From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-186.mta0.migadu.com (out-186.mta0.migadu.com [91.218.175.186]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 17FDF3ACEF6 for ; Wed, 25 Feb 2026 12:15:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.186 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772021723; cv=none; b=lHqNW6jOdvH77nXcifOLaPdSxVZH/Q0EQmG4MDpt05hmGXln31SPKHCe5m/LYyHbDysGdvByGbm6WhnyTCdb3ESXHUBD4DsWqSCdd6wh2OSVb3jzks1BCMeqGOGiQQFzsjrsW6EDdDyGBt8J+I3ObGF3kKq/2qwABhrH2bq+TS0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772021723; c=relaxed/simple; bh=SoIASlQHQkl2l2grY9dFi8gICeiVCKREbAsjLqwKoCw=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=AYDRArKwJWZminGV3/SbzvR0QDeCoTJhG8A6tjhSTPd6L2q3LZPoSkmJUKxLD9BpWxnVwqAIxXf3uCmjY2i4NDbYCusORRvS/L0ckbbjWsOJynvlFFXXMhM61hHOne+VWHYw7gAEKTKK6v7/F7Pscq1jWcf2EyolpcsRLHp8sRY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=m3eC2bCW; arc=none smtp.client-ip=91.218.175.186 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="m3eC2bCW" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1772021719; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=CSBZzSEqUJvM4KWzWVEwXGx1jyBYabVLcqc6HiuSlLA=; b=m3eC2bCWO6AO1U3jzWKMT3OOQzW83GzyuMmktszlvw+E43I1R/1KIGM73H9DC2K02Si/jE muAL3KY0eeTOvMKYX1V6z10ZmqjOdV96yjD9lpR4Aq8YVXiZuS5f9jZnHd0XNPzn3umM3x XV2GaBSkQhnjDadJFjqp9Viq+fAss5E= From: Jiayuan Chen To: bpf@vger.kernel.org Cc: jiayuan.chen@linux.dev, Jiayuan Chen , syzbot+2b3391f44313b3983e91@syzkaller.appspotmail.com, Sebastian Andrzej Siewior , Alexei Starovoitov , Daniel Borkmann , "David S. Miller" , Jakub Kicinski , Jesper Dangaard Brouer , John Fastabend , Stanislav Fomichev , Andrii Nakryiko , Martin KaFai Lau , Eduard Zingerman , Song Liu , Yonghong Song , KP Singh , Hao Luo , Jiri Olsa , Clark Williams , Steven Rostedt , Thomas Gleixner , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev Subject: [PATCH bpf v4 1/2] bpf: cpumap: fix race in bq_flush_to_queue on PREEMPT_RT Date: Wed, 25 Feb 2026 20:14:55 +0800 Message-ID: <20260225121459.183121-2-jiayuan.chen@linux.dev> In-Reply-To: <20260225121459.183121-1-jiayuan.chen@linux.dev> References: <20260225121459.183121-1-jiayuan.chen@linux.dev> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT From: Jiayuan Chen On PREEMPT_RT kernels, the per-CPU xdp_bulk_queue (bq) can be accessed concurrently by multiple preemptible tasks on the same CPU. The original code assumes bq_enqueue() and __cpu_map_flush() run atomically with respect to each other on the same CPU, relying on local_bh_disable() to prevent preemption. However, on PREEMPT_RT, local_bh_disable() only calls migrate_disable() (when PREEMPT_RT_NEEDS_BH_LOCK is not set) and does not disable preemption, which allows CFS scheduling to preempt a task during bq_flush_to_queue(), enabling another task on the same CPU to enter bq_enqueue() and operate on the same per-CPU bq concurrently. This leads to several races: 1. Double __list_del_clearprev(): after bq->count is reset in bq_flush_to_queue(), a preempting task can call bq_enqueue() -> bq_flush_to_queue() on the same bq when bq->count reaches CPU_MAP_BULK_SIZE. Both tasks then call __list_del_clearprev() on the same bq->flush_node, the second call dereferences the prev pointer that was already set to NULL by the first. 2. bq->count and bq->q[] races: concurrent bq_enqueue() can corrupt the packet queue while bq_flush_to_queue() is processing it. The race between task A (__cpu_map_flush -> bq_flush_to_queue) and task B (bq_enqueue -> bq_flush_to_queue) on the same CPU: Task A (xdp_do_flush) Task B (cpu_map_enqueue) ---------------------- ------------------------ bq_flush_to_queue(bq) spin_lock(&q->producer_lock) /* flush bq->q[] to ptr_ring */ bq->count = 0 spin_unlock(&q->producer_lock) bq_enqueue(rcpu, xdpf) <-- CFS preempts Task A --> bq->q[bq->count++] = xdpf /* ... more enqueues until full ... */ bq_flush_to_queue(bq) spin_lock(&q->producer_lock) /* flush to ptr_ring */ spin_unlock(&q->producer_lock) __list_del_clearprev(flush_node) /* sets flush_node.prev = NULL */ <-- Task A resumes --> __list_del_clearprev(flush_node) flush_node.prev->next = ... /* prev is NULL -> kernel oops */ Fix this by adding a local_lock_t to xdp_bulk_queue and acquiring it in bq_enqueue() and __cpu_map_flush(). These paths already run under local_bh_disable(), so use local_lock_nested_bh() which on non-RT is a pure annotation with no overhead, and on PREEMPT_RT provides a per-CPU sleeping lock that serializes access to the bq. To reproduce, insert an mdelay(100) between bq->count = 0 and __list_del_clearprev() in bq_flush_to_queue(), then run reproducer provided by syzkaller. Fixes: 3253cb49cbad ("softirq: Allow to drop the softirq-BKL lock on PREEMPT_RT") Reported-by: syzbot+2b3391f44313b3983e91@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/69369331.a70a0220.38f243.009d.GAE@google.com/T/ Reviewed-by: Sebastian Andrzej Siewior Signed-off-by: Jiayuan Chen Signed-off-by: Jiayuan Chen --- kernel/bpf/cpumap.c | 17 +++++++++++++++-- 1 file changed, 15 insertions(+), 2 deletions(-) diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c index 04171fbc39cb..32b43cb9061b 100644 --- a/kernel/bpf/cpumap.c +++ b/kernel/bpf/cpumap.c @@ -29,6 +29,7 @@ #include #include #include +#include #include #include #include @@ -52,6 +53,7 @@ struct xdp_bulk_queue { struct list_head flush_node; struct bpf_cpu_map_entry *obj; unsigned int count; + local_lock_t bq_lock; }; /* Struct for every remote "destination" CPU in map */ @@ -451,6 +453,7 @@ __cpu_map_entry_alloc(struct bpf_map *map, struct bpf_cpumap_val *value, for_each_possible_cpu(i) { bq = per_cpu_ptr(rcpu->bulkq, i); bq->obj = rcpu; + local_lock_init(&bq->bq_lock); } /* Alloc queue */ @@ -722,6 +725,8 @@ static void bq_flush_to_queue(struct xdp_bulk_queue *bq) struct ptr_ring *q; int i; + lockdep_assert_held(&bq->bq_lock); + if (unlikely(!bq->count)) return; @@ -749,11 +754,15 @@ static void bq_flush_to_queue(struct xdp_bulk_queue *bq) } /* Runs under RCU-read-side, plus in softirq under NAPI protection. - * Thus, safe percpu variable access. + * Thus, safe percpu variable access. PREEMPT_RT relies on + * local_lock_nested_bh() to serialise access to the per-CPU bq. */ static void bq_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_frame *xdpf) { - struct xdp_bulk_queue *bq = this_cpu_ptr(rcpu->bulkq); + struct xdp_bulk_queue *bq; + + local_lock_nested_bh(&rcpu->bulkq->bq_lock); + bq = this_cpu_ptr(rcpu->bulkq); if (unlikely(bq->count == CPU_MAP_BULK_SIZE)) bq_flush_to_queue(bq); @@ -774,6 +783,8 @@ static void bq_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_frame *xdpf) list_add(&bq->flush_node, flush_list); } + + local_unlock_nested_bh(&rcpu->bulkq->bq_lock); } int cpu_map_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_frame *xdpf, @@ -810,7 +821,9 @@ void __cpu_map_flush(struct list_head *flush_list) struct xdp_bulk_queue *bq, *tmp; list_for_each_entry_safe(bq, tmp, flush_list, flush_node) { + local_lock_nested_bh(&bq->obj->bulkq->bq_lock); bq_flush_to_queue(bq); + local_unlock_nested_bh(&bq->obj->bulkq->bq_lock); /* If already running, costs spin_lock_irqsave + smb_mb */ wake_up_process(bq->obj->kthread); -- 2.43.0