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 -- To unsubscribe from this list: send the line "unsubscribe linux-input" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
