ipv4: try to cache dst_entries which would cause a redirect

Not caching dst_entries which cause redirects could be exploited by hosts
on the same subnet, causing a severe DoS attack. This effect aggravated
since commit f88649721268999 ("ipv4: fix dst race in sk_dst_get()").

Lookups causing redirects will be allocated with DST_NOCACHE set which
will force dst_release to free them via RCU.  Unfortunately waiting for
RCU grace period just takes too long, we can end up with >1M dst_entries
waiting to be released and the system will run OOM. rcuos threads cannot
catch up under high softirq load.

Attaching the flag to emit a redirect later on to the specific skb allows
us to cache those dst_entries thus reducing the pressure on allocation
and deallocation.

This issue was discovered by Marcelo Leitner.

Cc: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Marcelo Leitner <mleitner@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 5 deletions

diff --git a/include/net/ip.h b/include/net/ip.h
index 0bb620702929..f7cbd703d15d 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -39,11 +39,12 @@ struct inet_skb_parm {
         struct ip_options       opt;            /* Compiled IP options          */
         unsigned char           flags;
 
-#define IPSKB_FORWARDED         1
-#define IPSKB_XFRM_TUNNEL_SIZE  2
-#define IPSKB_XFRM_TRANSFORMED  4
-#define IPSKB_FRAG_COMPLETE     8
-#define IPSKB_REROUTED          16
+#define IPSKB_FORWARDED         BIT(0)
+#define IPSKB_XFRM_TUNNEL_SIZE  BIT(1)
+#define IPSKB_XFRM_TRANSFORMED  BIT(2)
+#define IPSKB_FRAG_COMPLETE     BIT(3)
+#define IPSKB_REROUTED          BIT(4)
+#define IPSKB_DOREDIRECT        BIT(5)
 
         u16                     frag_max_size;
 };