Code:  int fscrypt_process_policy(struct inode *inode,
 				const struct fscrypt_policy *policy)
 {
 	if (policy->version != 0)
 		return -EINVAL;
 
	if (!inode_has_encryption_context(inode)) {
		if (!inode->i_sb->s_cop->empty_dir)
			return -EOPNOTSUPP;
		if (!inode->i_sb->s_cop->empty_dir(inode))
			return -ENOTEMPTY;
		return create_encryption_context_from_policy(inode, policy);
	}

	if (is_encryption_context_consistent_with_policy(inode, policy))
		return 0;

	printk(KERN_WARNING "%s: Policy inconsistent with encryption context\n",
	       __func__);
	return -EINVAL;
}

Vulnerability Type: DoS 
CWE ID: CWE-264
Summary: A missing authorization check in the fscrypt_process_policy function in fs/crypto/policy.c in the ext4 and f2fs filesystem encryption support in the Linux kernel before 4.7.4 allows a user to assign an encryption policy to a directory owned by a different user, potentially creating a denial of service.
Commit Message: fscrypto: add authorization check for setting encryption policy

On an ext4 or f2fs filesystem with file encryption supported, a user
could set an encryption policy on any empty directory(*) to which they
had readonly access.  This is obviously problematic, since such a
directory might be owned by another user and the new encryption policy
would prevent that other user from creating files in their own directory
(for example).

Fix this by requiring inode_owner_or_capable() permission to set an
encryption policy.  This means that either the caller must own the file,
or the caller must have the capability CAP_FOWNER.

(*) Or also on any regular file, for f2fs v4.6 and later and ext4
    v4.8-rc1 and later; a separate bug fix is coming for that.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Cc: stable@vger.kernel.org # 4.1+; check fs/{ext4,f2fs}
Signed-off-by: Theodore Ts'o <tytso@mit.edu>