Bug 1906157 - after su from root to a normal user mesg is unable to show current status
Summary: after su from root to a normal user mesg is unable to show current status
Alias: None
Product: Red Hat Enterprise Linux 8
Classification: Red Hat
Component: util-linux
Version: 8.3
Hardware: Unspecified
OS: Unspecified
Target Milestone: rc
: 8.0
Assignee: Karel Zak
QA Contact: Radka Brychtova
Depends On:
Blocks: 2053536
TreeView+ depends on / blocked
Reported: 2020-12-09 18:59 UTC by Michele Casaburo
Modified: 2022-03-05 06:12 UTC (History)
7 users (show)

Fixed In Version: util-linux-2.32.1-28.el8
Doc Type: If docs needed, set a value
Doc Text:
Clone Of:
: 2053536 (view as bug list)
Last Closed: 2021-11-09 20:02:21 UTC
Type: Bug
Target Upstream Version:

Attachments (Terms of Use)

System ID Private Priority Status Summary Last Updated
Red Hat Knowledge Base (Solution) 6471561 0 None None None 2022-03-05 06:12:02 UTC
Red Hat Product Errata RHBA-2021:4490 0 None None None 2021-11-09 20:02:28 UTC

Internal Links: 2036583

Description Michele Casaburo 2020-12-09 18:59:49 UTC
Description of problem:

After "su" from root to a normal user mesg is unable to retrieve current permissions as it cannot open the pty

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

How reproducible:

Steps to Reproduce:
1. clean RHEL 8.3 install
2. login as root
3. create a normal user
# useradd -m testuser
3. su to the user
# su testuser
4. run mesg
$ mesg

Actual results:
$ mesg
mesg: cannot open /dev/pts/0: Permission denied

Expected results:
$ mesg
is y

Additional info:

Comment 2 Karel Zak 2021-02-23 09:10:43 UTC
Sure, this is expected behavior. su(1) changes the user and group ID, but it still uses the original terminal with the original permissions.

Since v2.31 su(1) provides a new command-line option --pty to create a pseudo-terminal for the session, but it does not change permissions for the pty device. It seems like a disadvantage that I can fix in the upstream tree. 

Anyway, won't fix for RHEL8.

Comment 4 Karel Zak 2021-04-12 08:04:54 UTC
I have investigated the problem more closely and it seems we can fix it :-)

I thought the problem is only the relation between terminal and mesg(1) permissions, but the issue is that the new mesg(1) always tries to open the terminal device. It seems unnecessary. All we need is to call stat(2) in this case.

Sorry for the previous CLOSE/UPSTREAM, reopening.

Fixed also in the upstream tree by commit https://github.com/karelzak/util-linux/commit/c0246ce059503bbc078122a50d564ca36a66f348

Comment 15 errata-xmlrpc 2021-11-09 20:02:21 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 (util-linux 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.


Comment 18 Karel Zak 2022-02-15 09:16:03 UTC
Please, see my answer for bug #2053536.

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