[PATCH v3 3/6] blk-crypto: make blk_crypto_evict_key() more robust

[Eric Biggers <ebiggers@kernel.org>]

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 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.

Also improve some comments.

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 | 46 +++++++++++++++++---------------------
 block/blk-crypto.c         | 28 ++++++++++++++++-------
 2 files changed, 41 insertions(+), 33 deletions(-)

diff --git a/block/blk-crypto-profile.c b/block/blk-crypto-profile.c
index 0307fb0d95d34..3290c03c9918d 100644
--- a/block/blk-crypto-profile.c
+++ b/block/blk-crypto-profile.c
@@ -354,28 +354,16 @@ 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)
 {
 	struct blk_crypto_keyslot *slot;
-	int err = 0;
+	int err;
 
 	if (profile->num_slots == 0) {
 		if (profile->ll_ops.keyslot_evict) {
@@ -389,22 +377,30 @@ 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 (!slot) {
+		/*
+		 * Not an error, since a key not in use by I/O is not guaranteed
+		 * to be in a keyslot.  There can be more keys than keyslots.
+		 */
+		err = 0;
+		goto out;
+	}
 
 	if (WARN_ON_ONCE(atomic_read(&slot->slot_refs) != 0)) {
+		/* BUG: key is still in use by I/O */
 		err = -EBUSY;
-		goto out_unlock;
+		goto out_remove;
 	}
 	err = profile->ll_ops.keyslot_evict(profile, key,
 					    blk_crypto_keyslot_index(slot));
-	if (err)
-		goto out_unlock;
-
+out_remove:
+	/*
+	 * Callers 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;
-	err = 0;
-out_unlock:
+out:
 	blk_crypto_hw_exit(profile);
 	return err;
 }
diff --git a/block/blk-crypto.c b/block/blk-crypto.c
index e800f305e9eda..4d760b092deb9 100644
--- a/block/blk-crypto.c
+++ b/block/blk-crypto.c
@@ -400,15 +400,19 @@ 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.
+ *
+ * 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.
  */
 void blk_crypto_evict_key(struct block_device *bdev,
 			  const struct blk_crypto_key *key)
@@ -420,6 +424,14 @@ void blk_crypto_evict_key(struct block_device *bdev,
 		err = __blk_crypto_evict_key(q->crypto_profile, key);
 	else
 		err = blk_crypto_fallback_evict_key(key);
+	/*
+	 * An error can only occur here if the key failed to be evicted from a
+	 * keyslot (due to a hardware or driver issue) or is allegedly still in
+	 * use by I/O (due to a kernel bug).  Even in these cases, the key is
+	 * still unlinked from the keyslot management structures, and the caller
+	 * is allowed and expected to free it right away.  There's nothing
+	 * callers can do to handle errors, so just log them and return void.
+	 */
 	if (err)
 		pr_warn_ratelimited("%pg: error %d evicting key\n", bdev, err);
 }
-- 
2.39.2