isc-dhcp-client denied by apparmor

I forgot to add that this is an up-to-date Ubuntu 20.04.2

Status changed to 'Confirmed' because the bug affects multiple users.

I have been doing Xenial -> Bionic -> Focal release upgrades and started running into this apparmor issue with dhclient after reaching Focal.

I don't know the root issue, and it doesn't appear to impact functionality (at least as far as I can tell on my hosts). However, I am currently dealing with this by adding the following to /etc/apparmor.d/local/sbin.dhclient:

    @{PROC}/[0-9]*/task/[0-9]*/comm rw,

Denying
  /proc/1095210/task/1095213/comm

prevents the task from introspecting (reading), and changing (write) the command text associated with the task. In this case it would appear one thread is attempting to change the comm of another thread in the process (this is generally allowed), see man 5 proc

Reading isn't considered a problem, writing is more interesting. A process may use it to provide more info, like this is a sandbox thread, renderer etc. But a compromised task could also use it to masquerade its comm as another task. It won't hide the task but could make it harder to kill or monitor if the user is looking for the default comm value.

Denying this is not directly related to lack of IPv4 (kernel wise it has no relation), but application behavior could be such that when it fails to access the comm field it handles the error poorly resulting in it failing to setup IPv4.

I would recommend a modification to the above rule, changing [0-9]* to @{pids}

  @{PROC}/@{pids}/task/[0-9]*/comm rw,

Does adding the above rule fix the IPv4 issue for you? From comment #3 I assume no, but maybe I am reading it wrong. Are there any other denials in your logs?

As for disabling AppArmor for dhclient? It is precisely the the type of service (network facing root service) I would not recommend disabling the apparmor profile for. Instead updating the profile is the way to go.

Okay adding the suggested rule

works for me. So it would seem dhclient is treating denied access to comm as a fatal error.

Interestingly I also had it throw a rejection for capability sys_module

[ 1645.480546] audit: type=1400 audit(1616847221.859:73): apparmor="DENIED" operation="capable" profile="/{,usr/}sbin/dhclient" pid=3380 comm="dhclient" capability=16 capname="sys_module"

which should not generally be required anymore. See https://bugs.launchpad.net/ubuntu/+source/isc-dhcp/+bug/1759032

To further elaborate on why dhclient is accessing the comm

$ pstree -at 3395
dhclient ens3
  ├─{isc-socket}
  ├─{isc-timer}
  └─{isc-worker0000}

where 3395 is the process. It has 3 additional threads and it is providing functional names for them.

Hi John,

Thank you for your detailed response and education on command text. I was able to use pstree to see the thread command text behavior change before and after the AppArmor rule addition. I feel better now that I understand what is going on!

Merge upstream https://gitlab.com/apparmor/apparmor/-/merge_requests/730

it will be part of the next apparmor point releases

#4 Does not work for me.

My /etc/apparmor.d/sbin.dhclient partly looks like this:

@{PROC}/[0-9]*/net/ r,
  @{PROC}/[0-9]*/net/** r,
# @{PROC}/[0-9]*/task/[0-9]*/comm rw,
  @{PROC}/@{pids}*/task/[0-9]*/comm rw,

Maybe I'm missing something?

Note to everyone watching this bug:

The file that John modified above is in the "extra profiles" section of the upstream AppArmor source repository. It may be found on an Ubuntu system at

    /usr/share/apparmor/extra-profiles/sbin.dhclient

and in jammy, it has his fix.

However, the isc-dhcp-client package provides its own separate profile, which is installed at

    /etc/apparmor.d/sbin.dhclient

and is quite different.

Most people are likely going to be using this latter one, as it is enabled by default. So they will not receive the benefit of John's fix. I've confirmed that the original "DENIED" messages still occur on jammy.

This bug was fixed in the package isc-dhcp - 4.4.1-2.3ubuntu4

---------------
isc-dhcp (4.4.1-2.3ubuntu4) kinetic; urgency=medium

  * d/apparmor/sbin.dhclient: fix apparmor="DENIED" errors (LP: #1918410)

 -- Lukas Märdian <email address hidden> Tue, 21 Jun 2022 12:21:09 +0200

Hello Anisse, or anyone else affected,

Accepted isc-dhcp into jammy-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/isc-dhcp/4.4.1-2.3ubuntu2.1 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested, what testing has been performed on the package and change the tag from verification-needed-jammy to verification-done-jammy. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed-jammy. In either case, without details of your testing we will not be able to proceed.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance for helping!

N.B. The updated package will be released to -updates after the bug(s) fixed by this package have been verified and the package has been in -proposed for a minimum of 7 days.

Hello Anisse, or anyone else affected,

Accepted isc-dhcp into impish-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/isc-dhcp/4.4.1-2.3ubuntu1.1 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested, what testing has been performed on the package and change the tag from verification-needed-impish to verification-done-impish. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed-impish. In either case, without details of your testing we will not be able to proceed.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance for helping!

N.B. The updated package will be released to -updates after the bug(s) fixed by this package have been verified and the package has been in -proposed for a minimum of 7 days.

Hello Anisse, or anyone else affected,

Accepted isc-dhcp into focal-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/isc-dhcp/4.4.1-2.1ubuntu5.20.04.3 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested, what testing has been performed on the package and change the tag from verification-needed-focal to verification-done-focal. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed-focal. In either case, without details of your testing we will not be able to proceed.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance for helping!

N.B. The updated package will be released to -updates after the bug(s) fixed by this package have been verified and the package has been in -proposed for a minimum of 7 days.

All autopkgtests for the newly accepted isc-dhcp (4.4.1-2.3ubuntu1.1) for impish have finished running.
The following regressions have been reported in tests triggered by the package:

network-manager/1.32.12-0ubuntu1 (ppc64el)
initramfs-tools/0.140ubuntu6.1 (amd64)
systemd/248.3-1ubuntu8.6 (amd64)

Please visit the excuses page listed below and investigate the failures, proceeding afterwards as per the StableReleaseUpdates policy regarding autopkgtest regressions [1].

https://people.canonical.com/~ubuntu-archive/proposed-migration/impish/update_excuses.html#isc-dhcp

[1] https://wiki.ubuntu.com/StableReleaseUpdates#Autopkgtest_Regressions

Thank you!

Autopkgtest regressions got resolved by re-triggering.

I've tested isc-dhcp 4.4.1-2.3ubuntu2.1 from jammy-proposed. Works as expected.

root@jj-dhcp:~# apt list isc-dhcp-client -a
Listing... Done
isc-dhcp-client/jammy-proposed,now 4.4.1-2.3ubuntu2.1 amd64 [installed]

I can see no 'dhclient' entries in dmesg after the initial "apparmor_parser -r /etc/apparmor.d/sbin.dhclient" reload (neither after 'netplan apply', nor after reboot):

$ dmesg | grep dhclient
[...]
[325421.280166] audit: type=1400 audit(1655986215.663:4563): apparmor="STATUS" operation="profile_replace" label="lxd-jj-dhcp_</var/snap/lxd/common/lxd>//&:lxd-jj-dhcp_<var-snap-lxd-common-lxd>:unconfined" name="/{,usr/}sbin/dhclient" pid=1553222 comm="apparmor_parser"

dynamic IP address is assigned via NetworkManager:
$ ip addr show eth0
[...]
174: eth0@if175: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 00:16:3e:2f:0b:19 brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet 10.238.94.79/24 brd 10.238.94.255 scope global dynamic noprefixroute eth0
       valid_lft 3596sec preferred_lft 3596sec

$ nmcli -o
eth0: connected to netplan-eth0
        "eth0"
        ethernet (veth), 00:16:3E:2F:0B:19, sw, mtu 1500
        ip4 default
        inet4 10.238.94.79/24
[...]

I've tested isc-dhcp 4.4.1-2.1ubuntu5.20.04.3 from focal-proposed. Works as expected.

root@ff-dhcp:~# apt list isc-dhcp-client
Listing... Done
isc-dhcp-client/focal-proposed,now 4.4.1-2.1ubuntu5.20.04.3 amd64 [installed,automatic]

I can see no 'dhclient' entries in dmesg after the initial "apparmor_parser -r /etc/apparmor.d/sbin.dhclient" reload (neither after 'netplan apply', nor after reboot):

$ dmesg | grep dhclient
[...]
[327577.704123] audit: type=1400 audit(1655988372.103:4665): apparmor="STATUS" operation="profile_replace" label="lxd-ff-dhcp_</var/snap/lxd/common/lxd>//&:lxd-ff-dhcp_<var-snap-lxd-common-lxd>:unconfined" name="/{,usr/}sbin/dhclient" pid=1595322 comm="apparmor_parser"

dynamic IP address is assigned via NetworkManager:
$ ip addr show eth0
[...]
178: eth0@if179: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 00:16:3e:0a:83:de brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet 10.238.94.241/24 brd 10.238.94.255 scope global dynamic noprefixroute eth0
       valid_lft 3509sec preferred_lft 3509sec

$ nmcli -o
eth0: connected to netplan-eth0
        "eth0"
        ethernet (veth), 00:16:3E:0A:83:DE, sw, mtu 1500
        ip4 default, ip6 default
        inet4 10.238.94.241/24
[...]

I've tested isc-dhcp 4.4.1-2.3ubuntu1.1 from impish-proposed. Works as expected.

root@ii-dhcp:~# apt list isc-dhcp-client
Listing... Done
isc-dhcp-client/impish-proposed,now 4.4.1-2.3ubuntu1.1 amd64 [installed]

I can see no 'dhclient' entries in dmesg after the initial "apparmor_parser -r /etc/apparmor.d/sbin.dhclient" reload (neither after 'netplan apply', nor after reboot):

$ dmesg | grep dhclient
[...]
[328094.549824] audit: type=1400 audit(1655988888.955:4760): apparmor="STATUS" operation="profile_load" label="lxd-ii-dhcp_</var/snap/lxd/common/lxd>//&:lxd-ii-dhcp_<var-snap-lxd-common-lxd>:unconfined" name="/{,usr/}sbin/dhclient" pid=1609944 comm="apparmor_parser"

dynamic IP address is assigned via NetworkManager:
$ ip addr show eth0
[...]
182: eth0@if183: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 00:16:3e:e1:06:8a brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet 10.238.94.149/24 brd 10.238.94.255 scope global dynamic noprefixroute eth0
       valid_lft 3589sec preferred_lft 3589sec

$ nmcli -o
eth0: connected to netplan-eth0
        "eth0"
        ethernet (veth), 00:16:3E:E1:06:8A, sw, mtu 1500
        ip4 default, ip6 default
        inet4 10.238.94.149/24
[...]

This bug was fixed in the package isc-dhcp - 4.4.1-2.3ubuntu2.1

---------------
isc-dhcp (4.4.1-2.3ubuntu2.1) jammy; urgency=medium

  * d/apparmor/sbin.dhclient: fix apparmor="DENIED" errors (LP: #1918410)

 -- Lukas Märdian <email address hidden> Tue, 21 Jun 2022 12:39:11 +0200

The verification of the Stable Release Update for isc-dhcp has completed successfully and the package is now being released to -updates. Subsequently, the Ubuntu Stable Release Updates Team is being unsubscribed and will not receive messages about this bug report. In the event that you encounter a regression using the package from -updates please report a new bug using ubuntu-bug and tag the bug report regression-update so we can easily find any regressions.

This bug was fixed in the package isc-dhcp - 4.4.1-2.3ubuntu1.1

---------------
isc-dhcp (4.4.1-2.3ubuntu1.1) impish; urgency=medium

  * d/apparmor/sbin.dhclient: fix apparmor="DENIED" errors (LP: #1918410)

 -- Lukas Märdian <email address hidden> Tue, 21 Jun 2022 12:42:51 +0200

This bug was fixed in the package isc-dhcp - 4.4.1-2.1ubuntu5.20.04.3

---------------
isc-dhcp (4.4.1-2.1ubuntu5.20.04.3) focal; urgency=medium

  * d/apparmor/sbin.dhclient: fix apparmor="DENIED" errors (LP: #1918410)

 -- Lukas Märdian <email address hidden> Tue, 21 Jun 2022 12:44:45 +0200