[PATCH 6.18 328/641] xfrm: fix ip_rt_bug race in icmp_route_lookup reverse path

Greg Kroah-Hartman
2026-02-25T06:56:33Z

6.18-stable review patch. If anyone has any objections, please let me know.

------------------

From: Jiayuan Chen <jiayuan.chen@shopee.com>

[ Upstream commit 81b84de32bb27ae1ae2eb9acf0420e9d0d14bf00 ]

icmp_route_lookup() performs multiple route lookups to find a suitable
route for sending ICMP error messages, with special handling for XFRM
(IPsec) policies.

The lookup sequence is:
1. First, lookup output route for ICMP reply (dst = original src)
2. Pass through xfrm_lookup() for policy check
3. If blocked (-EPERM) or dst is not local, enter "reverse path"
4. In reverse path, call xfrm_decode_session_reverse() to get fl4_dec
which reverses the original packet's flow (saddr<->daddr swapped)
5. If fl4_dec.saddr is local (we are the original destination), use
__ip_route_output_key() for output route lookup
6. If fl4_dec.saddr is NOT local (we are a forwarding node), use
ip_route_input() to simulate the reverse packet's input path
7. Finally, pass rt2 through xfrm_lookup() with XFRM_LOOKUP_ICMP flag

The bug occurs in step 6: ip_route_input() is called with fl4_dec.daddr
(original packet's source) as destination. If this address becomes local
between the initial check and ip_route_input() call (e.g., due to
concurrent "ip addr add"), ip_route_input() returns a LOCAL route with
dst.output set to ip_rt_bug.

This route is then used for ICMP output, causing dst_output() to call
ip_rt_bug(), triggering a WARN_ON:

------------[ cut here ]------------
WARNING: net/ipv4/route.c:1275 at ip_rt_bug+0x21/0x30, CPU#1
Call Trace:
<TASK>
ip_push_pending_frames+0x202/0x240
icmp_push_reply+0x30d/0x430
__icmp_send+0x1149/0x24f0
ip_options_compile+0xa2/0xd0
ip_rcv_finish_core+0x829/0x1950
ip_rcv+0x2d7/0x420
__netif_receive_skb_one_core+0x185/0x1f0
netif_receive_skb+0x90/0x450
tun_get_user+0x3413/0x3fb0
tun_chr_write_iter+0xe4/0x220
...

Fix this by checking rt2->rt_type after ip_route_input(). If it's
RTN_LOCAL, the route cannot be used for output, so treat it as an error.

The reproducer requires kernel modification to widen the race window,
making it unsuitable as a selftest. It is available at:

https://gist.github.com/mrpre/eae853b72ac6a750f5d45d64ddac1e81

Reported-by: syzbot+e738404dcd14b620923c@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/000000000000b1060905eada8881@google.com/T/
Closes: https://lore.kernel.org/r/20260128090523.356953-1-jiayuan.chen@linux.dev
Fixes: 8b7817f3a959 ("[IPSEC]: Add ICMP host relookup support")
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Link: https://patch.msgid.link/20260206050220.59642-1-jiayuan.chen@linux.dev
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/ipv4/icmp.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)

diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index 8e10e9e7676c5..9323ee0a6ac48 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c
@@ -554,6 +554,21 @@ static struct rtable *icmp_route_lookup(struct net *net, struct flowi4 *fl4,
/* steal dst entry from skb_in, don't drop refcnt */
skb_dstref_steal(skb_in);
skb_dstref_restore(skb_in, orefdst);
+
+ /*
+ * At this point, fl4_dec.daddr should NOT be local (we
+ * checked fl4_dec.saddr above). However, a race condition
+ * may occur if the address is added to the interface
+ * concurrently. In that case, ip_route_input() returns a
+ * LOCAL route with dst.output=ip_rt_bug, which must not
+ * be used for output.
+ */
+ if (!err && rt2 && rt2->rt_type == RTN_LOCAL) {
+ net_warn_ratelimited("detected local route for %pI4 during ICMP sending, src %pI4\n",
+ &fl4_dec.daddr, &fl4_dec.saddr);
+ dst_release(&rt2->dst);
+ err = -EINVAL;
+ }
}

if (err)
--
2.51.0