This bug was fixed in the package pulseaudio - 1:13.99.1-1ubuntu3.8
---------------
pulseaudio (1:13.99.1-1ubuntu3.8) focal-security; urgency=medium
* SECURITY UPDATE: don't rely on SCM_CREDENTIALS to detect snap confined
clients (LP: #1895928)
- d/p/0409-pa-client-peer-credentials.patch: drop patch
- d/p/0409-fix-arg-parsing-after-async-hook.patch: remains of old 0409
patch not related to pa_creds.
- d/p/0410-pa-client-peer-apparmor-label.patch: new patch, records
AppArmor label in pa_client struct for native connections using
aa_getpeercon.
- d/p/0702-add-snappy-policy-module.patch: use the AppArmor
label in the pa_client rather than looking it up via the process ID
from SCM_CREDENTIALS.
- CVE-2020-16123
* Don't block classic snaps from module loading/unloading (LP: #1886854)
- d/p/0702-add-snappy-policy-module.patch: replace
deny_to_snaps_hook with a version that allows classic snaps.
-- James Henstridge <james.henstri...@canonical.com> Thu, 15 Oct 2020
17:23:31 -0400
** Changed in: pulseaudio (Ubuntu)
Status: New => Fix Released
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https://bugs.launchpad.net/bugs/1886854
Title:
Race in load-module snap policy check in classic confinement
Status in pulseaudio package in Ubuntu:
Fix Released
Bug description:
SUMMARY
=========
When running a snap in classic confinement, that needs access to
PA_COMMAND_LOAD_MODULE and PA_COMMAND_UNLOAD_MODULE. These sometimes succeed
and sometimes fail with "Access denied".
After running "pacmd unload-module module-snap-policy" and unloading
the snap policy module, these work reliably.
I have verified this in a fresh install of Ubuntu 20.04 in a VM.
STEPS TO REPRODUCE
=========
a) Either build a snap with classic confinement that sends these commands on
the pulseaudio native protocol socket. (This is how I found the bug)
b) Or, what I did here to easier reproduce, abuse the sandbox of a random
classic snap:
Download the attached bug.tgz with a minimal reproducer. It contains
the source code for a program that sends load and unload commands to
pulse. Unfortunately `pacmd` has a pid-file check that fails inside
the sandbox and doesn't work. The reproducer does essentially the same
as "pacmd load/unload-module" though.
(a pre-compiled x64 binary is also included in case you don't have a
go compiler and dare to run an untrusted binary in a VM)
Unpack the tgz, build it, if necessary with "go mod download && go
build"
Grab a random classic mode snap to use its sandbox as a test bed:
$ sudo snap install atom --classic
atom 1.48.0 from Snapcrafters installed
Open a shell in its sandbox:
snap run --shell atom
Navigate to the compiled binary and execute it a few times:
user@user-Standard-PC-Q35-ICH9-2009:~/bug$ ./bug
2020/07/08 18:46:10 PulseAudio connection created successfully
2020/07/08 18:46:10 Couldn't load module, error message: PulseAudio
error: commandLoadModule -> Access denied
user@user-Standard-PC-Q35-ICH9-2009:~/bug$ ./bug
2020/07/08 18:46:11 PulseAudio connection created successfully
Loaded Module sucessfully at index: 40
user@user-Standard-PC-Q35-ICH9-2009:~/bug$ ./bug
2020/07/08 18:46:12 PulseAudio connection created successfully
Loaded Module sucessfully at index: 41
user@user-Standard-PC-Q35-ICH9-2009:~/bug$ ./bug
2020/07/08 18:46:12 PulseAudio connection created successfully
2020/07/08 18:46:12 Couldn't load module, error message: PulseAudio
error: commandLoadModule -> Access denied
user@user-Standard-PC-Q35-ICH9-2009:~/bug$ ./bug
2020/07/08 18:46:14 PulseAudio connection created successfully
2020/07/08 18:46:14 Couldn't load module, error message: PulseAudio
error: commandLoadModule -> Access denied
user@user-Standard-PC-Q35-ICH9-2009:~/bug$ ./bug
2020/07/08 18:46:14 PulseAudio connection created successfully
2020/07/08 18:46:14 Couldn't load module, error message: PulseAudio
error: commandLoadModule -> Access denied
user@user-Standard-PC-Q35-ICH9-2009:~/bug$ ./bug
2020/07/08 18:46:15 PulseAudio connection created successfully
2020/07/08 18:46:15 Couldn't load module, error message: PulseAudio
error: commandLoadModule -> Access denied
Succeeds and fails apparently at random.
Now from a non-sandboxed shell, run
pacmd unload-module module-snap-policy
to unload the snap-policy module from pulseaudio, now run ./bug a few
more times. It now succeeds reliably, every time.
Side note, with the real program on my actual machine, the race seems
to behave slightly differently. It seems not to work the first time an
application is started, but closing it and reopening it seems to make
it work pretty reliably afterwards. Restarting "snapd", causes the
following run the snap to fail again.
EXPECTED BEHAVIOUR
==================
The pulseaudio snap policy module should correctly determine and
enforce it's policy.
ACTUAL BEHAVIOUR
================
The pulseaudio snap policy module seemingly at random denies access
when the snap has the permissions to do an operation.
ADDITIONAL INFORMATION
======================
$ lsb_release -rd
Description: Ubuntu 20.04 LTS
Release: 20.04
$ apt-cache policy pulseaudio
pulseaudio:
Installed: 1:13.99.1-1ubuntu3.3
Candidate: 1:13.99.1-1ubuntu3.3
Version table:
*** 1:13.99.1-1ubuntu3.3 500
500 http://ch.archive.ubuntu.com/ubuntu focal-updates/main amd64
Packages
100 /var/lib/dpkg/status
1:13.99.1-1ubuntu3.2 500
500 http://security.ubuntu.com/ubuntu focal-security/main amd64
Packages
1:13.99.1-1ubuntu3 500
500 http://ch.archive.ubuntu.com/ubuntu focal/main amd64 Packages
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