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Subject[PATCH 5.10 367/599] ipv4: Fix route lookups when handling ICMP redirects and PMTU updates
From: Guillaume Nault <>

[ 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

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

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 <>
Reviewed-by: David Ahern <>
Signed-off-by: Jakub Kicinski <>
Signed-off-by: Sasha Levin <>
net/ipv4/route.c | 18 ++++++++++++++----
1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index ce787c386793..c72d0de8bf71 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -529,6 +529,15 @@ void __ip_select_ident(struct net *net, struct iphdr *iph, int segs)

+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 ?
static void __build_flow_key(const struct net *net, struct flowi4 *fl4,
const struct sock *sk,
const struct iphdr *iph,
@@ -853,6 +862,7 @@ 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);

@@ -1077,6 +1087,7 @@ 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))
@@ -1151,6 +1162,8 @@ 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);
@@ -2524,7 +2537,6 @@ static struct rtable *__mkroute_output(const struct fib_result *res,
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,
@@ -2534,9 +2546,7 @@ 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) ?
+ ip_rt_fix_tos(fl4);

rth = ip_route_output_key_hash_rcu(net, fl4, &res, skb);

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