Fix blk-crypto keyslot race condition

From: Eric Biggers <[email protected]>

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 [email protected]

Signed-off-by Eric Biggers <[email protected]>

---
 block/blk-crypto-internal.h | 15 +++++++++++----
 block/blk-crypto.c          | 24 ++++++++++++------------
 block/blk-mq.c              | 15 ++++++++++++++-
 3 files changed, 37 insertions(+), 17 deletions(-)

block/blk-crypto-internal.h +11 -4

index a8cdaf26851e1..73609902349b6 100644
@@ -153,14 +153,21 @@ 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_is_encrypted(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 +206,7 @@ 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;
 }
 

block/blk-crypto.c +12 -12

index 45378586151f7..8e5612364c48c 100644
@@ -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)
+{
        mempool_free(rq->crypt_ctx, bio_crypt_ctx_pool);
-       blk_crypto_rq_set_defaults(rq);
+       rq->crypt_ctx = NULL;
+
+       /* The keyslot, if one was needed, should have been released earlier. */
+       if (WARN_ON_ONCE(rq->crypt_keyslot))
+               __blk_crypto_rq_put_keyslot(rq);
 }
 
 /**

block/blk-mq.c +14 -2

index d3494a796ba80..738e81f518227 100644
@@ -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_is_encrypted(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

From: Eric Biggers <[email protected]>

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 [email protected]

Signed-off-by Eric Biggers <[email protected]>

---
 block/blk-crypto-profile.c | 50 +++++++++++++++-----------------------
 block/blk-crypto.c         | 23 +++++++++++-------
 2 files changed, 33 insertions(+), 40 deletions(-)

block/blk-crypto-profile.c +10 -22

index 0307fb0d95d34..1b20ead59f39b 100644
@@ -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(

block/blk-crypto-profile.c +0 -9

-       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;
 }

block/blk-crypto.c +14 -10

index 8e5612364c48c..caa86a210cb6c 100644
@@ -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

Quoted 18 lines

int __blk_crypto_evict_key(struct blk_crypto_profile *profile,
const struct blk_crypto_key *key)

block/blk-crypto-profile.c +6 -6

@@ -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(

block/blk-crypto-profile.c int __blk_crypto_evict_key(struct blk_crypto_profile *profile, · line 395

Quoted 10 lines

block/blk-crypto-profile.c +9 -0

+                                       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 <[email protected]>

Thanks,
Huck

block/blk-crypto-profile.c int __blk_crypto_evict_key(struct blk_crypto_profile *profile, · line 398

Quoted 12 lines

block/blk-crypto-profile.c +0 -9

-       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;
 }

Quoted 38 lines · Nathan Huckleberry

Quoted 18 lines

int __blk_crypto_evict_key(struct blk_crypto_profile *profile,
const struct blk_crypto_key *key)

block/blk-crypto-profile.c +6 -6

@@ -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(

block/blk-crypto-profile.c int __blk_crypto_evict_key(struct blk_crypto_profile *profile, · line 395

Quoted 10 lines

block/blk-crypto-profile.c +9 -0

+                                       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 <[email protected]>

Thanks,
Huck

Quoted 12 lines

block/blk-crypto-profile.c int __blk_crypto_evict_key(struct blk_crypto_profile *profile, · line 398

Quoted 12 lines

block/blk-crypto-profile.c +0 -9

-       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;
 }

I'm not sure what you're referring to. The !slot case is handled correctly, and it's the same as before.

- Eric