

[Kleber Sacilotto de Souza]

From: Eric Dumazet <edumazet at google.com>

CVE-2017-5970

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 at google.com>
Reported-by: Andrey Konovalov <andreyknvl at google.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
(cherry picked from commit 34b2cef20f19c87999fff3da4071e66937db9644)
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza at canonical.com>
---
 net/ipv4/ip_sockglue.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index 4aeb2131242b..b86d12793328 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -1080,7 +1080,14 @@ 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,
-- 
2.14.1