linux-kernel - Re: [PATCH v2] tty: Permit some TIOCL_SETSEL modes without CAP_SYS

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Message-ID: <491f3df9de6593df8e70dbe77614b026@finder.org>
Date: Sat, 22 Feb 2025 13:07:50 -0800
From: Jared Finder <jared@...der.org>
To: Günther Noack <gnoack3000@...il.com>
Cc: hanno@...eck.de, kees@...nel.org, gnoack@...gle.com,
 gregkh@...uxfoundation.org, jannh@...gle.com, jirislaby@...nel.org,
 linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
 regressions@...ts.linux.dev, stable@...r.kernel.org
Subject: Re: [PATCH v2] tty: Permit some TIOCL_SETSEL modes without
 CAP_SYS_ADMIN

On 2025-02-20 16:10, Günther Noack wrote:
> 
> Jared, can you please confirm whether Emacs works now with this patch
> in the kernel?
> 
> I am asking this because I realized that the patch had a bug.  We are
> erring in the "secure" direction, but not all TIOCL_SELMOUSEREPORT
> invocations work without CAP_SYS_ADMIN.

I confirmed that Emacs worked fine with 6.14-rc1.  My understanding is 
that the Emacs process relies only on TIOCL_SELPOINTER which it needs to 
do to draw the mouse pointer after Emacs' redisplay.  It's fine for 
TIOCL_SELMOUSEREPORT to not work in an unpriviliged Emacs.

> If this specific selection mode is not needed by Emacs, I think *the
> best thing would be to keep it guarded by CAP_SYS_ADMIN, after all*.

This sounds good to me.

Reading over a documentation proposal for TIOCL_SELMOUSEREPORT 
(https://lkml.org/lkml/2020/7/6/249), I can not imagine how a userspace 
program that was not acting as the mouse daemon could successfully use 
SELMOUSEREPORT as the mouse daemon will be fighting with it.  Any 
legitimate setting of mouse state (for example, setting the mouse x/y 
coordinate) would need to be done with the mouse daemon in the loop, in 
which case the mouse daemon might as well send the message itself.

   -- MJF