Re: [PATCH v10 02/12] block: Keyslot Manager for Inline Encryption

From: Satya Tangirala <satyat@google.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: linux-block@vger.kernel.org, linux-scsi@vger.kernel.org,
	linux-fscrypt@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-f2fs-devel@lists.sourceforge.net,
	linux-ext4@vger.kernel.org,
	Barani Muthukumaran <bmuthuku@qti.qualcomm.com>,
	Kuohong Wang <kuohong.wang@mediatek.com>,
	Kim Boojin <boojin.kim@samsung.com>,
	Eric Biggers <ebiggers@google.com>
Subject: Re: [PATCH v10 02/12] block: Keyslot Manager for Inline Encryption
Date: Tue, 28 Apr 2020 02:14:41 +0000	[thread overview]
Message-ID: <20200428021441.GA52406@google.com> (raw)
In-Reply-To: <20200422092250.GA12290@infradead.org>

On Wed, Apr 22, 2020 at 02:22:50AM -0700, Christoph Hellwig wrote:
> > +bool blk_ksm_crypto_cfg_supported(struct blk_keyslot_manager *ksm,
> > +				  const struct blk_crypto_config *cfg)
> > +{
> > +	if (!ksm)
> > +		return false;
> > +	return (ksm->crypto_modes_supported[cfg->crypto_mode] &
> > +		cfg->data_unit_size) &&
> > +	       (ksm->max_dun_bytes_supported >= cfg->dun_bytes);
> 
> Nit: why not expand this a bit to be more readable:
> 
> 	if (!(ksm->crypto_modes_supported[cfg->crypto_mode] &
> 			cfg->data_unit_size))
> 		return false;
> 	if (ksm->max_dun_bytes_supported < cfg->dun_bytes)
> 		return false;
> 	return true;
> 
> > +int blk_ksm_evict_key(struct blk_keyslot_manager *ksm,
> > +		      const struct blk_crypto_key *key)
> > +{
> > +	struct blk_ksm_keyslot *slot;
> > +	int err = 0;
> > +
> > +	blk_ksm_hw_enter(ksm);
> > +	slot = blk_ksm_find_keyslot(ksm, key);
> > +	if (!slot)
> > +		goto out_unlock;
> > +
> > +	if (atomic_read(&slot->slot_refs) != 0) {
> > +		err = -EBUSY;
> > +		goto out_unlock;
> > +	}
> 
> This check looks racy.
Yes, this could in theory race with blk_ksm_put_slot (e.g. if it's
called while there's still IO in flight/IO that just finished) - But
it's currently only called by fscrypt when a key is being destroyed,
which only happens after all the inodes using that key are evicted, and
no data is in flight, so when this function is called, slot->slot_refs
will be 0. In particular, this function should only be called when the
key isn't being used for IO anymore. I'll add a WARN_ON_ONCE and also
make the assumption clearer. We could also instead make this wait for
the slot_refs to become 0 and then evict the key instead of just
returning -EBUSY as it does now, but I'm not sure if it's really what
we want to do/worth doing right now...