write says write: effective gid does not match group of /dev/pts/5

[Robert Dinse]

I am logged in as 'nanook', I am in the tty group:
id
uid=2000(nanook) gid=1000(shell) groups=1000(shell),4(adm),5(tty),24(cdrom),27(sudo),30(dip),46(plugdev),118(lpadmin),128(sambashare),135(lxd)
Yet when I say,
write nanook
I get
write: effective gid does not match group of /dev/pts/5
ls -l /dev/pts/5
crw--w---- 1 nanook tty 136, 5 May 2 18:51 /dev/pts/5
So something is wrong. And since it seems to involve permissions there may be security issues.

ProblemType: Bug
DistroRelease: Ubuntu 22.04
Package: util-linux 2.37.2-4ubuntu3.4
ProcVersionSignature: Ubuntu 5.15.0-102.112-generic 5.15.148
Uname: Linux 5.15.0-102-generic x86_64
ApportVersion: 2.20.11-0ubuntu82.5
Architecture: amd64
CasperMD5CheckResult: unknown
CurrentDesktop: MATE
Date: Thu May 2 18:47:55 2024
SourcePackage: util-linux
UpgradeStatus: No upgrade log present (probably fresh install)

[Seth Arnold (seth-arnold)]

Hello Robert, I believe this is intentional.

If you wish to restore the previous behavior, it should be sufficient to change /usr/bin/write.ul to root:tty 02755.

Thanks

[Alex Murray (alexmurray)]

For context, this change was introduced in https://ubuntu.com/security/notices/USN-6719-2

[Robert Dinse (nanook)]

This does not really fix security because one aspect of security is availability to legitimate users and this breaks the latter. Seems there are better ways to fix such as restricting what this can write to with apparmor? Or just fixing the coding so it does parse the input more carefully.