os-prober exits prematurely with "logger: socket /dev/log: Protocol wrong type for socket"

[Dara Poon]

Failure occurs on Ubuntu 16.04 with the apparmor-profiles-2.10.95-0ubuntu2.10 package installed.

Running update-grub will run /usr/bin/os-prober, which spews about a dozen of the following line to stderr:

    logger: socket /dev/log: Protocol wrong type for socket

… but fails to report the existence of some installed operating systems as expected.

Furthermore, /var/log/messages contains:

    kernel: audit: type=1400 audit(1556043066.679:11460): apparmor="ALLOWED" operation="sendmsg" info="Failed name lookup - disconnected path" error=-13 profile="syslog-ng" name="dev/log" pid=28566 comm="logger" requested_mask="r" denied_mask="r" fsuid=0 ouid=0

Here is a stripped-down skeleton of the /usr/bin/os-prober script, which demonstrates the problem:

    #!/bin/sh
    set -e -x

    newns () {
      [ "$OS_PROBER_NEWNS" ] || exec /usr/lib/os-prober/newns "$0" "$@"
    }

    log() {
      logger -t "$(basename "$0")" "$@"
    }

    debug() {
      log "debug: $@"
    }

    ls -l /dev/log
    debug "Hello world"
    newns "$@"

The expected behavior is that it should write "debug: os-prober-testcase Hello world" to /var/log/messages twice. However, it only succeeds in writing "Hello world" once. After the script respawns itself with /usr/lib/os-prober/newns (which is like `unshare -m`), the second attempt to write to /dev/log fails as described above.

Since the os-prober Bash script runs with the -e flag, any error, even just a logging error, causes the script to terminate prematurely. (Arguably, the log() function should call `logger -t "$(basename "$0")" "$@" || :` so that logging failures aren't fatal.)

The fix, for me, is to edit /etc/apparmor.d/sbin.syslog-ng, and change

    profile syslog-ng /{usr/,}sbin/syslog-ng flags=(complain) {\
      …
    }

to

    profile syslog-ng /{usr/,}sbin/syslog-ng flags=(complain,attach_disconnected) {
      …
    }

… then run `aa-complain sbin.syslog-ng` and `service syslog-ng restart`, before running update-grub again. I assume that similar fixes would be required for the other logging daemons.

[Dimitri John Ledkov (xnox)]

yeah i get apparmor="DENIED" info="Failed name loookup - disconnected path", which breaks os-prober.

[Andreas Hasenack (ahasenack)]

I'll try to reproduce it and amend the profile as needed.

[Dimitri John Ledkov (xnox)]

Steps i can reproduce:

wget https://bugs.launchpad.net/ubuntu/+source/os-prober/+bug/1826294/+attachment/5652492/+files/reproducer.sh

chmod +x reproducer.sh

sudo journalctl -f -e &

clear

sudo ./reproducer.sh

[Dimitri John Ledkov (xnox)]

This is Ubuntu desktop install. rsyslog.service is active and running, systemd-journald-dev-log.socket is running.

[John Johansen (jjohansen)]

The syslog-ng profile needs flags=(attach_disconnected) added to it

[Andreas Hasenack (ahasenack)]

Sorry, works just fine in this lunar vm here:

# id; logger before; ./reproducer.sh ; logger after
uid=0(root) gid=0(root) groups=0(root)
+ set -e
+ . /usr/share/os-prober/common.sh
+ cleanup_tmpdir=false
+ progname=
+ type mapdevfs
+ newns
+ [ ]
+ exec /usr/lib/os-prober/newns ./reproducer.sh
+ set -e
+ . /usr/share/os-prober/common.sh
+ cleanup_tmpdir=false
+ progname=
+ type mapdevfs
+ newns
+ [ 1 ]
+ log hello
+ cache_progname
+ progname=reproducer.sh
+ logger -t reproducer.sh hello

journal:
Mar 07 18:25:21 l-rsyslog-osprober root[2073]: before
Mar 07 18:25:21 l-rsyslog-osprober reproducer.sh[2075]: hello
Mar 07 18:25:21 l-rsyslog-osprober root[2076]: after

# uname -r
6.1.0-16-generic
# ps axwZ|grep rsyslog|grep -v grep
rsyslogd (enforce) 687 ? Ssl 0:00 /usr/sbin/rsyslogd -n -iNONE

systemd-journald-dev-log.socket is also running.

But I tried on amd64. Trying on arm64 will take more time.

[Launchpad Janitor (janitor)]

This bug was fixed in the package os-prober - 1.81ubuntu2

---------------
os-prober (1.81ubuntu2) lunar; urgency=medium

  * Ignore logger socket-errors to workaround apparmor denials. (LP: #1826294)

 -- Dimitri John Ledkov <email address hidden> Tue, 07 Mar 2023 13:28:14 +0000

[Dimitri John Ledkov (xnox)]

> The syslog-ng profile needs flags=(attach_disconnected) added to it

I failed to reproduce this with syslog-ng, but i guess i didn't configure syslog-ng correctly to attempt attaching to /dev/log

I am also not sure of os-prober usage of logger is correct, and if it actually wants to use that all, or if it should simply use journald only, or nothing at all.

Reading attach_disconnected sounds scary, i'm not sure what os-prober can do better here. bind-mount /dev/log socket into it's new mount namespace?

[Andreas Hasenack (ahasenack)]

I'm not really looking into syslog-ng. The apparmor profile change I did was to rsyslog, and that's where I tried to reproduce this issue.