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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , netdev@vger.kernel.org Subject: [PATCH] net: fix fanout UAF in packet_release() via NETDEV_UP race Date: Thu, 19 Mar 2026 22:06:10 +0200 Message-ID: <20260319200610.25101-1-echelonh@gmail.com> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit `packet_release()` has a race window where `NETDEV_UP` can re-register a socket into a fanout group's `arr[]` array. The re-registration is not cleaned up by `fanout_release()`, leaving a dangling pointer in the fanout array. `packet_release()` does NOT zero `po->num` in its `bind_lock` section. After releasing `bind_lock`, `po->num` is still non-zero and `po->ifindex` still matches the bound device. A concurrent `packet_notifier(NETDEV_UP)` that already found the socket in `sklist` can re-register the hook. For fanout sockets, this re-registration calls `__fanout_link(sk, po)` which adds the socket back into `f->arr[]` and increments `f->num_members`, but does NOT increment `f->sk_ref`. The fix sets `po->num` to zero in `packet_release` while `bind_lock` is held to prevent NETDEV_UP from linking, preventing the race window. This bug was found following an additional audit with Claude Code based on CVE-2025-38617. Link: https://blog.calif.io/p/a-race-within-a-race-exploiting-cve Fixes: ce06b03e60fc ("packet: Add helpers to register/unregister ->prot_hook") Signed-off-by: Yochai Eisenrich --- net/packet/af_packet.c | 1 + 1 file changed, 1 insertion(+) diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c index 72d0935139f0..bb2d88205e5a 100644 --- a/net/packet/af_packet.c +++ b/net/packet/af_packet.c @@ -3135,6 +3135,7 @@ static int packet_release(struct socket *sock) spin_lock(&po->bind_lock); unregister_prot_hook(sk, false); + WRITE_ONCE(po->num, 0); packet_cached_dev_reset(po); if (po->prot_hook.dev) { -- 2.53.0