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

[Stefan Hajnoczi]

On Wed, Jul 13, 2011 at 11:12 AM, Andrew Griffiths
<address@hidden> wrote:
> Once you have code execution in the process, you can modify the others
> threads execution (if required) to execute your own code. With full
> capabilities, it would be trivial to escape from a chroot on a normal
> Linux kernel (grsecurity with appropriate kernel chroot restrictions
> enabled would reduce the avenues available for escaping.).
>
> I seem to recall other distro's handle thread privileges differently.

Hi Andrew,
I think what Chris meant is that libvirt does not use -runas at all.
It drops privileges (including initgroups(3)) itself *before* invoking
QEMU.  So I think his statement is simply that libvirt (commonly used
in KVM deployments) is not affected.

Stefan

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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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