From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from localhost.localdomain (unknown [147.136.157.0]) (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 A41592F7AB1; Tue, 23 Dec 2025 05:14:22 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=147.136.157.0 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766466864; cv=none; b=ViEvKIOP99TYCV5lldhUaj47+PxmoCNJyyelrUi2sJjbOE2GM1Njfh4Xxy0qECMxV6sdJy36CAv2/4CWp657fv/hXDLp4+HKyw38uOLQAVCas5ZAzXglhGXKa3Y2t6yUDKl6AdzNwy/0rS7Jk5ot2fjOiAJIO+wsP31LJBJUjBA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766466864; c=relaxed/simple; bh=ehR8ygWgcSZgp7QcRdkbVgjM7oRsnj7ikVKj0+gVjIU=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=Guwt4Z0OaZ9iUn/YUHJ3DdVKdVQs1DMMcy9P93GBMeh3VCdu2iGW/n5/tjZzPB+X5IOhO1qXyPj1rAClyophQG5CI7bid2xsndsb2ahKmx5VPAJlgY9gErV4LgOSCJdHwbNYDjhZqKo5lnl+g4WFrHuo/7AgnEVInYAztTNGijk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.dev; spf=none smtp.mailfrom=localhost.localdomain; arc=none smtp.client-ip=147.136.157.0 Authentication-Results: smtp.subspace.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=localhost.localdomain Received: by localhost.localdomain (Postfix, from userid 1007) id BFEF88AA382; Tue, 23 Dec 2025 13:14:20 +0800 (+08) From: Jiayuan Chen To: netdev@vger.kernel.org, jiayuan.chen@linux.dev Cc: syzbot+9b35e9bc0951140d13e6@syzkaller.appspotmail.com, "David S. Miller" , David Ahern , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev Subject: [PATCH net v4] ipv6: fix a BUG in rt6_get_pcpu_route() under PREEMPT_RT Date: Tue, 23 Dec 2025 13:14:12 +0800 Message-ID: <20251223051413.124687-1-jiayuan.chen@linux.dev> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On PREEMPT_RT kernels, after rt6_get_pcpu_route() returns NULL, the current task can be preempted. Another task running on the same CPU may then execute rt6_make_pcpu_route() and successfully install a pcpu_rt entry. When the first task resumes execution, its cmpxchg() in rt6_make_pcpu_route() will fail because rt6i_pcpu is no longer NULL, triggering the BUG_ON(prev). It's easy to reproduce it by adding mdelay() after rt6_get_pcpu_route(). Using preempt_disable/enable is not appropriate here because ip6_rt_pcpu_alloc() may sleep. Fix this by handling the cmpxchg() failure gracefully on PREEMPT_RT: free our allocation and return the existing pcpu_rt installed by another task. The BUG_ON is replaced by WARN_ON_ONCE for non-PREEMPT_RT kernels where such races should not occur. Link: https://syzkaller.appspot.com/bug?extid=9b35e9bc0951140d13e6 Fixes: d2d6422f8bd1 ("x86: Allow to enable PREEMPT_RT.") Reported-by: syzbot+9b35e9bc0951140d13e6@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/6918cd88.050a0220.1c914e.0045.GAE@google.com/T/ Signed-off-by: Jiayuan Chen --- v2 -> v4: Use correct Fixes tag; Replace BUG_ON by WARN_ON_ONCE. https://lore.kernel.org/netdev/20251219025140.77695-1-jiayuan.chen@linux.dev/ v1 -> v2: Drop migrate_{disable, enabled} suggested by Steven and Paolo. https://lore.kernel.org/all/20251209124805.379112-1-jiayuan.chen@linux.dev/T/ --- net/ipv6/route.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) diff --git a/net/ipv6/route.c b/net/ipv6/route.c index aee6a10b112a..a3e051dc66ee 100644 --- a/net/ipv6/route.c +++ b/net/ipv6/route.c @@ -1470,7 +1470,18 @@ static struct rt6_info *rt6_make_pcpu_route(struct net *net, p = this_cpu_ptr(res->nh->rt6i_pcpu); prev = cmpxchg(p, NULL, pcpu_rt); - BUG_ON(prev); + if (unlikely(prev)) { + /* + * Another task on this CPU already installed a pcpu_rt. + * This can happen on PREEMPT_RT where preemption is possible. + * Free our allocation and return the existing one. + */ + WARN_ON_ONCE(!IS_ENABLED(CONFIG_PREEMPT_RT)); + + dst_dev_put(&pcpu_rt->dst); + dst_release(&pcpu_rt->dst); + return prev; + } if (res->f6i->fib6_destroying) { struct fib6_info *from; -- 2.43.0