CVE-2026-45905

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 packets 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 packets 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 packets 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: 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 its 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

Weakness

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.

Affected Software

Extended Description

A race condition occurs within concurrent environments, and it is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc. A race condition violates these properties, which are closely related:

A race condition exists when an “interfering code sequence” can still access the shared resource, violating exclusivity. The interfering code sequence could be “trusted” or “untrusted.” A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.

Potential Mitigations

• Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring.
• Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/29.html

References

• https://git.kernel.org/stable/c/1c9ef28f643cce34a6a6c36c8f4d6d60a60db7e1
• https://git.kernel.org/stable/c/2c1f59005da9dd4b07b26984fd719e36557dc57c
• https://git.kernel.org/stable/c/423ce12d10b426709489d6b84fdaa6d2f31c5652
• https://git.kernel.org/stable/c/81b84de32bb27ae1ae2eb9acf0420e9d0d14bf00
• https://git.kernel.org/stable/c/9a95ec9144eeff1fc6fbcc21b677e322c6f1430b
• https://git.kernel.org/stable/c/b04061f89ffc6168e7ec3c71d0086ec3c3797228