[PATCH net] ipv4: icmp: reject non-unicast input routes in icmp_route_lookup

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
local, broadcast, multicast and unreachable addresses it still succeeds
and returns an input route whose dst.output is set to ip_rt_bug().  The
existing check only rejects RTN_LOCAL routes, so the other 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 ]------------

If the original source address is unroutable, ip_route_input() returns a
RTN_UNREACHABLE input route with err == 0, and the ICMP reply is then sent
through ip_rt_bug().

Only RTN_UNICAST input routes have dst.output set to ip_output() and are
safe to use for output.  Reject any input route that is not RTN_UNICAST.

Fixes: 8b7817f3a959 ("[IPSEC]: Add ICMP host relookup support")
Signed-off-by: Dong Chenchen <redacted>
---
 net/ipv4/icmp.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index 0caedfc7ca92..d58ac2f23f9f 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c

@@ -584,12 +584,14 @@ static struct rtable *icmp_route_lookup(struct net *net, struct flowi4 *fl4,
 		 * 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.
+		 * concurrently, or if fl4_dec.daddr is otherwise unroutable.
+		 * In that case, ip_route_input() returns an input route
+		 * (RTN_LOCAL, RTN_BROADCAST, RTN_UNREACHABLE or
+		 * RTN_MULTICAST) with dst.output set to 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",
+		if (!err && rt2 && rt2->rt_type != RTN_UNICAST) {
+			net_warn_ratelimited("detected unusable input route for %pI4 during ICMP sending, src %pI4\n",
 					     &fl4_dec.daddr, &fl4_dec.saddr);
 			dst_release(&rt2->dst);
 			err = -EINVAL;

-- 
2.25.1