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Date: Tue, 22 Aug 2023 08:51:03 -0400
From: Boris Lukashev <>
Subject: Re: [PATCH] Restrict access to TIOCLINUX

See this asked a lot, and as someone who works mostly in languages with robust test facilities, I gotta ask: why isn't kernel code mandated to be submitted with tests?

The maple tree thing was kind of a mess and was a much bigger change than the stuff people push in these security machinations. With the pushback upstream gives on this stuff its no wonder that actual progress on kernsec is made by an out of tree third party. Upstream does not make it easy to improve boundaries and controls or establish coherent awareness.

Introducing tests and a public, expandable, well-defined threat model against which defensive measures are built would permit smaller/piecemeal efforts to cobble together a stronger overall posture (and automate regression testing for new features which are not necessarily seen as being in the security domain).


On August 22, 2023 8:07:24 AM EDT, Greg KH <> wrote:
>On Fri, Aug 18, 2023 at 06:10:18PM +0200, Günther Noack wrote:
>> Hello!
>> +CC the people involved in TIOCSTI
>> This patch seems sensible to me --
>> and I would like to kindly ask you to reconsider it.
>> On Sun, Apr 02, 2023 at 07:23:44PM +0200, Greg KH wrote:
>> > On Sun, Apr 02, 2023 at 07:16:52PM +0200, Hanno Böck wrote:
>> > > On Sun, 2 Apr 2023 16:55:01 +0200
>> > > Greg KH <> wrote:
>> > > 
>> > > > You just now broke any normal user programs that required this (or the
>> > > > other ioctls), and so you are going to have to force them to be run
>> > > > with CAP_SYS_ADMIN permissions? 
>> > > 
>> > > Are you aware of such normal user programs?
>> > > It was my impression that this is a relatively obscure feature and gpm
>> > > is pretty much the only tool using it.
>> > 
>> > "Pretty much" does not mean "none" :(
>> This patch only affects TIOCLINUX subcodes which are responsible for text
>> The only program that I am aware of which uses cut&paste on the console is gpm.
>> My web searches for these subcode names have only surfaced Linux header files
>> and discussions about their security problems.
>Is gpm running with the needed permissions already?
>> > > > And you didn't change anything for programs like gpm that already had
>> > > > root permission (and shouldn't that permission be dropped anyway?)
>> > > 
>> > > Well, you could restrict all that to a specific capability. However, it
>> > > is my understanding that the existing capability system is limited in
>> > > the number of capabilities and new ones should only be introduced in
>> > > rare cases. It does not seem a feature probably few people use anyway
>> > > deserves a new capability.
>> > 
>> > I did not suggest that a new capability be created for this, that would
>> > be an abust of the capability levels for sure.
>> > 
>> > > Do you have other proposals how to fix this issue? One could introduce
>> > > an option like for TIOCSTI that allows disabling selection features by
>> > > default.
>> > 
>> > What exact issue are you trying to fix here?
>> It's the same problem as with TIOCSTI, which got (optionally) disabled for
>> non-CAP_SYS_ADMIN in commit 83efeeeb3d04 ("tty: Allow TIOCSTI to be disabled")
>> and commit 690c8b804ad2 ("TIOCSTI: always enable for CAP_SYS_ADMIN").
>> The number of exploits which have used TIOCSTI in the past is long[1] and has
>> affected multiple sandboxing and sudo-like tools.  If the user is using the
>> console, TIOCLINUX's cut&paste functionality can replace TIOCSTI in these
>> exploits.
>> We have this problem with the Landlock LSM as well, with both TIOCSTI and these
>> TIOCLINUX subcodes.
>> Here is an example scenario:
>> * User runs a vulnerable version of the "ping" command from the console.
>Don't do that :)
>> * The "ping" command is a hardened version which puts itself into a Landlock
>>   sandbox, but it still has the TTY FD through stdout.
>> * Ping gets buffer-overflow-exploited by an attacker through ping responses.
>You allowed a root-permissioned program to accept unsolicted network
>code, why is it the kernel's issue here?
>> * The attacker can't directly access the file system, but the attacker can
>>   escape the sandbox by controlling the surrounding (non-sandboxed) shell on its
>>   terminal through TIOCLINUX.
>> The ping example is not completely made up -- FreeBSD had such a vulnerability
>> in its ping utility in 2022[2].  The impact of the vulnerability was mitigated
>> by FreeBSD's Capsicum sandboxing.
>> The correct solution for the problem on Linux is to my knowledge to create a
>> pty/tty pair, but that is somewhat impractical for small utilities like ping, in
>> order to restrict themselves (they would need to create a sidecar process to
>> shovel the data back and forth).  Workarounds include setsid() and seccomp-bpf,
>> but they also have their limits and are not a clean solution.  We've previously
>> discussed it in [3].
>> I do believe that requiring CAP_SYS_ADMIN for TIOCLINUX's TIOCL_PASTESEL subcode
>> would be a better approach than so many sudo-style and sandboxing tools having
>> to learn this lesson the hard way.  Can we please reconsider this patch?
>Have you verified that nothing will break with this?
>If so, it needs to be submitted in a form that could be accepted (this
>one was not, so I couldn't take it even if I wanted to), and please add
>a tested-by from you and we will be glad to reconsider it.
>greg k-h

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