Code: int simple_set_acl(struct inode *inode, struct posix_acl *acl, int type)
{
 	int error;
 
 	if (type == ACL_TYPE_ACCESS) {
		error = posix_acl_equiv_mode(acl, &inode->i_mode);
		if (error < 0)
			return 0;
		if (error == 0)
			acl = NULL;
 	}
 
 	inode->i_ctime = current_time(inode);
	set_cached_acl(inode, type, acl);
	return 0;
}

Vulnerability Type: +Priv 
CWE ID: 
Summary: The simple_set_acl function in fs/posix_acl.c in the Linux kernel before 4.9.6 preserves the setgid bit during a setxattr call involving a tmpfs filesystem, which allows local users to gain group privileges by leveraging the existence of a setgid program with restrictions on execute permissions.  NOTE: this vulnerability exists because of an incomplete fix for CVE-2016-7097.
Commit Message: tmpfs: clear S_ISGID when setting posix ACLs

This change was missed the tmpfs modification in In CVE-2016-7097
commit 073931017b49 ("posix_acl: Clear SGID bit when setting
file permissions")
It can test by xfstest generic/375, which failed to clear
setgid bit in the following test case on tmpfs:

  touch $testfile
  chown 100:100 $testfile
  chmod 2755 $testfile
  _runas -u 100 -g 101 -- setfacl -m u::rwx,g::rwx,o::rwx $testfile

Signed-off-by: Gu Zheng <guzheng1@huawei.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>