Re: [RFC] Input: implement sysrq as an input handler

Dmitry Torokhov
2010-04-01T15:42:30Z

Hi Pavel,

On Thu, Apr 01, 2010 at 03:34:55PM +0200, Pavel Machek wrote:
> Hi!
>
> > > root isn't really a problem from a security PoV (well, maybe it is if the
> > > operation isn't constrained by capabilities). SAK can't protect you from
> > > root.
> > >
> > > _Normal_ userspace behaviour running a root process is a problem if it
> > > blocks these handles, though, both for SAK and regular SysRQ. I have lost
> > > count of how many times SysRQ+SUB delivered me from filesystem corruption
> > > and very annoying problems, both at home and at work.
> > >
> > > We are sort of trusting userspace to not break the one way out from severly
> > > hung systems while doing its normal day-to-day operations (as opposed to
> > > deliberately disabling SysRQ or remapping SAK, etc).
>
> If userspace disables sysrq during normal operation, that makes it
> useless.
>
> If normal user could do that, that's a security problem.
>

Yes, and...? This patch does not change the way one enables, disables,
intercepts, etc. SysRq and SAK compared to how it was handled when SysRq
was part of keyboard _input handler_. The only thisng this patch does is
moving the code into a _separate_ input handler.

> > > > That would require moving "these things", including their state
> > > > machines, into input core otherwise it would not know what events can be
> > > > trappable and which should be passed through. Or we should get rid of
> > > > EVIOCGRAB.
> > >
> > > Maybe we can add a flags field to input devices and input handlers, to be
> > > able to have the core behave differently when needed, without moving
> > > everything into the input core? Would that work, or would it need too much
> > > churn in the core?
> >
> > The problem is that device does not know what SysRq and especially SAK are.
> > User can reassign key codes and key symbols easily.
>
> That was not case in original implementation; it had hardcoded keymap.

The earth was also flat back then and the only keyboard was AT one. SAK
was always part of keymap so could be reassinged at any time.

>
> > I don't think we had any issues like this since 2.5 so I would not worry
> > about userspace too much. If anything we just need to review what stuff
> > we run as root (we do that anyway, right?).
>
> Hehe. If X can break sysrq, that's both X and sysrq problem.

Root can disable Sysrq... News at 11.

--
Dmitry