From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-171.mta0.migadu.com (out-171.mta0.migadu.com [91.218.175.171]) (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 4DFBC3B38B5; Thu, 26 Mar 2026 07:59:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.171 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774511959; cv=none; b=TMgWzorttxm7uThzslp/B2E9rrJkXB99N0kKItzeZYVRnAmBiE4QSEs2xl0dqHI5BoMFqb+ReZZHNRIFeIMvzlVgBKpMh2ASDZELBo7CF5hKZ1gM76VfuA34xMkTuTLIq8ZX15pA98k+vDAezGvz/s0K6QQ0Ni7LH6L6U36OjdU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774511959; c=relaxed/simple; bh=vO8PIpxJovXOwq92AzoKj+UrdmM7MxtrDETvJjpsmvs=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=WQeAuHNif4PhR6jMjPs5mcdgbetPBsu1dJqYXjmEkmLlcCTNfkZ1gqu/PaVhRlPBJnksPtN4IUaedioz7nKbsfnawsAxbsb+FJPkSgDy662CeLcF+EUp9NHV3HOrm8bc1n+IMgvXEcPjB46OzyoSTJGW5prIGWpo114kcGZ1cu4= 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=E/5bo7P6; arc=none smtp.client-ip=91.218.175.171 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="E/5bo7P6" Message-ID: <5ba172d5-5551-40fb-a3d8-521c471a334d@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1774511955; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=34Zx/NqkbD/+leDsZuDXNlhFwVlMr8IpFSvBRz0tHhQ=; b=E/5bo7P6e5pgPv3sAS2MuZHLCiADlWMvV6uDpbA/Ad/S47UDicT7OOA1xEUQY7HKend4bM f+3DP6eO6gU7g02ZMMSti/U05BwFzjy2QJtV0+th7jitylV7uAjpsGwbkcwrS8RErMOTze Lv2mFdDRn5g3GxSq4xv1+C2qm4OxcTs= Date: Thu, 26 Mar 2026 15:59:04 +0800 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH net] ipv6: fix data race in fib6_metric_set() using cmpxchg To: Hangbin Liu Cc: "David S. Miller" , David Ahern , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , David Ahern , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Fei Liu References: <20260326-b4-fib6_metric_set-kmemleak-v1-1-c89fc1b312c0@gmail.com> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Jiayuan Chen In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 3/26/26 3:13 PM, Hangbin Liu wrote: > On Thu, Mar 26, 2026 at 06:44:43AM +0000, Hangbin Liu wrote: >> On Thu, Mar 26, 2026 at 02:23:15PM +0800, Jiayuan Chen wrote: >>> On 3/26/26 12:22 PM, Hangbin Liu wrote: >>>> fib6_metric_set() may be called concurrently from softirq context without >>>> holding the FIB table lock. A typical path is: >>>> >>>> ndisc_router_discovery() >>>> spin_unlock_bh(&table->tb6_lock) <- lock released >>>> fib6_metric_set(rt, RTAX_HOPLIMIT, ...) <- lockless call >>>> >>>> When two CPUs process Router Advertisement packets for the same router >>>> simultaneously, they can both arrive at fib6_metric_set() with the same >>>> fib6_info pointer whose fib6_metrics still points to dst_default_metrics. >>>> >>>> if (f6i->fib6_metrics == &dst_default_metrics) { /* both CPUs: true */ >>>> struct dst_metrics *p = kzalloc_obj(*p, GFP_ATOMIC); >>>> refcount_set(&p->refcnt, 1); >>>> f6i->fib6_metrics = p; /* CPU1 overwrites CPU0's p -> p0 leaked */ >>>> } >>>> >>>> The dst_metrics allocated by the losing CPU has refcnt=1 but no pointer >>>> to it anywhere in memory, producing a kmemleak report: >>>> >>>> unreferenced object 0xff1100025aca1400 (size 96): >>>> comm "softirq", pid 0, jiffies 4299271239 >>>> backtrace: >>>> kmalloc_trace+0x28a/0x380 >>>> fib6_metric_set+0xcd/0x180 >>>> ndisc_router_discovery+0x12dc/0x24b0 >>>> icmpv6_rcv+0xc16/0x1360 >>>> >>>> Fix this by replacing the plain pointer store with cmpxchg() and free >>>> the allocation safely when competition failed. >>>> >>>> Fixes: d4ead6b34b67 ("net/ipv6: move metrics from dst to rt6_info") >>>> Reported-by: Fei Liu >>>> Signed-off-by: Hangbin Liu >>>> --- >>>> net/ipv6/ip6_fib.c | 6 ++++-- >>>> 1 file changed, 4 insertions(+), 2 deletions(-) >>>> >>>> diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c >>>> index dd26657b6a4a..64de761f40d5 100644 >>>> --- a/net/ipv6/ip6_fib.c >>>> +++ b/net/ipv6/ip6_fib.c >>>> @@ -730,14 +730,16 @@ void fib6_metric_set(struct fib6_info *f6i, int metric, u32 val) >>>> if (!f6i) >>>> return; >>>> - if (f6i->fib6_metrics == &dst_default_metrics) { >>>> + if (READ_ONCE(f6i->fib6_metrics) == &dst_default_metrics) { >>>> + struct dst_metrics *dflt = (struct dst_metrics *)&dst_default_metrics; >>>> struct dst_metrics *p = kzalloc_obj(*p, GFP_ATOMIC); >>>> if (!p) >>>> return; >>>> refcount_set(&p->refcnt, 1); >>>> - f6i->fib6_metrics = p; >>>> + if (cmpxchg(&f6i->fib6_metrics, dflt, p) != dflt) >>>> + kfree(p); >>>> } >>> >>> [...] >>> >>>> f6i->fib6_metrics->metrics[metric - 1] = val; >>> Suggest using marked accessors to suppress KCSAN warnings: >>> >>> struct dst_metrics *m = READ_ONCE(f6i->fib6_metrics); >>> WRITE_ONCE(m->metrics[metric - 1], val); >> Thanks, I will update this in next version. > BTW, do we really need to WRITE_ONCE here? What if the `val` are different > on 2 CPUs? This would hide the problem, right? > > Thanks > Hangbin If concurrent writes to the same memory by two CPUs are considered a bug, the solution is prevention, not detection by KCSAN. Furthermore, WRITE_ONCE also helps suppress warnings from read-write races. For example, commit 6e84fc395e90 demonstrates this.