linux-kernel - Re: [PATCH v8] KEYS: Add a list for unreferenced keys

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Message-ID: <Z_nCLHD33VR3un3O@kernel.org>
Date: Sat, 12 Apr 2025 04:30:20 +0300
From: Jarkko Sakkinen <jarkko@...nel.org>
To: David Howells <dhowells@...hat.com>
Cc: keyrings@...r.kernel.org, Jarkko Sakkinen <jarkko.sakkinen@...nsys.com>,
	stable@...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 v8] KEYS: Add a list for unreferenced keys

On Fri, Apr 11, 2025 at 11:37:25PM +0300, Jarkko Sakkinen wrote:
> On Fri, Apr 11, 2025 at 04:59:11PM +0100, David Howells wrote:
> > Jarkko Sakkinen <jarkko@...nel.org> wrote:
> > 
> > > +	spin_lock_irqsave(&key_graveyard_lock, flags);
> > > +	list_splice_init(&key_graveyard, &graveyard);
> > > +	spin_unlock_irqrestore(&key_graveyard_lock, flags);
> > 
> > I would wrap this bit in a check to see if key_graveyard is empty so that we
> > can avoid disabling irqs and taking the lock if the graveyard is empty.
> 
> Can do, and does make sense.
> 
> > 
> > > +		if (!refcount_inc_not_zero(&key->usage)) {
> > 
> > Sorry, but eww.  You're going to wangle the refcount twice on every key on the
> > system every time the gc does a pass.  Further, in some cases inc_not_zero is
> > not the fastest op in the world.
> 
> One could alternatively "test_bit(KEY_FLAG_FINAL_PUT, &key->flags)) &&
> !refcount_inc_not_zero(&key->usage))" without mb() on either side and

Refactoring the changes to key_put() would be (draft):

void key_put(struct key *key)
{
	unsigned long flags;

	if (!key)
		return;

	key_check(key);

	if (!refcount_dec_and_test(&key->usage))
		return;

	local_irq_save(flags);

	set_bit(KEY_FLAG_FINAL_PUT, &key->flags);

	/* Remove user's key tracking and quota: */
	if (test_bit(KEY_FLAG_IN_QUOTA, &key->flags)) {
		spin_lock(&key->user->lock);
		key->user->qnkeys--;
		key->user->qnbytes -= key->quotalen;
		spin_unlock(&key->user->lock);
	}

	spin_lock(&key_graveyard_lock);
	list_add_tail(&key->graveyard_link, &key_graveyard);
	spin_unlock(&key_graveyard_lock);

	schedule_work(&key_gc_work);

	local_irq_restore(flags);
}

And:

static bool key_get_gc(struct key *key)
{
	return !test_bit(KEY_FLAG_FINAL_PUT, &key->flags)) &&	/* fast path */
	       refcount_inc_not_zero(&key->usage)		/* slow path */
}

In the gc-loop:

	if (!key_get_gc(&key)) {
		key = NULL;
		gc_state |= KEY_GC_REAP_AGAIN;
		goto skip_dead_key;
 	}

[none yet compiled]

BR, Jarkko