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

Eric Biggers
2020-04-28T02:46:18Z

On Tue, Apr 28, 2020 at 02:14:41AM +0000, Satya Tangirala wrote:
> > > +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...

Note that we're holding down_write(&ksm->lock) here, which synchronizes with
someone getting the keyslot (in particular, incrementing its refcount from 0)
because that uses down_read(&ksm->lock).

So I don't think there's a race. The behavior is just that if someone tries to
evict a key that's still in-use, then we'll correctly fail to evict the key.

"Evicting a key that's still in-use" isn't supposed to happen, so printing a
warning is a good idea. But I think it needs to be pr_warn_once(), not
WARN_ON_ONCE(), because WARN_ON_ONCE() is for kernel bugs only, not userspace
bugs. It's theoretically possible for userspace to cause the same key to be
used multiple times on the same disk but via different blk_crypto_key's. The
keyslot manager will put these in the same keyslot, but there will be a separate
eviction attempt for each blk_crypto_key.

For example, with fscrypt with -o inlinecrypt and blk-crypto-fallback, userspace
could create an encrypted file using FSCRYPT_MODE_ADIANTUM and flags == 0, then
get its encryption nonce and derive the file's encryption key. Then in another
directory, they could set FSCRYPT_MODE_ADIANTUM and flags ==
FSCRYPT_POLICY_FLAG_DIRECT_KEY, and use the other file's encryption key as the
*master* key.

That would be totally insane for userspace to do. But it's possible, so we
can't use WARN_ON_ONCE().

- Eric