linux-kernel - Re: [PATCH] KEYS: Reduce smp_mb() calls in key

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aBeY4d3eC2aOthx-@kernel.org>
Date: Sun, 4 May 2025 19:42:09 +0300
From: Jarkko Sakkinen <jarkko@...nel.org>
To: David Howells <dhowells@...hat.com>
Cc: keyrings@...r.kernel.org, Lukas Wunner <lukas@...ner.de>,
	Ignat Korchagin <ignat@...udflare.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>,
	Peter Huewe <peterhuewe@....de>, Jason Gunthorpe <jgg@...pe.ca>,
	Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Mimi Zohar <zohar@...ux.ibm.com>, linux-crypto@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-integrity@...r.kernel.org,
	linux-security-module@...r.kernel.org
Subject: Re: [PATCH] KEYS: Reduce smp_mb() calls in key_put()

On Sat, May 03, 2025 at 11:19:21PM +0100, David Howells wrote:
> Jarkko Sakkinen <jarkko@...nel.org> wrote:
> 
> > 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. */
> >  			}
> > 			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.
> 
> spin_unlock() is semi-permeable, so stuff after it can leak into the inside of
> it up as far as the spin_lock().  With your change, the garbage collector can
> no longer guarantee that key_put() will have done with accessing key->user
> when it sees KEY_FLAG_FINAL_PUT is set.
> 
> So, NAK on this patch, I think.  If you want a second opinion, I'd suggest
> waving it in front of Paul McKenney.

Fair enough.

If I revisit this in a way or another,  I'll cc to him for comments but
for this I'll buy what you said.

> 
> Possibly we only need smp_mb() in the IN_QUOTA branch in key_put().
> 
> David

Thank you for the comments.

BR, Jarkko