linux-kernel - Re: VT_PROCESS, VT_LOCKSWITCH capabilities

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Message-ID: <46B05817.5070305@benkstein.net>
Date:	Wed, 01 Aug 2007 11:53:27 +0200
From:	Frank Benkstein <frank-lkml@...kstein.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org
Subject: Re: VT_PROCESS, VT_LOCKSWITCH capabilities

Andrew Morton wrote:
> On Wed, 01 Aug 2007 00:22:38 +0200 Frank Benkstein <frank-lkml@...kstein.net> wrote:
> 
>> I wonder why there are different permissions needed for VT_PROCESS
>> (access to the current virtual console) and VT_LOCKSWITCH
>> (CAP_SYS_TTY_CONFIG).
>>
> Perhaps the issue with VT_LOCKSWITCH is that its effects will persist after
> the user has logged out?  So user A is effectively altering user B's
> console, hence suitable capabilities are needed?
> 
> Is the current code actually causing any observable problem?

Both controls can be used to deny service to other users.  For example:
user B locks his X session or current console and walks off to lunch.
User A walks up to user B's machine, switches to another console, logs
in and execs program_that_does_vt_process.  User B will not be able to
continue work unless he/she can get user A or someone with CAP_KILL to
kill the program.  If remote logins aren't allowed, the only way I see
to use the machine again is to reboot.

I think VT_PROCESS (or VT_SETMODE respectively) should be protected with
the same level of security as VT_LOCKSWITCH, i.e. CAP_SYS_TTY_CONFIG.
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