From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 0F9622C3770; Tue, 6 Jan 2026 17:32:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767720744; cv=none; b=Ixo2eBYzwOBbf6O00n0J4+UoQ6sDn57QPt1z4fRc1O6wZaPeTS+aa/wcjJl7/BKrvolV0gDX5+nXz8R+FmDEpblGN3JXrfKkQMVxbz12MOzIGWTynyDPwlI+iyKKzNGRUqpoBIUxROIibXVU+nCm6HGzVUNGLoLxtQxheVKm3ag= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767720744; c=relaxed/simple; bh=C1ePl8ydEQqWgoRvTjenL2Wb58ob1Yyq/e1yTFOzBpo=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=LP+9HFln6NQ1bcv1FzvwC/tzM8NoRtD256f7E4zes8Au2QCFwnOdwkzXPs27glYsnDaP1eMy+PNooYLq86ftouEzYzs0toHT3IsQzKQ6J82T+nTV97WCx6/LkQxiVvfgEgJN6gx/KAzy5L48BxbJLOTSIxdIPE5m/IjTMmTwxSI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=UfY/f+Hk; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="UfY/f+Hk" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 195D5C116C6; Tue, 6 Jan 2026 17:32:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1767720743; bh=C1ePl8ydEQqWgoRvTjenL2Wb58ob1Yyq/e1yTFOzBpo=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=UfY/f+HkO6HuvoNmiUoR6HdDPGP6aVMwTZ2RDCBJoJMWunuqXYOquywXJLgGV1DSx ZxnvRZmPGn3STeOlKkoUXbR7Z2ICMILL3lC2sv/ccI1hOtDe27mCE6IOSfjJztT5fG GomBLBvldbXIbHjxivcbGyq8x1FQVUmjUJ+HaC+0= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, syzbot+9b35e9bc0951140d13e6@syzkaller.appspotmail.com, Jiayuan Chen , Paolo Abeni , Sasha Levin Subject: [PATCH 6.12 329/567] ipv6: fix a BUG in rt6_get_pcpu_route() under PREEMPT_RT Date: Tue, 6 Jan 2026 18:01:51 +0100 Message-ID: <20260106170503.505024650@linuxfoundation.org> X-Mailer: git-send-email 2.52.0 In-Reply-To: <20260106170451.332875001@linuxfoundation.org> References: <20260106170451.332875001@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.12-stable review patch. If anyone has any objections, please let me know. ------------------ From: Jiayuan Chen [ Upstream commit 1adaea51c61b52e24e7ab38f7d3eba023b2d050d ] 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 Link: https://patch.msgid.link/20251223051413.124687-1-jiayuan.chen@linux.dev Signed-off-by: Paolo Abeni Signed-off-by: Sasha Levin --- 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 22866444efc0..276fa74af206 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; -- 2.51.0