CVE-2026-80850

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

tcp: fix AO info use-after-free in tcp ao connect init()

tcp v4 connect() adds a SYN-SENT socket to the ehash before calling tcp connect(). If TCP-AO is configured, tcp connect() first verifies that a key matches the peer and the bound device's current L3 master. tcp ao connect init() later resolves the L3 master again and removes keys which do not match it.

The socket lock does not stabilize the bound device's VRF membership. Detaching the device from its VRF between the initial validation and the L3-master calculation in tcp ao connect init() can therefore make the validation succeed while initialization observes the default L3 domain and removes the only key. The subsequent AO lookup then fails, so the no-key path clears tp->ao info and frees it directly.

The receive path can find the socket in the ehash and load tp->ao info under RCU before acquiring the socket lock. A reader which loaded the old pointer can thus continue into tcp inbound ao hash() after the direct free.

The issue was found during a static audit of TCP-AO object lifetime. An unprivileged reproducer in self-created user and network namespaces raced connect() with detaching a veth from its VRF while sending TCP-AO segments. It triggered the same KASAN report on two fresh boots:

BUG: KASAN: slab-use-after-free in tcp inbound ao hash+0x585/0x19f0 Write of size 8 at addr ffff88800bf88128 by task tcp ao vrf race/232

Call Trace: tcp inbound ao hash+0x585/0x19f0 tcp inbound hash+0x677/0xa80 tcp v4 rcv+0x1c3e/0x3ab0

Allocated by task 235: tcp ao alloc info+0x43/0xf0 tcp ao add cmd+0xdf7/0x13b0 do tcp setsockopt+0x168c/0x2640

Freed by task 235: kfree+0x1b8/0x550 tcp connect+0x252/0x4f00 tcp v4 connect+0x1114/0x1720

The bad address is 40 bytes inside the freed 128-byte object, matching the tcp ao info counters.key not found field. The two runs used 1000 attempts each, reached the no-key path 366 and 411 times, and produced one and two KASAN reports respectively. With this change, the same reproducer reached the no-key path 366 times in 1000 attempts without a KASAN report or oops.

Use tcp ao destroy sock() for the no-key path. It unpublishes the AO info, updates the socket memory and static-key accounting, and defers the free until after an RCU grace period.

Also drop the WARN ON ONCE() and its stale comment. The VRF detach race makes the no-key state reachable during normal operation, so it is a handled condition rather than an impossible assertion. On panic on warn kernels the WARN would turn this handled race into a kernel panic.