fscrypt: clean up some BUG_ON()s in block encryption/decryption

Replace some BUG_ON()s with WARN_ON_ONCE() and returning an error code,
and move the check for len divisible by FS_CRYPTO_BLOCK_SIZE into
fscrypt_crypt_block() so that it's done for both encryption and
decryption, not just encryption.
Reviewed-by: Chandan Rajendra <chandan@linux.ibm.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs/crypto/crypto.c

@@ -162,7 +162,10 @@ int fscrypt_crypt_block(const struct inode *inode, fscrypt_direction_t rw,
 	struct crypto_skcipher *tfm = ci->ci_ctfm;
 	int res = 0;
 
-	BUG_ON(len == 0);
+	if (WARN_ON_ONCE(len <= 0))
+		return -EINVAL;
+	if (WARN_ON_ONCE(len % FS_CRYPTO_BLOCK_SIZE != 0))
+		return -EINVAL;
 
 	fscrypt_generate_iv(&iv, lblk_num, ci);
 
@@ -225,8 +228,6 @@ struct page *fscrypt_encrypt_page(const struct inode *inode,
 	struct page *ciphertext_page = page;
 	int err;
 
-	BUG_ON(len % FS_CRYPTO_BLOCK_SIZE != 0);
-
 	if (inode->i_sb->s_cop->flags & FS_CFLG_OWN_PAGES) {
 		/* with inplace-encryption we just encrypt the page */
 		err = fscrypt_crypt_block(inode, FS_ENCRYPT, lblk_num, page,
@@ -238,7 +239,8 @@ struct page *fscrypt_encrypt_page(const struct inode *inode,
 		return ciphertext_page;
 	}
 
-	BUG_ON(!PageLocked(page));
+	if (WARN_ON_ONCE(!PageLocked(page)))
+		return ERR_PTR(-EINVAL);
 
 	/* The encryption operation will require a bounce page. */
 	ciphertext_page = fscrypt_alloc_bounce_page(gfp_flags);
@@ -275,8 +277,9 @@ EXPORT_SYMBOL(fscrypt_encrypt_page);
 int fscrypt_decrypt_page(const struct inode *inode, struct page *page,
 			unsigned int len, unsigned int offs, u64 lblk_num)
 {
-	if (!(inode->i_sb->s_cop->flags & FS_CFLG_OWN_PAGES))
-		BUG_ON(!PageLocked(page));
+	if (WARN_ON_ONCE(!PageLocked(page) &&
+			 !(inode->i_sb->s_cop->flags & FS_CFLG_OWN_PAGES)))
+		return -EINVAL;
 
 	return fscrypt_crypt_block(inode, FS_DECRYPT, lblk_num, page, page,
 				   len, offs, GFP_NOFS);