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>

3 files changed, 44 insertions, 56 deletions

diff --git a/fs/crypto/inline_crypt.c b/fs/crypto/inline_crypt.c
index 47324062fee5..66b9c9150fed 100644
--- a/fs/crypto/inline_crypt.c
+++ b/fs/crypto/inline_crypt.c
@@ -22,22 +22,14 @@
 
 #include "fscrypt_private.h"
 
-static struct block_device **fscrypt_get_devices(struct super_block *sb,
-						 unsigned int *num_devs)
+static unsigned int
+fscrypt_get_devices(struct super_block *sb,
+		    struct block_device *devs[FSCRYPT_MAX_DEVICES])
 {
-	struct block_device **devs;
-
-	if (sb->s_cop->get_devices) {
-		devs = sb->s_cop->get_devices(sb, num_devs);
-		if (devs)
-			return devs;
-	}
-	devs = kmalloc_obj(*devs);
-	if (!devs)
-		return ERR_PTR(-ENOMEM);
+	if (sb->s_cop->get_devices)
+		return sb->s_cop->get_devices(sb, devs);
 	devs[0] = sb->s_bdev;
-	*num_devs = 1;
-	return devs;
+	return 1;
 }
 
 static unsigned int fscrypt_get_dun_bytes(const struct fscrypt_inode_info *ci)
@@ -96,7 +88,7 @@ int fscrypt_select_encryption_impl(struct fscrypt_inode_info *ci,
 	const struct inode *inode = ci->ci_inode;
 	struct super_block *sb = inode->i_sb;
 	struct blk_crypto_config crypto_cfg;
-	struct block_device **devs;
+	struct block_device *devs[FSCRYPT_MAX_DEVICES];
 	unsigned int num_devs;
 	unsigned int i;
 
@@ -135,20 +127,15 @@ int fscrypt_select_encryption_impl(struct fscrypt_inode_info *ci,
 	crypto_cfg.key_type = is_hw_wrapped_key ?
 		BLK_CRYPTO_KEY_TYPE_HW_WRAPPED : BLK_CRYPTO_KEY_TYPE_RAW;
 
-	devs = fscrypt_get_devices(sb, &num_devs);
-	if (IS_ERR(devs))
-		return PTR_ERR(devs);
-
+	num_devs = fscrypt_get_devices(sb, devs);
 	for (i = 0; i < num_devs; i++) {
 		if (!blk_crypto_config_supported(devs[i], &crypto_cfg))
-			goto out_free_devs;
+			return 0;
 	}
 
 	fscrypt_log_blk_crypto_impl(ci->ci_mode, devs, num_devs, &crypto_cfg);
 
 	ci->ci_inlinecrypt = true;
-out_free_devs:
-	kfree(devs);
 
 	return 0;
 }
@@ -164,7 +151,7 @@ int fscrypt_prepare_inline_crypt_key(struct fscrypt_prepared_key *prep_key,
 	enum blk_crypto_key_type key_type = is_hw_wrapped ?
 		BLK_CRYPTO_KEY_TYPE_HW_WRAPPED : BLK_CRYPTO_KEY_TYPE_RAW;
 	struct blk_crypto_key *blk_key;
-	struct block_device **devs;
+	struct block_device *devs[FSCRYPT_MAX_DEVICES];
 	unsigned int num_devs;
 	unsigned int i;
 	int err;
@@ -182,17 +169,12 @@ int fscrypt_prepare_inline_crypt_key(struct fscrypt_prepared_key *prep_key,
 	}
 
 	/* Start using blk-crypto on all the filesystem's block devices. */
-	devs = fscrypt_get_devices(sb, &num_devs);
-	if (IS_ERR(devs)) {
-		err = PTR_ERR(devs);
-		goto fail;
-	}
+	num_devs = fscrypt_get_devices(sb, devs);
 	for (i = 0; i < num_devs; i++) {
 		err = blk_crypto_start_using_key(devs[i], blk_key);
 		if (err)
 			break;
 	}
-	kfree(devs);
 	if (err) {
 		fscrypt_err(inode, "error %d starting to use blk-crypto", err);
 		goto fail;
@@ -210,20 +192,21 @@ void fscrypt_destroy_inline_crypt_key(struct super_block *sb,
 				      struct fscrypt_prepared_key *prep_key)
 {
 	struct blk_crypto_key *blk_key = prep_key->blk_key;
-	struct block_device **devs;
+	struct block_device *devs[FSCRYPT_MAX_DEVICES];
 	unsigned int num_devs;
 	unsigned int i;
 
 	if (!blk_key)
 		return;
 
-	/* Evict the key from all the filesystem's block devices. */
-	devs = fscrypt_get_devices(sb, &num_devs);
-	if (!IS_ERR(devs)) {
-		for (i = 0; i < num_devs; i++)
-			blk_crypto_evict_key(devs[i], blk_key);
-		kfree(devs);
-	}
+	/*
+	 * Evict the key from all the filesystem's block devices.
+	 * This *must* be done before the key is freed.
+	 */
+	num_devs = fscrypt_get_devices(sb, devs);
+	for (i = 0; i < num_devs; i++)
+		blk_crypto_evict_key(devs[i], blk_key);
+
 	kfree_sensitive(blk_key);
 }
 
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 2b8d96411156..9760e4efeffe 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -3749,24 +3749,27 @@ static bool f2fs_has_stable_inodes(struct super_block *sb)
 	return true;
 }
 
-static struct block_device **f2fs_get_devices(struct super_block *sb,
-					      unsigned int *num_devs)
+static unsigned int
+f2fs_get_devices(struct super_block *sb,
+		 struct block_device *devs[FSCRYPT_MAX_DEVICES])
 {
 	struct f2fs_sb_info *sbi = F2FS_SB(sb);
-	struct block_device **devs;
+	int ndevs;
 	int i;
 
-	if (!f2fs_is_multi_device(sbi))
-		return NULL;
+	static_assert(MAX_DEVICES <= FSCRYPT_MAX_DEVICES);
 
-	devs = kmalloc_objs(*devs, sbi->s_ndevs);
-	if (!devs)
-		return ERR_PTR(-ENOMEM);
+	if (!f2fs_is_multi_device(sbi)) {
+		devs[0] = sb->s_bdev;
+		return 1;
+	}
+	ndevs = sbi->s_ndevs;
+	if (WARN_ON_ONCE(ndevs > FSCRYPT_MAX_DEVICES))
+		ndevs = FSCRYPT_MAX_DEVICES;
 
-	for (i = 0; i < sbi->s_ndevs; i++)
+	for (i = 0; i < ndevs; i++)
 		devs[i] = FDEV(i).bdev;
-	*num_devs = sbi->s_ndevs;
-	return devs;
+	return ndevs;
 }
 
 static const struct fscrypt_operations f2fs_cryptops = {
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,