[gentoo-user] [NOTABUG] Re: Root can't write to files owned by others?

[Dr Rainer Woitok]

Greetings,

On Wednesday, 2022-03-09 19:28:49 +0100, I myself wrote:

> ...
> until recently my system behaves sort of strangely:
>
>    $ touch /tmp/file
>    $ ls -l /tmp/file
>    -rw------- 1 rainer rainer 0 2022-03-09 19:06 /tmp/file
>    $ echo x | sudo tee /tmp/file
>    Password:
>    tee: /tmp/file: Permission denied
>    x
>    $ chmod a+w /tmp/file
>    $ ls -l /tmp/file
>    -rw--w--w- 1 rainer rainer 0 2022-03-09 19:06 /tmp/file
>    $ echo x | sudo tee /tmp/file
>    tee: /tmp/file: Permission denied
>    x
>    $
>
> Since when can't root write to files  it doesn't own?   And not even, if
> the file has write permission for everybody?

Turns out it's not a bug but a feature,  more precisely a security feat-
ure.  Its purpose is  to prevent an unprivileged  local user from luring
user "root" into  writing possibly  sensitive information  to an already
existing file owned by and thus accessible to the local attacker.

This protection feature was introduced  as an option deactivated by def-
ault way back in some 4.* kernel,  and somewhere between kernels 5.15.19
and 5.15.26 the default was changed to on.

If you want or have to,  you can again disable  this feature  at runtime
using

   # sysctl fs.protected_regular=0

or permanently from the next boot onwards via

   # echo fs.protected_regular = 0 >> /etc/sysctl.d/local.conf

Personally, I've decided to again boot kernel 5.15.26,  to leave the new
default setting  (even on my laptop),  and to adapt  my script which had
problems with this new behaviour accordingly.

Your milage may vary.

Sincerely,
  Rainer