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Date: Sun, 6 Mar 2011 11:54:27 +0100
From: Florian Zumbiehl <florz@...rz.de>
To: Solar Designer <solar@...nwall.com>
Cc: oss-security@...ts.openwall.com,
	"Steven M. Christey" <coley@...us.mitre.org>,
	Stefan Fritsch <sf@...itsch.de>, Jan Kaluza <jkaluza@...hat.com>,
	Paul Martin <pm@...ian.org>, Petr Uzel <petr.uzel@...e.cz>,
	Thomas Biege <thomas@...e.de>
Subject: Re: CVE Request -- logrotate -- nine issues

Hi,

> If a log file directory is writable by the service pseudo-user, and you
> make both the service and patched logrotate work with it safely, there's
> still the problem of how the sysadmin can access those logs safely.

I always suspected logfiles weren't there just to be rotated ...

> The admin could "su" to the service pseudo-user, but that allows the
> compromised service to attack the admin's terminal, accessing the fd via
> /proc or ptrace of a process such as "less" running under the "su"

/proc/<pid>/fd/? I think that doesn't work. I don't know the exact
semantics, but from experience there seem to be some more checks in place
than in a "remote-dup()". That doesn't change anything about ptrace()
of course ...

> session.  Only "su" itself is immune from such attacks (since it has its
> "dumpable" flag cleared); its child processes are not.  The attacker
> would be able to print control characters directly to the terminal fd,
> and to issue ioctl's on it, changing the terminal mode.  This might have
> a security impact worse than DoS.  Sebastian - you could want to comment
> on this (I recall your research).

DoS is obvious, worse than that I'd be highly interested in details.

> A more reliable solution (in terms of being safe from the attacks being
> discussed here even when the sysadmins don't specifically try to access
> log files safely) is simply to have services create log files before
> they drop root (and indeed to have the directory only writable by root).
> This is the case for all services that we currently have in Owl.  By the
> way, our syslogd runs as non-root, but it starts up as root and it
> creates its log files in /var/log if necessary - before dropping to the
> syslogd pseudo-user.  In my opinion, that's how it should be done.

That could be problematic with services that either generally create
log files dynamically or that implement some dynamic config reload that
is intended to retain current processing state/connections/whatever.
The latter could be solved by pre-creating log files before initiating
the reload, of course.

> If a service starts up as non-root right away (such as via "su" in its
> startup script), then there's also a problem with its pidfile.  It has
> to write the pidfile as non-root, and thus into a directory writable by
> the service pseudo-user (well, or rewrite a file in-place, which has
> its own issues).  However, the pidfile is likely to be accessed by other
> parts of the system - such as by startup scripts that use
> /etc/init.d/functions and by programs invoked from there (such as
> start-stop-daemon).  Such accesses are likely to be made as root, and
> the file contents are parsed by scripts/programs running as root.  If
> such parsing is not robust when faced with incorrectly formatted input,
> we may have a local root vulnerability.  And we almost certainly have
> DoS potential via spoofed PIDs (have another service killed) and via
> links to device files, like what I described above.

Well, _actually_ pid files should just be deprecated, given that PIDs
are only really usable for anything in the parent process ... ;-)

> On Fri, Mar 04, 2011 at 06:58:17PM +0100, Florian Zumbiehl wrote:
> [...]
> > it is planned to add a new config directive that allows to specify
> > the credentials to be used for manipulating specific sets of log files,
> > thus obviating the need for separate logrotate invocations but still
> > letting the kernel take care of separating privileges.
> 
> This sounds good to me, but it does not solve other problems I pointed
> out above.  Would you also add a similar option to start-stop-daemon and
> to the daemon() function in /etc/init.d/functions (or whatever a given

You mean for pid file accesses?

> As to services that a non-root user may want to run on their own, the
> user would not be able to use the new user-switching feature of logrotate
> anyway (no root access).  So the user will run an instance of logrotate
> under their account, which is already possible.

A similar problem could exist there with group-writable log directories
when multiple users are in the group.

> > I guess I don't really have much of an opinion on that. The vulnerabilities
> > should be fixed, and probably in a way that breaks existing setups as
> > little as possible, I don't really care which side is declared defective
> > and subsequently fixed in order to achieve that ;-)
> 
> I care because, in this case, I think that "which side is declared
> defective" affects whether the vulnerabilities are fixed for real or
> not.  Thus, one side (service packages) needs vulnerability fixes and
> the other (logrotate) may use some hardening (but that's tricky).

Yep, I see your point and thus now am of the opinion that the affected
service packages should be fixed.

However, I think that still #6 (shell injection) and #7 (logrotate
DoS with strange characters in file names) should be considered
vulnerabilities in logrotate: It would be reasonable to assume that you
can use user input that's a valid (slash-less) filename as a (part of a)
log file name (assuming that the program is running as the same user that
inspects and rotates the logs, so the log directory being writable by
the program would not be insecure per-se) without that file name being
interpreted by a shell or causing logrotate to stop functioning,
respectively.

> Maybe logrotate should simply refuse to run when the target directory is
> writable by other than the user running logrotate (typically root), with
> an option to accept the risk and force logrotate to run anyway.  Sure,
> this would break existing security-broken setups, but it would also
> force package maintainers to fix their packages in this respect. ;-)

Precisely for the latter reason it should be done ;-)

> I am sorry for the length of this message, yet I hope it helps.

Yes, thanks a lot--without it I'd probably have ignored the fact that
people do occasionally actually want to look at log files ;-)

Florian

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