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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Simon Horman Subject: [PATCH net v4 1/5] amt: fix use-after-free when a Membership Update re-arms a freed tunnel Date: Tue, 13 Oct 2026 17:54:03 +0000 Message-ID: <20261013175407.947606-2-omar@blockcast.net> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20261013175407.947606-1-omar@blockcast.net> References: <20261013175407.947606-1-omar@blockcast.net> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit amt_update_handler() refreshes a relay tunnel's garbage-collection timer with mod_delayed_work(&tunnel->gc_wq, ...) on a tunnel found by a lockless list_for_each_entry_rcu() walk, holding only rcu_read_lock(). Nothing serialises this against amt_tunnel_expire(), which runs from that same gc_wq: once expire has taken amt->lock, done list_del_rcu() and queued kfree_rcu(), a concurrent Update that already cached the tunnel pointer re-arms gc_wq on an object that is about to be freed. After the grace period the delayed_work timer is left armed in freed memory, so it fires into a use-after-free (and double-list_del_rcu/nr_tunnels underflow on the way). Make amt_tunnel_expire() the only context that frees a tunnel on the timeout path and let it re-arm itself: the Update path only stamps tunnel->last_seen, and expire re-reads last_seen under amt->lock to decide whether to re-arm or free. The Update path therefore never touches gc_wq, so it can no longer re-queue the work of a tunnel being torn down. amt_dev_stop() is the other freer, and it walked tunnel_list without amt->lock while a running gc_wq could free the next entry under it. Mark every tunnel dead under amt->lock first, so a gc_wq that is already running sees ->dead and bails instead of freeing in parallel; then unlink the tunnels one at a time with list_del_rcu() under amt->lock, and cancel, clear and free each one outside it. The tunnel is freed exactly once, and because readers walk tunnel_list locklessly under RCU, the entries are unlinked in place rather than spliced onto a local list head. Fixes: cbc21dc1cfe9 ("amt: add data plane of amt interface") Assisted-by: LLM Signed-off-by: Omar Ramadan --- drivers/net/amt.c | 63 +++++++++++++++++++++++++++++++++++++++++------ include/net/amt.h | 11 +++++++++ 2 files changed, 67 insertions(+), 7 deletions(-) diff --git a/drivers/net/amt.c b/drivers/net/amt.c index b53f8ec..530650d 100644 --- a/drivers/net/amt.c +++ b/drivers/net/amt.c @@ -1328,8 +1328,30 @@ static void amt_tunnel_expire(struct work_struct *work) struct amt_tunnel_list, gc_wq); struct amt_dev *amt = tunnel->amt; + unsigned long deadline; spin_lock_bh(&amt->lock); + /* amt_dev_stop() has taken ownership of the teardown. */ + if (tunnel->dead) { + spin_unlock_bh(&amt->lock); + return; + } + /* A Membership Update may have refreshed the tunnel after this work + * was queued. Re-arm from last_seen rather than letting + * amt_update_handler() re-arm gc_wq: this handler is then the only + * context that frees the tunnel (and the only one amt_dev_stop() has + * to cancel), which closes the use-after-free where a concurrent + * Update re-armed gc_wq on a tunnel this function had already + * list_del_rcu()'d and kfree_rcu()'d. + */ + deadline = READ_ONCE(tunnel->last_seen) + + msecs_to_jiffies(amt_gmi(amt)) * 3; + if (time_before(jiffies, deadline)) { + mod_delayed_work(amt_wq, &tunnel->gc_wq, deadline - jiffies); + spin_unlock_bh(&amt->lock); + return; + } + tunnel->dead = true; rcu_read_lock(); list_del_rcu(&tunnel->list); amt->nr_tunnels--; @@ -2494,9 +2516,13 @@ static bool amt_update_handler(struct amt_dev *amt, struct sk_buff *skb) if (tunnel->ip4 == saddr) { if ((nonce == tunnel->nonce && response_mac == tunnel->mac)) { - mod_delayed_work(amt_wq, &tunnel->gc_wq, - msecs_to_jiffies(amt_gmi(amt)) - * 3); + /* Refresh liveness only. gc_wq is + * re-armed by amt_tunnel_expire() itself + * from last_seen, never from here, so this + * cannot re-queue the work of a tunnel + * that is being freed. + */ + WRITE_ONCE(tunnel->last_seen, jiffies); goto report; } else { netdev_dbg(amt->dev, "Invalid MAC\n"); @@ -2732,6 +2758,7 @@ static bool amt_request_handler(struct amt_dev *amt, struct sk_buff *skb) for (i = 0; i < amt->hash_buckets; i++) INIT_HLIST_HEAD(&tunnel->groups[i]); + tunnel->last_seen = jiffies; INIT_DELAYED_WORK(&tunnel->gc_wq, amt_tunnel_expire); list_add_tail_rcu(&tunnel->list, &amt->tunnel_list); @@ -3050,7 +3077,7 @@ static int amt_dev_open(struct net_device *dev) static int amt_dev_stop(struct net_device *dev) { struct amt_dev *amt = netdev_priv(dev); - struct amt_tunnel_list *tunnel, *tmp; + struct amt_tunnel_list *tunnel; struct sk_buff *skb; struct sock *sk; int i; @@ -3079,9 +3106,31 @@ static int amt_dev_stop(struct net_device *dev) amt->req_cnt = 0; WRITE_ONCE(amt->remote_ip, 0); - list_for_each_entry_safe(tunnel, tmp, &amt->tunnel_list, list) { - list_del_rcu(&tunnel->list); - amt->nr_tunnels--; + /* Mark every tunnel dead first, so a concurrent amt_tunnel_expire() + * sees ->dead and bails instead of freeing a tunnel in parallel; this + * path is then the sole freer. + */ + spin_lock_bh(&amt->lock); + list_for_each_entry(tunnel, &amt->tunnel_list, list) + tunnel->dead = true; + spin_unlock_bh(&amt->lock); + + /* Unlink one tunnel at a time with list_del_rcu() rather than splicing + * the list onto a local head: a lockless reader walking tunnel_list + * must always terminate at &amt->tunnel_list, never at a stack head. + */ + for (;;) { + spin_lock_bh(&amt->lock); + tunnel = list_first_entry_or_null(&amt->tunnel_list, + struct amt_tunnel_list, list); + if (tunnel) { + list_del_rcu(&tunnel->list); + amt->nr_tunnels--; + } + spin_unlock_bh(&amt->lock); + if (!tunnel) + break; + cancel_delayed_work_sync(&tunnel->gc_wq); amt_clear_groups(tunnel); kfree_rcu(tunnel, rcu); diff --git a/include/net/amt.h b/include/net/amt.h index 2846dde..0520121 100644 --- a/include/net/amt.h +++ b/include/net/amt.h @@ -240,6 +240,17 @@ struct amt_tunnel_list { u32 nr_sources; enum amt_status status; struct delayed_work gc_wq; + /* Last time a valid Membership Update refreshed this tunnel. gc_wq + * re-arms itself from this rather than being re-armed by the Update + * path, so the Update path cannot re-queue the work of a tunnel that + * amt_tunnel_expire() is freeing. + */ + unsigned long last_seen; + /* Set under amt->lock by whichever of amt_tunnel_expire() or + * amt_dev_stop() reaches the tunnel first; the other then leaves it + * alone, so the tunnel is freed exactly once. + */ + bool dead; __be16 source_port; __be32 ip4; __be32 nonce;