From nobody Fri Dec 19 09:42:06 2025 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 032FB481DD; Fri, 19 Dec 2025 03:47:15 +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=1766116037; cv=none; b=YNcLUbSYBY3rJJBD/yPpuk/9/22gQKtpu15Mo70t8LGbqP/HcQ+uO7tJiCAafu2zfDuGT06KTpH3G3dU5K8BZNZm0FTTMziXLh6pQsxREMa9U80Qqb0eH++k50kfMc2msXs3MjjXMj2urRkJpTIvCWoZQqzNjaZd+e3oleGp36Q= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766116037; c=relaxed/simple; bh=wWx04aQ6g06P4aDR+O3WO/Y3zLXaoh0Cs00LSg9xp4c=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=UjL+pSLMwMVQ11nsJp0NTTp2kh/1XD+34PhXj9IJOxGHJOX0zWgeXo1okzT86+qTTZiLa+Bhkj+73nrH+X0efNuOz2sKtktQQUYMJQ6coiEmqF/lpHS4G9C3g51ryHoUrZA9f/nqpVoghSQBawryIN8PffM3HhyH9I9q6mN/n0U= 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 0DC778B2A3F; Fri, 19 Dec 2025 10:52:02 +0800 (+08) From: Jiayuan Chen To: netdev@vger.kernel.org Cc: Jiayuan Chen , 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 , linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev Subject: [PATCH net v2] ipv6: fix a BUG in rt6_get_pcpu_route() under PREEMPT_RT Date: Fri, 19 Dec 2025 10:51:39 +0800 Message-ID: <20251219025140.77695-1-jiayuan.chen@linux.dev> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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: 1. Removing the BUG_ON and instead handling the race gracefully by freeing our allocation and returning the existing pcpu_rt when cmpxchg() fails. 2. Keeping the BUG_ON for non-PREEMPT_RT kernels, since preemption should not occur in this context and a cmpxchg failure would indicate a real bug. Link: https://syzkaller.appspot.com/bug?extid=3D9b35e9bc0951140d13e6 Fixes: 951f788a80ff ("ipv6: fix a BUG in rt6_get_pcpu_route()") Reported-by: syzbot+9b35e9bc0951140d13e6@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/6918cd88.050a0220.1c914e.0045.GAE@googl= e.com/T/ Signed-off-by: Jiayuan Chen --- 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..9e7afda7cba2 100644 --- a/net/ipv6/route.c +++ b/net/ipv6/route.c @@ -1470,7 +1470,18 @@ static struct rt6_info *rt6_make_pcpu_route(struct n= et *net, =20 p =3D this_cpu_ptr(res->nh->rt6i_pcpu); prev =3D 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. + */ + BUG_ON(!IS_ENABLED(CONFIG_PREEMPT_RT)); + + dst_dev_put(&pcpu_rt->dst); + dst_release(&pcpu_rt->dst); + return prev; + } =20 if (res->f6i->fib6_destroying) { struct fib6_info *from; --=20 2.43.0