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Bug 1919399 - SELinux is preventing /usr/lib/cups/daemon/cups-lpd from read access on the sock_file cups.sock
Summary: SELinux is preventing /usr/lib/cups/daemon/cups-lpd from read access on the s...
Keywords:
Status: CLOSED ERRATA
Alias: None
Product: Red Hat Enterprise Linux 8
Classification: Red Hat
Component: selinux-policy
Version: 8.3
Hardware: All
OS: Linux
medium
medium
Target Milestone: rc
: 8.5
Assignee: Zdenek Pytela
QA Contact: Milos Malik
URL:
Whiteboard:
Depends On:
Blocks: 1903942 1919401
TreeView+ depends on / blocked
 
Reported: 2021-01-22 19:18 UTC by Bryan Mason
Modified: 2024-12-20 19:33 UTC (History)
5 users (show)

Fixed In Version: selinux-policy-3.14.3-68.el8
Doc Type: No Doc Update
Doc Text:
Clone Of:
: 1919401 1947397 (view as bug list)
Environment:
Last Closed: 2021-11-09 19:42:29 UTC
Type: Bug
Target Upstream Version:
Embargoed:

Attachments (Terms of Use)


Links
System ID Private Priority Status Summary Last Updated
Red Hat Knowledge Base (Solution) 5729251 0 None None None 2021-01-22 20:17:45 UTC
Red Hat Product Errata RHBA-2021:4420 0 None None None 2021-11-09 19:42:47 UTC

Description Bryan Mason 2021-01-22 19:18:01 UTC 
Description of problem:

  SELinux prevents /usr/lib/cups/daemon/cups-lpd from reading
  /var/run/cups/cups.sock.  The message is:

    type=AVC msg=audit(1611337553.457:1827): avc: denied { read } for
    pid=19870 comm="cups-lpd" name="cups.sock" dev="tmpfs" ino=41315
    scontext=system_u:system_r:cupsd_lpd_t:s0
    tcontext=system_u:object_r:cupsd_var_run_t:s0
    tclass=sock_file permissive=0

  The existing policy allows getattr, open, write, and append, but not
  read:

    # sesearch --all | grep cupsd_lpd_t | grep cupsd_var_run_t
    allow cupsd_lpd_t cupsd_var_run_t:dir { getattr search open }; 
    allow cupsd_lpd_t cupsd_var_run_t:sock_file { write getattr append open };

  This prevents cups-lpd from initiating a connection with the local
  CUPS server (cupsd) over the domain socket /var/run/cups/cups.sock.

  As it turns out, cups-lpd recovers by initiating a network connection
  through localhost:631, so functionality isn't actually lost in the
  default CUPS configuration (which is configured to accept network
  connections on localhost), but it's still wrong.

Version-Release number of selected component (if applicable):

  selinux-policy-3.14.3-54.el8.noarch

How reproducible:

  100%

Steps to Reproduce:

 1. Install cups-lpd and enable/start cups-lpd.socket

    yum -y install cups-lpd
    systemctl enable cups-lpd.socket
    systemctl start cups-lpd.socket

 2. Create a test print queue:

    lpadmin -p test -E

 3. Use the CUPS lpd backend to print a file via LPD:

    DEVICE_URI=lpd://127.0.0.1/test \
    /usr/lib/cups/backend/lpd 1 user test 1 "" /etc/fstab
    
Actual results:

  File is processed, but SELinux denial is logged (as shown above) and
  connection is made through network layer.  The journal contains a
  record of this:

    cupsd[10227]: [Client 5] Accepted from localhost:55800 (IPv4)

Expected results:

  No SELinux denial.  Connection is made through the domain socket, as
  shown in the journal:

    cupsd[10227]: [Client 6] Accepted from localhost (Domain)

Additional info:

 * Creating and adding the following module fixed this in my testing:

    module cupslpd-read-cupssocket 1.0;
    
    require {
    	type cupsd_var_run_t;
    	type cupsd_lpd_t;
    	class sock_file read;
    }
    
    #============= cupsd_lpd_t ==============
    allow cupsd_lpd_t cupsd_var_run_t:sock_file read;

  After this, read access is allowed:

    # sesearch --allow | grep cupsd_lpd_t | grep cupsd_var_run_t
    allow cupsd_lpd_t cupsd_var_run_t:dir { getattr open search };
    allow cupsd_lpd_t cupsd_var_run_t:sock_file 
      { append getattr open read write };

 * This issue also exists in RHEL 7 (this is where it was originally
   reported), and Fedora 32.  The output below is from F32:

    # sesearch --allow | grep cupsd_lpd_t | grep cupsd_var_run_t
    allow cupsd_lpd_t cupsd_var_run_t:sock_file { append getattr open write };
Comment 1 Zdenek Pytela 2021-01-22 20:02:03 UTC 
This permission is not present in rawhide either.

Also note - although it is not this case - grep may filter out related rules where attributes are used so it's better use switches:

# sesearch -A -s cupsd_lpd_t -t cupsd_var_run_t -c sock_file
allow cupsd_lpd_t cupsd_var_run_t:sock_file { append getattr open write };

# sesearch -A -s cupsd_lpd_t -t cupsd_var_run_t -c dir
allow cupsd_lpd_t cupsd_var_run_t:dir { getattr open search };
allow nsswitch_domain pidfile:dir { getattr open search };
allow nsswitch_domain pidfile:dir { getattr open search }; [ authlogin_nsswitch_use_ldap ]:True
allow nsswitch_domain pidfile:dir { getattr open search }; [ nis_enabled ]:True
Comment 2 Milos Malik 2021-01-26 10:37:07 UTC 
Steps to Reproduce trigger the following SELinux denial:
----
type=PROCTITLE msg=audit(01/26/2021 05:35:20.011:960) : proctitle=/usr/lib/cups/daemon/cups-lpd 
type=SYSCALL msg=audit(01/26/2021 05:35:20.011:960) : arch=x86_64 syscall=access success=no exit=EACCES(Permission denied) a0=0x7f58c4cfd485 a1=R_OK a2=0x4 a3=0xffffffff items=0 ppid=1 pid=7625 auid=unset uid=lp gid=lp euid=lp suid=lp fsuid=lp egid=lp sgid=lp fsgid=lp tty=(none) ses=unset comm=cups-lpd exe=/usr/lib/cups/daemon/cups-lpd subj=system_u:system_r:cupsd_lpd_t:s0 key=(null) 
type=AVC msg=audit(01/26/2021 05:35:20.011:960) : avc:  denied  { read } for  pid=7625 comm=cups-lpd name=cups.sock dev="tmpfs" ino=51644 scontext=system_u:system_r:cupsd_lpd_t:s0 tcontext=system_u:object_r:cupsd_var_run_t:s0 tclass=sock_file permissive=0 
----

# rpm -qa selinux\* cups\* | sort
cups-2.2.6-38.el8.x86_64
cups-client-2.2.6-38.el8.x86_64
cups-filesystem-2.2.6-38.el8.noarch
cups-filters-1.20.0-24.el8.x86_64
cups-filters-libs-1.20.0-24.el8.x86_64
cups-ipptool-2.2.6-38.el8.x86_64
cups-libs-2.2.6-38.el8.x86_64
cups-lpd-2.2.6-38.el8.x86_64
selinux-policy-3.14.3-60.el8.noarch
selinux-policy-targeted-3.14.3-60.el8.noarch
#
Comment 3 Milos Malik 2021-01-26 10:38:38 UTC 
The only SELinux denial which appears in permissive mode is:
----
type=PROCTITLE msg=audit(01/26/2021 05:37:42.592:965) : proctitle=/usr/lib/cups/daemon/cups-lpd 
type=SYSCALL msg=audit(01/26/2021 05:37:42.592:965) : arch=x86_64 syscall=access success=yes exit=0 a0=0x7f6f50cee485 a1=R_OK a2=0x4 a3=0xffffffff items=0 ppid=1 pid=7634 auid=unset uid=lp gid=lp euid=lp suid=lp fsuid=lp egid=lp sgid=lp fsgid=lp tty=(none) ses=unset comm=cups-lpd exe=/usr/lib/cups/daemon/cups-lpd subj=system_u:system_r:cupsd_lpd_t:s0 key=(null) 
type=AVC msg=audit(01/26/2021 05:37:42.592:965) : avc:  denied  { read } for  pid=7634 comm=cups-lpd name=cups.sock dev="tmpfs" ino=51644 scontext=system_u:system_r:cupsd_lpd_t:s0 tcontext=system_u:object_r:cupsd_var_run_t:s0 tclass=sock_file permissive=1 
----
Comment 6 Zdenek Pytela 2021-04-08 19:48:32 UTC 
I've submitted a Fedora PR to address the issue:
https://github.com/fedora-selinux/selinux-policy/pull/687
Comment 7 Zdenek Pytela 2021-04-09 18:45:24 UTC 
commit 82affb22d6a89d87345c0a6b8805aede11aaba2d (HEAD -> rawhide, upstream/rawhide, upstream-rw/rawhide, origin/rawhide, origin/HEAD)
Author: Zdenek Pytela <zpytela>
Date:   Thu Apr 8 21:44:11 2021 +0200

    Allow cups-lpd read its private runtime socket files

    For /usr/lib/cups/daemon/cups-lpd to be able to initiate a connection
    with the local CUPS server (cupsd) over the /var/run/cups/cups.sock
    domain socket file, the read permission is required.

    Resolves: rhbz#1919399
Comment 18 errata-xmlrpc 2021-11-09 19:42:29 UTC 
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (selinux-policy bug fix and enhancement update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2021:4420
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