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

Dmitry Torokhov
2010-03-22T04:59:17Z

On Fri, Mar 19, 2010 at 11:53:29PM -0300, Henrique de Moraes Holschuh wrote:
> On Fri, 19 Mar 2010, Dmitry Torokhov wrote:
> > On Fri, Mar 19, 2010 at 01:06:41PM -0300, Henrique de Moraes Holschuh wrote:
> > > On Thu, 18 Mar 2010, Dmitry Torokhov wrote:
> > > > On Thu, Mar 18, 2010 at 09:00:43PM -0300, Henrique de Moraes Holschuh wrote:
> > > > > Any chance of the user being able to avoid the SysRQ events getting to the
> > > > > handle, e.g. by opening the input device in exclusive mode or something like
> > > > > that?
> > > >
> > > > Yes, it is a possible to suppress SysRq by grabbing an input device.
> > > > This possibility exisst with the current implementation too though -
> > > > after all legacy keyboard driver implemented as an input handler as
> > > > well.
> > > >
> > > > ... or am I answering a question different from the one you asked? ;)
> > >
> > > No, that's exactly what I wanted to know.
> > >
> > > What about SAK? That thing *has* to be untrappable.
> >
> > On what level untrapable? And what exactly is SAK? There is not a
> > special key, at least not in general case, it is an action assigned to a
> > key comboi. Root can "trap" legacy keyboard SAK with loadkeys; it can
> > also disable sysrq, unload modules and do other nasty things. But
> > ordinary users can not trap it.
>
> 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).
>
> > > Even for the SysRQ debug events, I'd feel better if we could have a class of
> > > system input handlers that cannot be suppressed to use for these things.
> >
> > 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.

>
> > Given the fact that event devices are accessible only to root I think
> > that current behavior is acceptable.
>
> I don't trust the class of programs that would want to open input devices as
> root in exclusive mode. Desktop fluff might decide to use EVIOCGRAB or open
> input devices in exclusive mode for some reason, and break SysRQ. I'd like
> to preserve the hability of userspace to EVIOCGRAB if it feels there's a
> need to, while preserving the kernel's hability to NEVER ignore SysRQ and
> SAK while enabled.

I am afraid that you chose wrong verb then. You can not _preserve_ what
you do not have - legacy keyboard driver is still an input handler, and
thus can still interfere with SysRq by grabbing input devices.

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?).

--
Dmitry