Re: [PATCH net v3] ipv4: icmp: reject RTN_UNREACHABLE input routes in icmp_route_lookup
From: Ido Schimmel <idosch@nvidia.com>
Date: 2026-08-26 15:40:33
On Tue, Aug 25, 2026 at 05:45:23PM +0800, Dong Chenchen wrote:
When the forward output route cannot be used in icmp_route_lookup(), it enters the "reverse path" and calls ip_route_input() on fl4_dec.daddr, the original packet's source address. ip_route_input() only returns an error for truly invalid packets. For unreachable addresses it will succeed and return an input route whose dst.output is set to ip_rt_bug(). The existing check only rejects RTN_LOCAL routes, so the RTN_UNREACHABLE route types can still be returned and later used for output, syzkaller triggering a WARN_ON_ONCE() in ip_rt_bug() as bellow: ------------[ cut here ]------------ WARNING: net/ipv4/route.c:1273 at ip_rt_bug+0x14/0x20 RIP: 0010:ip_rt_bug+0x14/0x20 Call Trace: ip_push_pending_frames+0xfa/0x100 __icmp_send+0x905/0xf10 ip_options_compile+0xc0/0xd0 ip_rcv_finish_core+0x321/0xae0 ip_rcv+0x1de/0x260 __netif_receive_skb_one_core+0x11a/0x130 netif_receive_skb+0x7b/0x260 tun_get_user+0x11bf/0x1c10 ------------[ cut here ]------------ Reject input route that is RTN_UNREACHABLE to fix it.
Nit: Also mention that the warning is only printed for RTN_LOCAL, as RTN_UNREACHABLE is not the result of a race condition.
quoted hunk ↗ jump to hunk
Fixes: 8b7817f3a959 ("[IPSEC]: Add ICMP host relookup support") Signed-off-by: Dong Chenchen <redacted> --- v2: - RTN_UNICAST judgement will reject bforward packet. Only reject RTN_UNREACHABLE v3: - Modify the comments to make them more accurate. - RTN_UNREACHABLE dont need warn log. --- net/ipv4/icmp.c | 17 ++++++++--------- 1 file changed, 8 insertions(+), 9 deletions(-)diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c index 0caedfc7ca92..7350f0380be1 100644 --- a/net/ipv4/icmp.c +++ b/net/ipv4/icmp.c@@ -581,16 +581,15 @@ static struct rtable *icmp_route_lookup(struct net *net, struct flowi4 *fl4, 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. + * At this point, fl4_dec.daddr should NOT be local (the + * address can be added to the interface concurrently) or unroutable. + * In that case, ip_route_input() returns a LOCAL or UNREACHABLE + * route with dst.output=ip_rt_bug, which must not be used for output. */
The code is fine, but the comment is a bit misleading (the "should NOT" part is only correct for "local"). I find this clearer: " fl4_dec.daddr is not expected to be local here, but it can be added to an interface concurrently, in which case ip_route_input() returns a LOCAL route. It can also fail to build a forwarding route towards fl4_dec.daddr, for example, when forwarding is disabled, and return an UNREACHABLE route. Both cases will result in a route with dst.output=ip_rt_bug, which must not be used for output. " Thanks!