[PATCH 3.18 06/20] ipv4: try to cache dst_entries which would cause a redirect

Greg Kroah-Hartman
2015-02-25T02:21:04Z

From: Hannes Frederic Sowa <hannes@stressinduktion.org>

[ Upstream commit df4d92549f23e1c037e83323aff58a21b3de7fe0 ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
include/net/ip.h | 11 ++++++-----
net/ipv4/ip_forward.c | 3 ++-
net/ipv4/route.c | 9 +++++----
3 files changed, 13 insertions(+), 10 deletions(-)

--- 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;
};
--- a/net/ipv4/ip_forward.c
+++ b/net/ipv4/ip_forward.c
@@ -129,7 +129,8 @@ int ip_forward(struct sk_buff *skb)
* We now generate an ICMP HOST REDIRECT giving the route
* we calculated.
*/
- if (rt->rt_flags&RTCF_DOREDIRECT && !opt->srr && !skb_sec_path(skb))
+ if (IPCB(skb)->flags & IPSKB_DOREDIRECT && !opt->srr &&
+ !skb_sec_path(skb))
ip_rt_send_redirect(skb);

skb->priority = rt_tos2priority(iph->tos);
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1554,11 +1554,10 @@ static int __mkroute_input(struct sk_buf

do_cache = res->fi && !itag;
if (out_dev == in_dev && err && IN_DEV_TX_REDIRECTS(out_dev) &&
+ skb->protocol == htons(ETH_P_IP) &&
(IN_DEV_SHARED_MEDIA(out_dev) ||
- inet_addr_onlink(out_dev, saddr, FIB_RES_GW(*res)))) {
- flags |= RTCF_DOREDIRECT;
- do_cache = false;
- }
+ inet_addr_onlink(out_dev, saddr, FIB_RES_GW(*res))))
+ IPCB(skb)->flags |= IPSKB_DOREDIRECT;

if (skb->protocol != htons(ETH_P_IP)) {
/* Not IP (i.e. ARP). Do not create route, if it is
@@ -2303,6 +2302,8 @@ static int rt_fill_info(struct net *net,
r->rtm_flags = (rt->rt_flags & ~0xFFFF) | RTM_F_CLONED;
if (rt->rt_flags & RTCF_NOTIFY)
r->rtm_flags |= RTM_F_NOTIFY;
+ if (IPCB(skb)->flags & IPSKB_DOREDIRECT)
+ r->rtm_flags |= RTCF_DOREDIRECT;

if (nla_put_be32(skb, RTA_DST, dst))
goto nla_put_failure;