ipv4: Fix route lookups when handling ICMP redirects and PMTU updates

BugLink: https://bugs.launchpad.net/bugs/1969110
[ Upstream commit 544b4dd568e3b09c1ab38a759d3187e7abda11a0 ]

The PMTU update and ICMP redirect helper functions initialise their fl4
variable with either __build_flow_key() or build_sk_flow_key(). These
initialisation functions always set ->flowi4_scope with
RT_SCOPE_UNIVERSE and might set the ECN bits of ->flowi4_tos. This is
not a problem when the route lookup is later done via
ip_route_output_key_hash(), which properly clears the ECN bits from
->flowi4_tos and initialises ->flowi4_scope based on the RTO_ONLINK
flag. However, some helpers call fib_lookup() directly, without
sanitising the tos and scope fields, so the route lookup can fail and,
as a result, the ICMP redirect or PMTU update aren't taken into
account.

Fix this by extracting the ->flowi4_tos and ->flowi4_scope sanitisation
code into ip_rt_fix_tos(), then use this function in handlers that call
fib_lookup() directly.

Note 1: We can't sanitise ->flowi4_tos and ->flowi4_scope in a central
place (like __build_flow_key() or flowi4_init_output()), because
ip_route_output_key_hash() expects non-sanitised values. When called
with sanitised values, it can erroneously overwrite RT_SCOPE_LINK with
RT_SCOPE_UNIVERSE in ->flowi4_scope. Therefore we have to be careful to
sanitise the values only for those paths that don't call
ip_route_output_key_hash().

Note 2: The problem is mostly about sanitising ->flowi4_tos. Having
->flowi4_scope initialised with RT_SCOPE_UNIVERSE instead of
RT_SCOPE_LINK probably wasn't really a problem: sockets with the
SOCK_LOCALROUTE flag set (those that'd result in RTO_ONLINK being set)
normally shouldn't receive ICMP redirects or PMTU updates.

Fixes: 4895c771c7f0 ("ipv4: Add FIB nexthop exceptions.")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit 9151982547ee92ed4a5ad5bfb609b3b5febca881)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 23833660584df57896e2659913a91b616cd06d89..ed9b6842a9a0c4d51f797af1dd455c607262b234 100644 (file)
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -506,6 +506,15 @@ void __ip_select_ident(struct net *net, struct iphdr *iph, int segs)
 }
 EXPORT_SYMBOL(__ip_select_ident);
 
+static void ip_rt_fix_tos(struct flowi4 *fl4)
+{
+       __u8 tos = RT_FL_TOS(fl4);
+
+       fl4->flowi4_tos = tos & IPTOS_RT_MASK;
+       fl4->flowi4_scope = tos & RTO_ONLINK ?
+                           RT_SCOPE_LINK : RT_SCOPE_UNIVERSE;
+}
+
 static void __build_flow_key(const struct net *net, struct flowi4 *fl4,
                             const struct sock *sk,
                             const struct iphdr *iph,
@@ -831,6 +840,7 @@ static void ip_do_redirect(struct dst_entry *dst, struct sock *sk, struct sk_buf
        rt = (struct rtable *) dst;
 
        __build_flow_key(net, &fl4, sk, iph, oif, tos, prot, mark, 0);
+       ip_rt_fix_tos(&fl4);
        __ip_do_redirect(rt, skb, &fl4, true);
 }
 
@@ -1055,6 +1065,7 @@ static void ip_rt_update_pmtu(struct dst_entry *dst, struct sock *sk,
        struct flowi4 fl4;
 
        ip_rt_build_flow_key(&fl4, sk, skb);
+       ip_rt_fix_tos(&fl4);
 
        /* Don't make lookup fail for bridged encapsulations */
        if (skb && netif_is_any_bridge_port(skb->dev))
@@ -1129,6 +1140,8 @@ void ipv4_sk_update_pmtu(struct sk_buff *skb, struct sock *sk, u32 mtu)
                        goto out;
 
                new = true;
+       } else {
+               ip_rt_fix_tos(&fl4);
        }
 
        __ip_rt_update_pmtu((struct rtable *)xfrm_dst_path(&rt->dst), &fl4, mtu);
@@ -2609,7 +2622,6 @@ add:
 struct rtable *ip_route_output_key_hash(struct net *net, struct flowi4 *fl4,
                                        const struct sk_buff *skb)
 {
-       __u8 tos = RT_FL_TOS(fl4);
        struct fib_result res = {
                .type           = RTN_UNSPEC,
                .fi             = NULL,
@@ -2619,9 +2631,7 @@ struct rtable *ip_route_output_key_hash(struct net *net, struct flowi4 *fl4,
        struct rtable *rth;
 
        fl4->flowi4_iif = LOOPBACK_IFINDEX;
-       fl4->flowi4_tos = tos & IPTOS_RT_MASK;
-       fl4->flowi4_scope = ((tos & RTO_ONLINK) ?
-                        RT_SCOPE_LINK : RT_SCOPE_UNIVERSE);
+       ip_rt_fix_tos(fl4);
 
        rcu_read_lock();
        rth = ip_route_output_key_hash_rcu(net, fl4, &res, skb);