fscrypt: Avoid dynamic allocation in fscrypt_get_devices()

When a blk_crypto_key starts being used or is evicted, fs/crypto/ calls
fscrypt_get_devices() to get the filesystem's list of block devices,
then iterates over them and calls blk_crypto_config_supported(),
blk_crypto_start_using_key(), or blk_crypto_evict_key() on each one.

Currently, the block device pointers are placed in a dynamically
allocated array.  This dynamic allocation is problematic because:

- It can fail, especially at the fscrypt_destroy_inline_crypt_key() call
  site when it's invoked for inode eviction under direct reclaim.

- fscrypt_destroy_inline_crypt_key() doesn't handle the failure.  It
  just zeroizes and frees the blk_crypto_key without calling
  blk_crypto_evict_key().  That causes a use-after-free.

For now, let's fix this in the straightforward and easily-backportable
way by switching to an on-stack array.  Currently the fscrypt
multi-device functionality is used only by f2fs, which has a hardcoded
limit of 8 block devices.  An on-stack array works fine for that.

(Of course, this solution won't scale up to large number of block
devices.  For that we'd need a different solution, like moving the block
device iteration into the filesystem.  Or in the case of btrfs, which
will only support blk-crypto-fallback, we should make it just call
blk-crypto-fallback directly, so the block devices won't be needed.)

Fixes: 22e9947a4b2b ("fscrypt: stop holding extra request_queue references")
Cc: stable@vger.kernel.org
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260713023708.9245-1-ebiggers%40kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/20260719055602.78828-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

1 files changed, 10 insertions, 8 deletions

diff --git a/include/linux/fscrypt.h b/include/linux/fscrypt.h
index 54712ec61ffb..f6b235cd72b4 100644
--- a/include/linux/fscrypt.h
+++ b/include/linux/fscrypt.h
@@ -57,6 +57,9 @@ struct fscrypt_name {
 /* Maximum value for the third parameter of fscrypt_operations.set_context(). */
 #define FSCRYPT_SET_CONTEXT_MAX_SIZE	40
 
+/* Maximum supported number of block devices per filesystem */
+#define FSCRYPT_MAX_DEVICES	8
+
 #ifdef CONFIG_FS_ENCRYPTION
 
 /* Crypto operations for filesystems */
@@ -181,21 +184,20 @@ struct fscrypt_operations {
 	bool (*has_stable_inodes)(struct super_block *sb);
 
 	/*
-	 * Return an array of pointers to the block devices to which the
-	 * filesystem may write encrypted file contents, NULL if the filesystem
-	 * only has a single such block device, or an ERR_PTR() on error.
+	 * Retrieve the list of block devices to which the filesystem may write
+	 * encrypted file contents.
 	 *
-	 * On successful non-NULL return, *num_devs is set to the number of
-	 * devices in the returned array.  The caller must free the returned
-	 * array using kfree().
+	 * This writes the block_device pointers to @devs and returns the count
+	 * (between 1 and FSCRYPT_MAX_DEVICES inclusively).
 	 *
 	 * If the filesystem can use multiple block devices (other than block
 	 * devices that aren't used for encrypted file contents, such as
 	 * external journal devices), and wants to support inline encryption,
 	 * then it must implement this function.  Otherwise it's not needed.
 	 */
-	struct block_device **(*get_devices)(struct super_block *sb,
-					     unsigned int *num_devs);
+	unsigned int (*get_devices)(
+		struct super_block *sb,
+		struct block_device *devs[FSCRYPT_MAX_DEVICES]);
 };
 
 int fscrypt_d_revalidate(struct inode *dir, const struct qstr *name,