On Fri, 30 Jun 2017 13:25:18 +0200 Axel Beckert wrote: > Hi, > > On Fri, Jun 30, 2017 at 02:50:48PM +0800, Clark Wang wrote: > > On Thu, Jun 29, 2017 at 7:52 PM, Axel Beckert <a...@deuxchevaux.org> wrote: > > > I know that behaviour of screen for ages and even saw people > > > recommending tmux because it doesn't seem to have this issue. > > > > > > > /dev/pts/14 is indeed not owned by user test, but this way screen > > > > have worked fine for as long as I can remember, so this access > > > > should not be mandatory. > > > > > > > > Any way to return the old behaviour? > > > > > > I'd rather be interested in how you got that working all these years. > > > :-) > > > > Just tried v4.5. /usr/bin/screen (rwxr-sr-x) does not work with su but > > /usr/local/bin/screen (rwsr-xr-x) I built from source works fine: > > > > # ls -Ll /usr/bin/screen /usr/local/bin/screen > > -rwxr-sr-x 1 root utmp 457608 2017-05-23 07:57 /usr/bin/screen > > -rwsr-xr-x 1 root staff 1441416 2017-01-19 13:59 /usr/local/bin/screen > > Thanks for the comparison. > > Ok, so the screen binary which is setuid root works and the one which > is not, doesn't. Sounds like a reason.
Setuid root is not necessary for this to work. In Gentoo I never had a problem with screen after su (except for 4.6.0 version). Looks like it works the following way: 1. screen is configured as follows: --with-pty-mode=0620 \ --with-pty-group=5 2. pty permissions are set to 0620 user:tty (tty gid is 5) using udev. screen binary is not SUID root for non-multiuser configurations: 2755 root:utmp for /usr/bin/screen 0775 root:utmp for /tmp/screen for utmp access If user chooses to enable multiuser support during package compilation, /usr/bin/screen perms are set to 4755, but I don't use that. > But I won't revert to setuid for the Debian package. In contrary, > Debian's screen package in the next stable release will contain, and also > already > 4.5.1 in the current Debian Testing and Unstable already contains > libutempter support to avoid issues like the privilege escalation in > 4.5.0. (While in 4.6.0 this doesn't seem to make a difference > anymore.) > > Kind regards, Axel Best regards, Andrew Savchenko
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