samba profile: missing rule for mkdir /var/cache/samba/printing

[ Impact ]

Users who chose to:

a) install apparmor-profiles (a package with extra optional apparmor profiles, including samba)

b) change the samba related profiles from complain (the default) to enforce mode

will find out that sharing a printing in samba and using it won't work.

In by itself this is *definitely* not worth an SRU for apparmor, which impacts all users of Ubuntu (because it's installed everywhere). But, if apparmor is to be updated for another more important reason, then this fix could be bundled together with it. Therefore I'm adding the block-proposed-kinetic tag to this bug.

[ Test Plan ]

sudo apt install apparmor-profiles apparmor-utils apparmor-profiles-extra
sudo apt install samba smbclient cups cups-client

Set a password for the samba "root" user:
printf "root\nroot\n" | sudo smbpasswd -a root

Create a fake printer:
sudo lpadmin -p testprinter -E -v /dev/null

Check it's there:
sudo lpstat -l -p testprinter

Probe it via samba:
rpcclient -Uroot%root localhost -c 'getprinter testprinter 2'
(some printer related output, or even an error, doesn't matter)

Check dmesg and look for an apparmor ALLOWED message:
[497031.827841] audit: type=1400 audit(1669215188.733:555): apparmor="ALLOWED" operation="mkdir" class="file" namespace="root//lxd-l-samba-apparmor_<var-snap-lxd-common-lxd>" profile="samba-rpcd-spoolss" name="/var/cache/samba/printing/" pid=388168 comm="rpcd_spoolss" requested_mask="c" denied_mask="c" fsuid=1000000 ouid=1000000

With the updated package, there should be no apparmor message for samba-rpcd-spoolss.

NOTE: since, for this test, we are not switching the apparmor profile to enforce mode, this means that the mkdir attempted by rpcd_spoolss will succeed, and if you try the rpcclient command one more time, there will be no further apparmor messages about it in the logs.

[ Where problems could occur ]

This change is adding an apparmor rule to a samba-related apparmor profile. Without this rule (and with the apparmor profile in confine mode), then printing does not work, so regressing that aspect of it is hard.

Maybe some exotic future security vulnerability could take advantage of this new apparmor rule which allows writing to (and therefore deleting from) /var/cache/samba/printing.

What's more likely perhaps (but still rare) is that an apparmor upgrade, which triggers all apparmor profiles to be reloaded, would find some error in an existing profile and fail to load it, and perhaps stop loading all other profiles after that, perhaps leaving the system without confinement. But this should be caught by the upgrade process since postinst would exit non-zero (hopefully).

[ Other Info ]
Not at this time.

[Original Description]

After the fix for bug #1990692, one more rule is needed it seems.

I put all samba profiles in enforce mode, and when I ran that final rpcclient command, got an error and an apparmor denied message:

Prep:
sudo apt install apparmor-profiles apparmor-utils apparmor-profiles-extra
sudo apt install samba smbclient cups cups-client

Set a password for the samba "root" user:
printf "root\nroot\n" | sudo smbpasswd -a root

Create a fake printer:
sudo lpadmin -p testprinter -E -v /dev/null

Check it's there:
sudo lpstat -l -p testprinter

$ rpcclient -Uroot%root localhost -c 'getprinter testprinter 2'
cli_rpc_pipe_open_noauth: rpc_pipe_bind for pipe spoolss failed with error NT_STATUS_CONNECTION_DISCONNECTED
do_cmd: Could not initialise spoolss. Error was NT_STATUS_CONNECTION_DISCONNECTED

[qua out 19 14:42:36 2022] audit: type=1400 audit(1666201357.627:342): apparmor="DENIED" operation="mkdir" class="file" namespace="root//lxd-k-samba-apparmor_<var-snap-lxd-common-lxd>" profile="samba-rpcd-spoolss" name="/var/cache/samba/printing/" pid=129107 comm="rpcd_spoolss" requested_mask="c" denied_mask="c" fsuid=1000000 ouid=1000000

And indeed, that directory wasn't created:
$ l /var/cache/samba/printing
ls: cannot access '/var/cache/samba/printing': No such file or directory
$ l /var/cache/samba/
total 16K
drwxr-xr-x 1 root root 48 Oct 19 17:42 .
drwxr-xr-x 1 root root 170 Oct 19 17:41 ..
-rw-r--r-- 1 root root 166 Oct 19 17:42 browse.dat
-rw-r--r-- 1 root root 8.7K Oct 19 17:42 smbprofile.tdb