From 214ddb05df3990c38e380df1cff33248306ab447 Mon Sep 17 00:00:00 2001 From: Yochai Eisenrich Date: Thu, 19 Mar 2026 22:06:10 +0200 Subject: [PATCH] net: fix fanout UAF in packet_release() via NETDEV_UP race [ Upstream commit 42156f93d123436f2a27c468f18c966b7e5db796 ] `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. Fixes: ce06b03e60fc ("packet: Add helpers to register/unregister ->prot_hook") Link: https://blog.calif.io/p/a-race-within-a-race-exploiting-cve Signed-off-by: Yochai Eisenrich Reviewed-by: Willem de Bruijn Link: https://patch.msgid.link/20260319200610.25101-1-echelonh@gmail.com Signed-off-by: Jakub Kicinski Signed-off-by: Sasha Levin (cherry picked from commit ee642b1962caa9aa231c01abbd58bc453ae6b66e) Orabug: 39250953 CVE: CVE-2026-31504 Change-Id: I002fefc6bd0bbe09ed5e2ad13009c783609ec04a Signed-off-by: Alok Tiwari Signed-off-by: Tashfin Shakeer Rhythm --- 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 f7e02f9f9ed1..9c6cc1cf547c 100644 --- a/net/packet/af_packet.c +++ b/net/packet/af_packet.c @@ -3117,6 +3117,7 @@ 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) {