Re: [PATCH 2/3] blk-crypto: make blk_crypto_evict_key() more robust

[Nathan Huckleberry <nhuck@google.com>]

Hi Eric,

On Thu, Mar 2, 2023 at 11:23 PM Eric Biggers <ebiggers@kernel.org> wrote:
>
> From: Eric Biggers <ebiggers@google.com>
>
> If blk_crypto_evict_key() sees that the key is still in-use (due to a
> bug) or that ->keyslot_evict failed, it currently just returns an error
> while leaving the key linked into the keyslot management structures.
>
> However, blk_crypto_evict_key() is only called in contexts such as inode
> eviction where failure is not an option.  So actually the caller
> proceeds with freeing the blk_crypto_key regardless of the return value
> of blk_crypto_evict_key().
>
> These two assumptions don't match, and the result is that there can be a
> use-after-free in blk_crypto_reprogram_all_keys() after one of these
> errors occurs.  (Note, these errors *shouldn't* happen; we're just
> talking about what happens if they do anyway.)
>
> Fix this by making blk_crypto_evict_key() unlink the key from the
> keyslot management structures even on failure.
>
> Fixes: 1b2628397058 ("block: Keyslot Manager for Inline Encryption")
> Cc: stable@vger.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
>  block/blk-crypto-profile.c | 50 +++++++++++++++-----------------------
>  block/blk-crypto.c         | 23 +++++++++++-------
>  2 files changed, 33 insertions(+), 40 deletions(-)
>
> diff --git a/block/blk-crypto-profile.c b/block/blk-crypto-profile.c
> index 0307fb0d95d34..1b20ead59f39b 100644
> --- a/block/blk-crypto-profile.c
> +++ b/block/blk-crypto-profile.c
> @@ -354,22 +354,10 @@ bool __blk_crypto_cfg_supported(struct blk_crypto_profile *profile,
>         return true;
>  }
>
> -/**
> - * __blk_crypto_evict_key() - Evict a key from a device.
> - * @profile: the crypto profile of the device
> - * @key: the key to evict.  It must not still be used in any I/O.
> - *
> - * If the device has keyslots, this finds the keyslot (if any) that contains the
> - * specified key and calls the driver's keyslot_evict function to evict it.
> - *
> - * Otherwise, this just calls the driver's keyslot_evict function if it is
> - * implemented, passing just the key (without any particular keyslot).  This
> - * allows layered devices to evict the key from their underlying devices.
> - *
> - * Context: Process context. Takes and releases profile->lock.
> - * Return: 0 on success or if there's no keyslot with the specified key, -EBUSY
> - *        if the keyslot is still in use, or another -errno value on other
> - *        error.
> +/*
> + * This is an internal function that evicts a key from an inline encryption
> + * device that can be either a real device or the blk-crypto-fallback "device".
> + * It is used only by blk_crypto_evict_key(); see that function for details.
>   */
>  int __blk_crypto_evict_key(struct blk_crypto_profile *profile,
>                            const struct blk_crypto_key *key)
> @@ -389,22 +377,22 @@ int __blk_crypto_evict_key(struct blk_crypto_profile *profile,
>
>         blk_crypto_hw_enter(profile);
>         slot = blk_crypto_find_keyslot(profile, key);
> -       if (!slot)
> -               goto out_unlock;
> -
> -       if (WARN_ON_ONCE(atomic_read(&slot->slot_refs) != 0)) {
> -               err = -EBUSY;
> -               goto out_unlock;
> +       if (slot) {
> +               if (WARN_ON_ONCE(atomic_read(&slot->slot_refs) != 0)) {
> +                       /* BUG: key is still in use by I/O */
> +                       err = -EBUSY;
> +               } else {
> +                       err = profile->ll_ops.keyslot_evict(
> +                                       profile, key,
> +                                       blk_crypto_keyslot_index(slot));
> +               }
> +               /*
> +                * Callers may free the key even on error, so unlink the key
> +                * from the hash table and clear slot->key even on error.
> +                */
> +               hlist_del(&slot->hash_node);
> +               slot->key = NULL;
>         }

The !slot case still needs to be handled. If profile->num_slots != 0
and !slot, we'll get an invalid index from blk_crypto_keyslot_index.

With that change,
Reviewed-by: Nathan Huckleberry <nhuck@google.com>

Thanks,
Huck

> -       err = profile->ll_ops.keyslot_evict(profile, key,
> -                                           blk_crypto_keyslot_index(slot));
> -       if (err)
> -               goto out_unlock;
> -
> -       hlist_del(&slot->hash_node);
> -       slot->key = NULL;
> -       err = 0;
> -out_unlock:
>         blk_crypto_hw_exit(profile);
>         return err;
>  }
> diff --git a/block/blk-crypto.c b/block/blk-crypto.c
> index 8e5612364c48c..caa86a210cb6c 100644
> --- a/block/blk-crypto.c
> +++ b/block/blk-crypto.c
> @@ -399,17 +399,22 @@ int blk_crypto_start_using_key(struct block_device *bdev,
>  }
>
>  /**
> - * blk_crypto_evict_key() - Evict a key from any inline encryption hardware
> - *                         it may have been programmed into
> - * @bdev: The block_device who's associated inline encryption hardware this key
> - *     might have been programmed into
> - * @key: The key to evict
> + * blk_crypto_evict_key() - Evict a blk_crypto_key from a block_device
> + * @bdev: a block_device on which I/O using the key may have been done
> + * @key: the key to evict
>   *
> - * Upper layers (filesystems) must call this function to ensure that a key is
> - * evicted from any hardware that it might have been programmed into.  The key
> - * must not be in use by any in-flight IO when this function is called.
> + * For a given block_device, this function removes the given blk_crypto_key from
> + * the keyslot management structures and evicts it from any underlying hardware
> + * keyslot(s) or blk-crypto-fallback keyslot it may have been programmed into.
>   *
> - * Return: 0 on success or if the key wasn't in any keyslot; -errno on error.
> + * Upper layers must call this before freeing the blk_crypto_key.  It must be
> + * called for every block_device the key may have been used on.  The key must no
> + * longer be in use by any I/O when this function is called.
> + *
> + * Context: May sleep.
> + * Return: 0 on success or if the key wasn't in any keyslot; -errno if the key
> + *        failed to be evicted from a keyslot or is still in-use.  Even on
> + *        "failure", the key is removed from the keyslot management structures.
>   */
>  int blk_crypto_evict_key(struct block_device *bdev,
>                          const struct blk_crypto_key *key)
> --
> 2.39.2
>