From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from Chamillionaire.breakpoint.cc (Chamillionaire.breakpoint.cc [91.216.245.30])
	(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 160132C11D9
	for <netdev@vger.kernel.org>; Tue, 10 Mar 2026 19:49:45 +0000 (UTC)
Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.216.245.30
ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116;
	t=1773172187; cv=none; b=qOEG4YfXorXyL4gpkH9mWF1JRepWkjr14AKp5bTPHcwELuHCF0B+o2S1pHOzeA3iCkHt1hZU0BM+MWRrW8RfwHpWjlNzoNO2JfAkh+1c8Si7k7LFC/aJr96huEQxQl68hf6+OWvKTM8UTHloJKMd2i4vGAgZ4V6+Jl0zYvgJJvQ=
ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org;
	s=arc-20240116; t=1773172187; c=relaxed/simple;
	bh=Cbk+TPCTTua5crdvvKusCVU5CfZ4/K0YX1H9qIVTQBs=;
	h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version:
	 Content-Type:Content-Disposition:In-Reply-To; b=FdgGJW0jEMnEadYx5wep2wKElWpjWlMkLXDMbqluiAFjtBztz0EMpsY3AOVDVhCdCILT4cEbkO7NVZRYbgF33wUS6BU7SVqs3PPwRB54K5s6kCuyBxhw8T3heBZD+PQekcnoCQaBRqR55VZWffz/87+PJFueIxq+BARjcAEol0A=
ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=strlen.de; spf=pass smtp.mailfrom=strlen.de; arc=none smtp.client-ip=91.216.245.30
Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=strlen.de
Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=strlen.de
Received: by Chamillionaire.breakpoint.cc (Postfix, from userid 1003)
	id 6CBC760345; Tue, 10 Mar 2026 20:49:44 +0100 (CET)
Date: Tue, 10 Mar 2026 20:49:44 +0100
From: Florian Westphal <fw@strlen.de>
To: Leon Romanovsky <leon@kernel.org>
Cc: Sabrina Dubroca <sd@queasysnail.net>, netdev@vger.kernel.org,
	Steffen Klassert <steffen.klassert@secunet.com>,
	Herbert Xu <herbert@gondor.apana.org.au>
Subject: Re: [PATCH ipsec-next 01/10] xfrm: state: fix sparse warnings on
 xfrm_state_hold_rcu
Message-ID: <abB12Ae769OivuYB@strlen.de>
References: <cover.1773051558.git.sd@queasysnail.net>
 <7388df7238672a92be0e4048f0225e6db294e736.1773051558.git.sd@queasysnail.net>
 <20260310103135.GB12611@unreal>
 <abABg6aO9twGIk7F@krikkit>
 <20260310182012.GF12611@unreal>
 <abBv6zR4Qq_lrzuC@strlen.de>
 <20260310194500.GO12611@unreal>
Precedence: bulk
X-Mailing-List: netdev@vger.kernel.org
List-Id: <netdev.vger.kernel.org>
List-Subscribe: <mailto:netdev+subscribe@vger.kernel.org>
List-Unsubscribe: <mailto:netdev+unsubscribe@vger.kernel.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20260310194500.GO12611@unreal>

Leon Romanovsky <leon@kernel.org> wrote:
> > You can only use xfrm_state_hold() if the refcount is already > 0.
> > xfrm_state_hold uses refcount_inc(), so you get a UaF warn splat
> > if this assuption doesn't hold true.
> 
> I know it, the thing that bothers me is that it is unclear how
> xfrm_state_hold_rcu() can have refcount equal to 0.
> 
> xfrm_state_put() decreases refcount and when it is zero, it calls
> to __xfrm_state_destroy(). The latter assumes that the state was
> already removed from various hlists.

Yes, insertion in the table means refcount is 1, but userspace
can zap states at any time, e.g.:

xfrm_del_sa -> xfrm_state_delete -> __xfrm_state_delete (which
unlinks from hash lists).

The last xfrm_state_put() in that function may cause 1 -> 0
transition.  Parallel lookup can still observe that state,
so it has to pretend it wasn't there to begin with.