Code: static int __f2fs_set_acl(struct inode *inode, int type,
			struct posix_acl *acl, struct page *ipage)
{
	int name_index;
	void *value = NULL;
	size_t size = 0;
	int error;

	switch (type) {
 	case ACL_TYPE_ACCESS:
 		name_index = F2FS_XATTR_INDEX_POSIX_ACL_ACCESS;
 		if (acl) {
			error = posix_acl_equiv_mode(acl, &inode->i_mode);
			if (error < 0)
 				return error;
 			set_acl_inode(inode, inode->i_mode);
			if (error == 0)
				acl = NULL;
 		}
 		break;
 
	case ACL_TYPE_DEFAULT:
		name_index = F2FS_XATTR_INDEX_POSIX_ACL_DEFAULT;
		if (!S_ISDIR(inode->i_mode))
			return acl ? -EACCES : 0;
		break;

	default:
		return -EINVAL;
	}

	if (acl) {
		value = f2fs_acl_to_disk(acl, &size);
		if (IS_ERR(value)) {
			clear_inode_flag(inode, FI_ACL_MODE);
			return (int)PTR_ERR(value);
		}
	}

	error = f2fs_setxattr(inode, name_index, "", value, size, ipage, 0);

	kfree(value);
	if (!error)
		set_cached_acl(inode, type, acl);

	clear_inode_flag(inode, FI_ACL_MODE);
	return error;
}

Vulnerability Type: +Priv 
CWE ID: CWE-285
Summary: The filesystem implementation in the Linux kernel through 4.8.2 preserves the setgid bit during a setxattr call, which allows local users to gain group privileges by leveraging the existence of a setgid program with restrictions on execute permissions.
Commit Message: posix_acl: Clear SGID bit when setting file permissions

When file permissions are modified via chmod(2) and the user is not in
the owning group or capable of CAP_FSETID, the setgid bit is cleared in
inode_change_ok().  Setting a POSIX ACL via setxattr(2) sets the file
permissions as well as the new ACL, but doesn't clear the setgid bit in
a similar way; this allows to bypass the check in chmod(2).  Fix that.

References: CVE-2016-7097
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>