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 BDD5D1E1DE7; Sun, 4 May 2025 16:56:03 +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=1746377763; cv=none; b=PkkSRO1BzNsgwPjF+gxwSa9uh01LnskU3w8ax080RpwCktwzbE1/XeDuQw8e3KnszupGYOo9S6gtisScXOT4UOrExol0P0cetXSo3x2yHBwGjbz9NPCCn8/E6m+0+tDtmESwWK41Xf62r7vDM6i99JVmbMXOdiSeVcpQxhF3ydY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1746377763; c=relaxed/simple; bh=7byS+mjeSFHrzb+IfI4pFxAaork3Tq6cUfMVQ3+Gz9Y=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=r/9DCSi2a+0/oK92onh1dppDoyHKvChvO/WB1GUl9PjFD6V6pqtaURyLPmkKuQfVohKe7K+LCxrrymM1KMMUMdzwntrHSPQ2I/764zryNbkdB81/QpgzdCKeUWEPhQNUn4soQVrjvamnRBmifiaKEIfEj0B4sirGhbcJF2A7tmQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=mYPp7SoF; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="mYPp7SoF" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9FD47C4CEE7; Sun, 4 May 2025 16:56:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1746377763; bh=7byS+mjeSFHrzb+IfI4pFxAaork3Tq6cUfMVQ3+Gz9Y=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=mYPp7SoFQgkP8sahvG/Tei97RR/8zhOMj0Tn7BI3CCTOTpgJihq65Tic8HqBDtw0Z HpbjfKZKMkyQbYT+x/m9OufNyXDhzdTmvo9lWAUn3ScsD6vXfrggpC2mU9jhHcMRXT 1HOGbKr8sjgIgZcPMjWQ+eehI3Ng3ZukcUqOnR+wc6ecsBUD0DVWmbRW8aOu/E0nn8 /QwUFwC94XFPjJVIzvyiQXU7XMGi46liIxxqTt/5pflc0qFXZm+QJZLWpFlh/VoOqn WSHY1YBDsuHx8qA4R++lGTjRCYxgsAKRi/qFHuQ3y1AcMLnONfknytVSlW0HeK9pcU vmxUuCeFjTGVw== Date: Sun, 4 May 2025 19:55:58 +0300 From: Jarkko Sakkinen To: Herbert Xu Cc: keyrings@vger.kernel.org, David Howells , Lukas Wunner , Ignat Korchagin , "David S. Miller" , Peter Huewe , Jason Gunthorpe , Paul Moore , James Morris , "Serge E. Hallyn" , James Bottomley , Mimi Zohar , linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org, linux-integrity@vger.kernel.org, linux-security-module@vger.kernel.org Subject: Re: [PATCH] KEYS: Reduce smp_mb() calls in key_put() Message-ID: References: <20250430152554.23646-1-jarkko@kernel.org> Precedence: bulk X-Mailing-List: linux-integrity@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Sat, May 03, 2025 at 11:02:57PM +0800, Herbert Xu wrote: > On Sat, May 03, 2025 at 05:39:16PM +0300, Jarkko Sakkinen wrote: > > On Wed, Apr 30, 2025 at 06:25:53PM +0300, Jarkko Sakkinen wrote: > > > Rely only on the memory ordering of spin_unlock() when setting > > > KEY_FLAG_FINAL_PUT under key->user->lock in key_put(). > > > > > > Signed-off-by: Jarkko Sakkinen > > > --- > > > security/keys/key.c | 6 ++++-- > > > 1 file changed, 4 insertions(+), 2 deletions(-) > > > > > > diff --git a/security/keys/key.c b/security/keys/key.c > > > index 7198cd2ac3a3..aecbd624612d 100644 > > > --- a/security/keys/key.c > > > +++ b/security/keys/key.c > > > @@ -656,10 +656,12 @@ void key_put(struct key *key) > > > spin_lock_irqsave(&key->user->lock, flags); > > > key->user->qnkeys--; > > > key->user->qnbytes -= key->quotalen; > > > + set_bit(KEY_FLAG_FINAL_PUT, &key->flags); > > > spin_unlock_irqrestore(&key->user->lock, flags); > > > + } else { > > > + set_bit(KEY_FLAG_FINAL_PUT, &key->flags); > > > + smp_mb(); /* key->user before FINAL_PUT set. */ > > > } > > > - smp_mb(); /* key->user before FINAL_PUT set. */ > > > - set_bit(KEY_FLAG_FINAL_PUT, &key->flags); > > > > Oops, my bad (order swap), sorry. Should have been: > > > > spin_unlock_irqrestore(&key->user->lock, flags); > > } else { > > smp_mb(); /* key->user before FINAL_PUT set. */ > > You can use smp_mb__before_atomic here as it is equivalent to > smp_mb in this situation. > > > } > > set_bit(KEY_FLAG_FINAL_PUT, &key->flags); > > > > Should spin_lock()/unlock() be good enough or what good does smp_mb() do > > in that branch? Just checking if I'm missing something before sending > > fixed version. > > I don't think spin_unlock alone is enough to replace an smp_mb. > A spin_lock + spin_unlock would be enough though. > > However, looking at the bigger picture this smp_mb looks bogus. > What exactly is it protecting against? > > The race condition that this is supposed to fix should have been > dealt with by the set_bit/test_bit of FINAL_PUT alone. I don't > see any point in having this smb_mb at all. smp_mb() there makes sure that key->user change don't spill between key_put() and gc. GC pairs smp_mb() in key_put() after FINAL_PUT to make sure that also in its side key->user changes have been walled before moving the key as part of unrefenced keys. See also [1]. It cleared this up for me. Here user->lock easily misleads to overlook the actual synchronization scheme. > > Cheers, > -- > Email: Herbert Xu > Home Page: http://gondor.apana.org.au/~herbert/ > PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt [1] https://lore.kernel.org/keyrings/1121543.1746310761@warthog.procyon.org.uk/ BR, Jarkko