Re: [PATCH v2 1/4] blk-mq: release crypto keyslot before reporting I/O complete

[Nathan Huckleberry <nhuck@google.com>]

On Wed, Mar 8, 2023 at 11:39 AM Eric Biggers <ebiggers@kernel.org> wrote:
>
> From: Eric Biggers <ebiggers@google.com>
>
> Once all I/O using a blk_crypto_key has completed, filesystems can call
> blk_crypto_evict_key().  However, the block layer currently doesn't call
> blk_crypto_put_keyslot() until the request is being freed, which happens
> after upper layers have been told (via bio_endio()) the I/O has
> completed.  This causes a race condition where blk_crypto_evict_key()
> can see 'slot_refs != 0' without there being an actual bug.
>
> This makes __blk_crypto_evict_key() hit the
> 'WARN_ON_ONCE(atomic_read(&slot->slot_refs) != 0)' and return without
> doing anything, eventually causing a use-after-free in
> blk_crypto_reprogram_all_keys().  (This is a very rare bug and has only
> been seen when per-file keys are being used with fscrypt.)
>
> There are two options to fix this: either release the keyslot before
> bio_endio() is called on the request's last bio, or make
> __blk_crypto_evict_key() ignore slot_refs.  Let's go with the first
> solution, since it preserves the ability to report bugs (via
> WARN_ON_ONCE) where a key is evicted while still in-use.
>
> Fixes: a892c8d52c02 ("block: Inline encryption support for blk-mq")
> Cc: stable@vger.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
>  block/blk-crypto-internal.h | 25 +++++++++++++++++++++----
>  block/blk-crypto.c          | 24 ++++++++++++------------
>  block/blk-merge.c           |  2 ++
>  block/blk-mq.c              | 15 ++++++++++++++-
>  4 files changed, 49 insertions(+), 17 deletions(-)
>
> diff --git a/block/blk-crypto-internal.h b/block/blk-crypto-internal.h
> index a8cdaf26851e..4f1de2495f0c 100644
> --- a/block/blk-crypto-internal.h
> +++ b/block/blk-crypto-internal.h
> @@ -65,6 +65,11 @@ static inline bool blk_crypto_rq_is_encrypted(struct request *rq)
>         return rq->crypt_ctx;
>  }
>
> +static inline bool blk_crypto_rq_has_keyslot(struct request *rq)
> +{
> +       return rq->crypt_keyslot;
> +}
> +
>  blk_status_t blk_crypto_get_keyslot(struct blk_crypto_profile *profile,
>                                     const struct blk_crypto_key *key,
>                                     struct blk_crypto_keyslot **slot_ptr);
> @@ -119,6 +124,11 @@ static inline bool blk_crypto_rq_is_encrypted(struct request *rq)
>         return false;
>  }
>
> +static inline bool blk_crypto_rq_has_keyslot(struct request *rq)
> +{
> +       return false;
> +}
> +
>  #endif /* CONFIG_BLK_INLINE_ENCRYPTION */
>
>  void __bio_crypt_advance(struct bio *bio, unsigned int bytes);
> @@ -153,14 +163,21 @@ static inline bool blk_crypto_bio_prep(struct bio **bio_ptr)
>         return true;
>  }
>
> -blk_status_t __blk_crypto_init_request(struct request *rq);
> -static inline blk_status_t blk_crypto_init_request(struct request *rq)
> +blk_status_t __blk_crypto_rq_get_keyslot(struct request *rq);
> +static inline blk_status_t blk_crypto_rq_get_keyslot(struct request *rq)
>  {
>         if (blk_crypto_rq_is_encrypted(rq))
> -               return __blk_crypto_init_request(rq);
> +               return __blk_crypto_rq_get_keyslot(rq);
>         return BLK_STS_OK;
>  }
>
> +void __blk_crypto_rq_put_keyslot(struct request *rq);
> +static inline void blk_crypto_rq_put_keyslot(struct request *rq)
> +{
> +       if (blk_crypto_rq_has_keyslot(rq))
> +               __blk_crypto_rq_put_keyslot(rq);
> +}
> +
>  void __blk_crypto_free_request(struct request *rq);
>  static inline void blk_crypto_free_request(struct request *rq)
>  {
> @@ -199,7 +216,7 @@ static inline blk_status_t blk_crypto_insert_cloned_request(struct request *rq)
>  {
>
>         if (blk_crypto_rq_is_encrypted(rq))
> -               return blk_crypto_init_request(rq);
> +               return blk_crypto_rq_get_keyslot(rq);
>         return BLK_STS_OK;
>  }
>
> diff --git a/block/blk-crypto.c b/block/blk-crypto.c
> index 45378586151f..d0c7feb447e9 100644
> --- a/block/blk-crypto.c
> +++ b/block/blk-crypto.c
> @@ -224,27 +224,27 @@ static bool bio_crypt_check_alignment(struct bio *bio)
>         return true;
>  }
>
> -blk_status_t __blk_crypto_init_request(struct request *rq)
> +blk_status_t __blk_crypto_rq_get_keyslot(struct request *rq)
>  {
>         return blk_crypto_get_keyslot(rq->q->crypto_profile,
>                                       rq->crypt_ctx->bc_key,
>                                       &rq->crypt_keyslot);
>  }
>
> -/**
> - * __blk_crypto_free_request - Uninitialize the crypto fields of a request.
> - *
> - * @rq: The request whose crypto fields to uninitialize.
> - *
> - * Completely uninitializes the crypto fields of a request. If a keyslot has
> - * been programmed into some inline encryption hardware, that keyslot is
> - * released. The rq->crypt_ctx is also freed.
> - */
> -void __blk_crypto_free_request(struct request *rq)
> +void __blk_crypto_rq_put_keyslot(struct request *rq)
>  {
>         blk_crypto_put_keyslot(rq->crypt_keyslot);
> +       rq->crypt_keyslot = NULL;
> +}
> +
> +void __blk_crypto_free_request(struct request *rq)
> +{
> +       /* The keyslot, if one was needed, should have been released earlier. */
> +       if (WARN_ON_ONCE(rq->crypt_keyslot))
> +               __blk_crypto_rq_put_keyslot(rq);
> +
>         mempool_free(rq->crypt_ctx, bio_crypt_ctx_pool);
> -       blk_crypto_rq_set_defaults(rq);
> +       rq->crypt_ctx = NULL;
>  }
>
>  /**
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 6460abdb2426..65e75efa9bd3 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -867,6 +867,8 @@ static struct request *attempt_merge(struct request_queue *q,
>         if (!blk_discard_mergable(req))
>                 elv_merge_requests(q, req, next);
>
> +       blk_crypto_rq_put_keyslot(next);
> +

This looks good to me, but it looks like there was a pre-existing bug
in the blk-merge code. The elv_merged_request function is only called
when the request does not merge. Does anyone know if that behavior is
correct?

>         /*
>          * 'next' is going away, so update stats accordingly
>          */
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index d0cb2ef18fe2..49825538d932 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -840,6 +840,12 @@ static void blk_complete_request(struct request *req)
>                 req->q->integrity.profile->complete_fn(req, total_bytes);
>  #endif
>
> +       /*
> +        * Upper layers may call blk_crypto_evict_key() anytime after the last
> +        * bio_endio().  Therefore, the keyslot must be released before that.
> +        */
> +       blk_crypto_rq_put_keyslot(req);
> +
>         blk_account_io_completion(req, total_bytes);
>
>         do {
> @@ -905,6 +911,13 @@ bool blk_update_request(struct request *req, blk_status_t error,
>                 req->q->integrity.profile->complete_fn(req, nr_bytes);
>  #endif
>
> +       /*
> +        * Upper layers may call blk_crypto_evict_key() anytime after the last
> +        * bio_endio().  Therefore, the keyslot must be released before that.
> +        */
> +       if (blk_crypto_rq_has_keyslot(req) && nr_bytes >= blk_rq_bytes(req))
> +               __blk_crypto_rq_put_keyslot(req);
> +
>         if (unlikely(error && !blk_rq_is_passthrough(req) &&
>                      !(req->rq_flags & RQF_QUIET)) &&
>                      !test_bit(GD_DEAD, &req->q->disk->state)) {
> @@ -2967,7 +2980,7 @@ void blk_mq_submit_bio(struct bio *bio)
>
>         blk_mq_bio_to_request(rq, bio, nr_segs);
>
> -       ret = blk_crypto_init_request(rq);
> +       ret = blk_crypto_rq_get_keyslot(rq);
>         if (ret != BLK_STS_OK) {
>                 bio->bi_status = ret;
>                 bio_endio(bio);
> --
> 2.39.2
>

This patch itself looks good to me.

Reviewed-by: Nathan Huckleberry <nhuck@google.com>