[PATH net] ipv4: keep skb->dst around in presence of IP options

[Eric Dumazet]

From: Eric Dumazet <eduma...@google.com>

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 <eduma...@google.com>
Reported-by: Andrey Konovalov <andreyk...@google.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 
8a4409dd390aac7fcb88383af1550a2f967d..c4fcf9d713cca64a3afaec1628692739d9fa 
100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -1243,7 +1243,14 @@ 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,