CVE-2026-74727: ovpn: skip rehash for peers already removed from by_id

From: Greg Kroah-Hartman <[email protected]>

Description
===========

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

ovpn: skip rehash for peers already removed from by_id

ovpn_nl_peer_set_doit() resolves the target peer via
ovpn_peer_get_by_id() before taking ovpn->lock. In the window between
the lookup (which only takes a refcount) and the subsequent
spin_lock_bh(&ovpn->lock), a concurrent OVPN_CMD_PEER_DEL, keepalive
expiry, or socket teardown can take ovpn->lock first, run
ovpn_peer_remove() to unhash the peer from all four tables (by_id,
by_vpn_addr4/6, by_transp_addr) and release the lock. set_doit then
acquires ovpn->lock and calls ovpn_peer_hash_vpn_ip(), which
re-inserts the now-removed peer back into the rehashing tables.

The same race affects the float path: ovpn_peer_endpoints_update()
holds only a refcount and acquires ovpn->lock very late (after async
AEAD decrypt and a netlink notification), then rehashes the peer
in the by_transp_addr table.

The resurrected peer becomes reachable again from the RX lookup
(ovpn_peer_get_by_transp_addr) and the TX VPN-IP lookup, even though
userspace believes it is gone. Once the data-path refcount drops the
peer is freed via call_rcu while the hash entries embedded in it
remain linked, opening a UAF window.

Bail out of the rehash when hash_entry_id is unhashed, mirroring
the sentinel already used by ovpn_peer_remove() to detect the
already-removed state. The check is safe under ovpn->lock, which
serializes every mutation of hash_entry_id, and is a no-op for the
add path because ovpn_peer_add_mp() inserts hash_entry_id before
calling ovpn_peer_hash_vpn_ip().

The Linux kernel CVE team has assigned CVE-2026-74727 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.16 with commit 1d36a36f6d5347360ef9681a05f6166683bafd1d and fixed in 6.18.45 with commit d20c181088984b6eaa8d7fe7cb5ab3510988df59
	Issue introduced in 6.16 with commit 1d36a36f6d5347360ef9681a05f6166683bafd1d and fixed in 7.1.9 with commit 66745480298775f188b2f5ad266643e85a90f73b
	Issue introduced in 6.16 with commit 1d36a36f6d5347360ef9681a05f6166683bafd1d and fixed in 7.2 with commit 33ec10567fe14456063daf549fdf1a4f53448e4c

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-74727
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/net/ovpn/peer.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/d20c181088984b6eaa8d7fe7cb5ab3510988df59
	https://git.kernel.org/stable/c/66745480298775f188b2f5ad266643e85a90f73b
	https://git.kernel.org/stable/c/33ec10567fe14456063daf549fdf1a4f53448e4c