Code: open_log_file(const char *name, const char *prog, const char *namespace, const char *instance)
{
	char *file_name;

	if (log_file) {
		fclose(log_file);
		log_file = NULL;
	}

	if (!name)
		return;
 
 	file_name = make_file_name(name, prog, namespace, instance);
 
	log_file = fopen(file_name, "a");
 	if (log_file) {
 		int n = fileno(log_file);
 		fcntl(n, F_SETFD, FD_CLOEXEC | fcntl(n, F_GETFD));
		fcntl(n, F_SETFL, O_NONBLOCK | fcntl(n, F_GETFL));
	}

	FREE(file_name);
}

Vulnerability Type: 
CWE ID: CWE-59
Summary: keepalived 2.0.8 didn't check for pathnames with symlinks when writing data to a temporary file upon a call to PrintData or PrintStats. This allowed local users to overwrite arbitrary files if fs.protected_symlinks is set to 0, as demonstrated by a symlink from /tmp/keepalived.data or /tmp/keepalived.stats to /etc/passwd.
Commit Message: When opening files for write, ensure they aren't symbolic links

Issue #1048 identified that if, for example, a non privileged user
created a symbolic link from /etc/keepalvied.data to /etc/passwd,
writing to /etc/keepalived.data (which could be invoked via DBus)
would cause /etc/passwd to be overwritten.

This commit stops keepalived writing to pathnames where the ultimate
component is a symbolic link, by setting O_NOFOLLOW whenever opening
a file for writing.

This might break some setups, where, for example, /etc/keepalived.data
was a symbolic link to /home/fred/keepalived.data. If this was the case,
instead create a symbolic link from /home/fred/keepalived.data to
/tmp/keepalived.data, so that the file is still accessible via
/home/fred/keepalived.data.

There doesn't appear to be a way around this backward incompatibility,
since even checking if the pathname is a symbolic link prior to opening
for writing would create a race condition.

Signed-off-by: Quentin Armitage <quentin@armitage.org.uk>