fix(falco_service): falco service needs to write under /sys/module/falco

[Andreagit97]

What type of PR is this?

/kind bug

Any specific area of the project related to this PR?

/area build

What this PR does / why we need it:

Trying to run the latest Falco deb/rpm package there is an issue:

Since this PR #2214, Falco service needs to write under /sys/module/falco/parameters/g_buffer_bytes_dim the variable buffer dimension. The problem is that right now the path /sys/module is mounted read-only so Falco will fail with Errno 30 in the attempt to write /sys/module/falco/parameters/g_buffer_bytes_dim:

Oct 07 23:41:26 ubuntu2204.localdomain falco[34605]: Falco version: 0.32.1-241+251b4d5 (x86_64)
Oct 07 23:41:26 ubuntu2204.localdomain falco[34605]: Fri Oct  7 23:41:26 2022: Falco version: 0.32.1-241+251b4d5 (x86_64)
Oct 07 23:41:26 ubuntu2204.localdomain falco[34605]: Fri Oct  7 23:41:26 2022: Falco initialized with configuration file: /etc/falco/falco.yaml
Oct 07 23:41:26 ubuntu2204.localdomain falco[34605]: Falco initialized with configuration file: /etc/falco/falco.yaml
Oct 07 23:41:26 ubuntu2204.localdomain falco[34605]: Loading rules from file /etc/falco/falco_rules.yaml
Oct 07 23:41:26 ubuntu2204.localdomain falco[34605]: Fri Oct  7 23:41:26 2022: Loading rules from file /etc/falco/falco_rules.yaml
Oct 07 23:41:26 ubuntu2204.localdomain falco[34605]: Loading rules from file /etc/falco/falco_rules.local.yaml
Oct 07 23:41:26 ubuntu2204.localdomain falco[34605]: Fri Oct  7 23:41:26 2022: Loading rules from file /etc/falco/falco_rules.local.yaml
Oct 07 23:41:26 ubuntu2204.localdomain falco[34605]: The chosen syscall buffer dimension is: 8388608 bytes (8 MBs)
Oct 07 23:41:26 ubuntu2204.localdomain falco[34605]: Fri Oct  7 23:41:26 2022: The chosen syscall buffer dimension is: 8388608 bytes (8 MBs)
Oct 07 23:41:26 ubuntu2204.localdomain falco[34605]: Fri Oct  7 23:41:26 2022: Starting health webserver with threadiness 6, listening on port 8765
Oct 07 23:41:26 ubuntu2204.localdomain falco[34605]: Starting health webserver with threadiness 6, listening on port 8765
Oct 07 23:41:26 ubuntu2204.localdomain falco[34605]: Fri Oct  7 23:41:26 2022: Enabled event sources: syscall
Oct 07 23:41:26 ubuntu2204.localdomain falco[34605]: Fri Oct  7 23:41:26 2022: Opening capture with Kernel module
Oct 07 23:41:26 ubuntu2204.localdomain falco[34605]: Enabled event sources: syscall
Oct 07 23:41:26 ubuntu2204.localdomain falco[34605]: Opening capture with Kernel module
Oct 07 23:41:26 ubuntu2204.localdomain falco[34605]: Trying to inject the Kernel module and opening the capture again...
Oct 07 23:41:26 ubuntu2204.localdomain falco[34605]: Fri Oct  7 23:41:26 2022: Trying to inject the Kernel module and opening the capture again...
Oct 07 23:41:26 ubuntu2204.localdomain falco[34605]: Error: unable to open '/sys/module/falco/parameters/g_buffer_bytes_dim': Errno 30. Please ensure the kernel module is already loaded.

Considering the falco service config:

[Unit]
Description=Falco: Container Native Runtime Security
Documentation=https://falco.org/docs/

[Service]
Type=simple
User=root
ExecStartPre=/sbin/modprobe falco
ExecStart=/usr/bin/falco --pidfile=/var/run/falco.pid
ExecStopPost=/sbin/rmmod falco
UMask=0077
TimeoutSec=30
RestartSec=15s
Restart=on-failure
PrivateTmp=true
NoNewPrivileges=yes
ProtectHome=read-only
ProtectSystem=full
ProtectKernelTunables=true
RestrictRealtime=true
RestrictAddressFamilies=~AF_PACKET

[Install]
WantedBy=multi-user.target

there are some solutions to this problem:

1. The most aggressive one is to put ProtectKernelTunables to false
2. [PROPOSED ONE] The cleanest one should be to inject the kmod in a separate unit and call the falco unit only when the path /sys/module/falco is already there, in this way the falco unit could set ReadWritePaths to only /sys/module/falco
3. The third solution sets ReadWritePaths to /sys/module because the falco subfolder is not yet created at startup-time and we cannot mkdir this directory into another unit because we cannot create folders under sys/module only the kernel can do that.

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

fix(falco_service): falco service needs to write under /sys/module/falco

[Andreagit97]

@leogr @FedeDP @happy-dude WDYT?

[Andreagit97]

PS: I forgot to mention that systemd fail at all when something in a ReadWritePath directive doesn't exist 👇
systemd/systemd#10972 (comment)

[Andreagit97]

The first commit propose the 3° solution

The third solution sets ReadWritePaths to /sys/module because the falco subfolder is not yet created at startup-time and we cannot mkdir this directory into another unit because we cannot create folders under sys/module only the kernel can do that.

The last commit propose the 2° solution that should be better 🤔

[PROPOSED ONE] The cleanest one should be to inject the kmod in a separate unit and call the falco unit only when the path /sys/module/falco is already there, in this way the falco unit could set ReadWritePaths to only /sys/module/falco

[happy-dude]

Does PermissionsStartOnly=true allow writing into /sys/module or is still mounted as read only?

While I don't love having multiple service units, I do appreciate the clarity that comes from splitting the modprobe tasks.
(I personally modify the service unit to remove the modprobe commands because I want to start the service in BPF mode.)

[FedeDP]

Hi @happy-dude !
I think PermissionsStartOnly=true is only useful for Pre/Post Exec, right?
In this case, it won't help because the write to /sys/module is being done by Falco itself (https://github.com/falcosecurity/libs/blob/master/userspace/libscap/engine/kmod/scap_kmod.c#L79).

Btw i am working on an improvements over the current falco systemd unit, to support bpf too. I will hopefully remember to tag you when the PR it's open :D

[FedeDP]

The changes LGTM. I'll wait some more feedback!

[jasondellaluce]

/milestone 0.33.0

[FedeDP]

See also #2242 .
I think this is safer for the imminent release though :)

[FedeDP]

/approve

[poiana]

LGTM label has been added.

Git tree hash: 43d9aa61422d96b75c563125b6151806a877ed1d

[jasondellaluce]

/approve

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Andreagit97, FedeDP, jasondellaluce

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