[Qemu-devel] [Bug 807893] Re: qemu privilege escalation

[Andrew Griffiths]

Actually, from a quick google perhaps ensuring all threads run after
chroot / dropping privileges might be a good idea.

- http://wiki.freebsd.org/Per-Thread%20Credentials
- http://www.cocoabuilder.com/archive/cocoa/33107-cthread-fork.html

though it looks like you might need to put in effort into getting per-
thread uid's for freebsd/macosx when they make that available, and
you're assuming they're running a recent glibc. Depending on complexity,
it can't hurt to ensure you're not going to hit into per-thread
uid/gid's. I'm of two minds about glibc doing this. This was a
particular favourite bug class of mine :)

It seems that there is a linux distro which uses uclibc, which does not
emulate the glibc behaviour:

http://dl-4.alpinelinux.org/alpine/v2.2/main/x86/  <-- has qemu
packages.

we can use http://paste.pocoo.org/raw/438497/ to emulate qemu's
behaviour

# ./test
[main] my [ug]id is 100/100
[thread] my [ug]id is 0/0

^-- the qemu thread would be running as root

running the same code under glibc (without grsecurity chroot_findtask),
and it will drop privileges as you'd expect on recent glibc.

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https://bugs.launchpad.net/bugs/807893

Title:
  qemu privilege escalation

Status in QEMU:
  Confirmed

Bug description:
  If qemu is started as root, with -runas, the extra groups is not
  dropped correctly

  /proc/`pidof qemu`/status
  ..
  Uid:    100     100     100     100
  Gid:    100     100     100     100
  FDSize: 32
  Groups: 0 1 2 3 4 6 10 11 26 27 
  ...

  The fix is to add initgroups() or setgroups(1, [gid]) where
  appropriate to os-posix.c.

  The extra gid's allow read or write access to other files (such as
  /dev etc).

  Emulating the qemu code:

  # python
  ...
  >>> import os
  >>> os.setgid(100)
  >>> os.setuid(100)
  >>> os.execve("/bin/sh", [ "/bin/sh" ], os.environ)
  sh-4.1$ xxd /dev/sda | head -n2
  0000000: eb48 9000 0000 0000 0000 0000 0000 0000  .H..............
  0000010: 0000 0000 0000 0000 0000 0000 0000 0000  ................
  sh-4.1$ ls -l /dev/sda
  brw-rw---- 1 root disk 8, 0 Jul  8 11:54 /dev/sda
  sh-4.1$ id
  uid=100(qemu00) gid=100(users) 
groups=100(users),0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel),11(floppy),26(tape),27(video)

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