xfrm: fix ip_rt_bug race in icmp_route_lookup reverse path

In the Linux kernel, the following vulnerability has been resolved:

xfrm: fix ip_rt_bug race in icmp_route_lookup reverse path

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