Re: [PATCH net v2] ipv4: icmp: reject RTN_UNREACHABLE input routes in icmp_route_lookup
From: Ido Schimmel <idosch@nvidia.com>
Date: 2026-08-17 14:15:49
On Sat, Aug 15, 2026 at 03:51:02PM +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 any input route that is RTN_UNREACHABLE to fix it.
Fixes: 8b7817f3a959 ("[IPSEC]: Add ICMP host relookup support")
Signed-off-by: Dong Chenchen <redacted>FYI, there's a new policy regarding AI review: " Patch authors are expected to proactively look into the AI-generated reviews and handle such feedback as any other kind of review: either debate it or address it. In both cases a reply on the mailing list is expected. " https://docs.kernel.org/next/process/maintainer-netdev.html#review-timelines
quoted hunk ↗ jump to hunk
--- net/ipv4/icmp.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c index 0caedfc7ca92..4703d8dd6b47 100644 --- a/net/ipv4/icmp.c +++ b/net/ipv4/icmp.c@@ -585,11 +585,11 @@ static struct rtable *icmp_route_lookup(struct net *net, struct flowi4 *fl4, * 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. + * LOCAL or UNREACHABLE route with dst.output=ip_rt_bug, + * which must not be used for output. */
Why would the addition of an address lead to RTN_UNREACHABLE? This seems to be true for RTN_LOCAL, but not for RTN_UNREACHABLE. Similarly, it doesn't seem like RTN_UNREACHABLE is the result of a race condition. The comment needs to be reworded to reflect that.
- if (!err && rt2 && rt2->rt_type == RTN_LOCAL) {
- net_warn_ratelimited("detected local route for %pI4 during ICMP sending, src %pI4\n",
+ if (!err && rt2 && (rt2->rt_type == RTN_LOCAL || rt2->rt_type == RTN_UNREACHABLE)) {
+ net_warn_ratelimited("detected unusable input route for %pI4 during ICMP sending, src %pI4\n",
&fl4_dec.daddr, &fl4_dec.saddr);The check is fine, but if RTN_UNREACHABLE is not the result of a race condition, then net_warn_ratelimited() should only be emitted for RTN_LOCAL.
dst_release(&rt2->dst); err = -EINVAL; -- 2.25.1