I think there's two issues at play here. The hooks we added for module loading/unloading as part of USN-4355-1 simply check if the client has an AppArmor label that looks like it belongs to a snap and denies access if found. This will also deny access to classic snaps, which is probably a mistake.
The race condition you've encountered is probably a case of "policy module not in effect" vs. "policy module in effect" rather than a race in the behaviour of the policy module itself. This probably indicates that Pulse is servicing client requests before it has completely started. For the first issue, we can make the hook request info about the snap and allow access to classic snaps. For the second, I think we just need to load module-snap-policy earlier during start up. -- You received this bug notification because you are a member of Desktop Packages, which is subscribed to pulseaudio in Ubuntu. https://bugs.launchpad.net/bugs/1886854 Title: Race in load-module snap policy check in classic confinement Status in pulseaudio package in Ubuntu: New 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 To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/pulseaudio/+bug/1886854/+subscriptions -- Mailing list: https://launchpad.net/~desktop-packages Post to : desktop-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~desktop-packages More help : https://help.launchpad.net/ListHelp
