ext4 crypto: allocate the right amount of memory for the on-disk symlink

Previously we were taking the required padding when allocating space
for the on-disk symlink.  This caused a buffer overrun which could
trigger a krenel crash when running fsstress.
Signed-off-by: NTheodore Ts'o <tytso@mit.edu>

fs/ext4/crypto_fname.c

@@ -262,8 +262,20 @@ u32 ext4_fname_crypto_round_up(u32 size, u32 blksize)
 	return ((size+blksize-1)/blksize)*blksize;
 }
 
-/**
- * ext4_fname_crypto_alloc_obuff() -
+unsigned ext4_fname_encrypted_size(struct inode *inode, u32 ilen)
+{
+	struct ext4_crypt_info *ci = EXT4_I(inode)->i_crypt_info;
+	int padding = 32;
+
+	if (ci)
+		padding = 4 << (ci->ci_flags & EXT4_POLICY_FLAGS_PAD_MASK);
+	if (ilen < EXT4_CRYPTO_BLOCK_SIZE)
+		ilen = EXT4_CRYPTO_BLOCK_SIZE;
+	return ext4_fname_crypto_round_up(ilen, padding);
+}
+
+/*
+ * ext4_fname_crypto_alloc_buffer() -
  *
  * Allocates an output buffer that is sufficient for the crypto operation
  * specified by the context and the direction.
@@ -271,15 +283,8 @@ u32 ext4_fname_crypto_round_up(u32 size, u32 blksize)
 int ext4_fname_crypto_alloc_buffer(struct inode *inode,
 				   u32 ilen, struct ext4_str *crypto_str)
 {
-	unsigned int olen;
-	int padding = 16;
-	struct ext4_crypt_info *ci = EXT4_I(inode)->i_crypt_info;
+	unsigned int olen = ext4_fname_encrypted_size(inode, ilen);
 
-	if (ci)
-		padding = 4 << (ci->ci_flags & EXT4_POLICY_FLAGS_PAD_MASK);
-	if (padding < EXT4_CRYPTO_BLOCK_SIZE)
-		padding = EXT4_CRYPTO_BLOCK_SIZE;
-	olen = ext4_fname_crypto_round_up(ilen, padding);
 	crypto_str->len = olen;
 	if (olen < EXT4_FNAME_CRYPTO_DIGEST_SIZE*2)
 		olen = EXT4_FNAME_CRYPTO_DIGEST_SIZE*2;

fs/ext4/ext4.h

@@ -2090,6 +2090,7 @@ static inline int ext4_sb_has_crypto(struct super_block *sb)
 /* crypto_fname.c */
 bool ext4_valid_filenames_enc_mode(uint32_t mode);
 u32 ext4_fname_crypto_round_up(u32 size, u32 blksize);
+unsigned ext4_fname_encrypted_size(struct inode *inode, u32 ilen);
 int ext4_fname_crypto_alloc_buffer(struct inode *inode,
 				   u32 ilen, struct ext4_str *crypto_str);
 int _ext4_fname_disk_to_usr(struct inode *inode,

fs/ext4/namei.c

@@ -3039,10 +3039,23 @@ static int ext4_symlink(struct inode *dir,
 
 	encryption_required = (ext4_encrypted_inode(dir) ||
 			       DUMMY_ENCRYPTION_ENABLED(EXT4_SB(dir->i_sb)));
-	if (encryption_required)
-		disk_link.len = encrypted_symlink_data_len(len) + 1;
-	if (disk_link.len > dir->i_sb->s_blocksize)
-		return -ENAMETOOLONG;
+	if (encryption_required) {
+		err = ext4_get_encryption_info(dir);
+		if (err)
+			return err;
+		if (ext4_encryption_info(dir) == NULL)
+			return -EPERM;
+		disk_link.len = (ext4_fname_encrypted_size(dir, len) +
+				 sizeof(struct ext4_encrypted_symlink_data));
+		sd = kzalloc(disk_link.len, GFP_KERNEL);
+		if (!sd)
+			return -ENOMEM;
+	}
+
+	if (disk_link.len > dir->i_sb->s_blocksize) {
+		err = -ENAMETOOLONG;
+		goto err_free_sd;
+	}
 
 	dquot_initialize(dir);
 
@@ -3073,18 +3086,14 @@ static int ext4_symlink(struct inode *dir,
 	if (IS_ERR(inode)) {
 		if (handle)
 			ext4_journal_stop(handle);
-		return PTR_ERR(inode);
+		err = PTR_ERR(inode);
+		goto err_free_sd;
 	}
 
 	if (encryption_required) {
 		struct qstr istr;
 		struct ext4_str ostr;
 
-		sd = kzalloc(disk_link.len, GFP_NOFS);
-		if (!sd) {
-			err = -ENOMEM;
-			goto err_drop_inode;
-		}
 		istr.name = (const unsigned char *) symname;
 		istr.len = len;
 		ostr.name = sd->encrypted_path;
@@ -3156,10 +3165,11 @@ static int ext4_symlink(struct inode *dir,
 err_drop_inode:
 	if (handle)
 		ext4_journal_stop(handle);
-	kfree(sd);
 	clear_nlink(inode);
 	unlock_new_inode(inode);
 	iput(inode);
+err_free_sd:
+	kfree(sd);
 	return err;
 }