Re: [RFC] prevent "dd if=/dev/mem" crash

Andrew Morton
2003-10-18T00:49:55Z

David Mosberger <davidm@napali.hpl.hp.com> wrote:
>
> >>>>> On Fri, 17 Oct 2003 16:55:43 -0700, Andrew Morton <akpm@osdl.org> said:
>
> >> If we really believe copy_*_user() must correctly handle *all* faults,
> >> isn't the "p >= __pa(high_memory)" test superfluous?
>
> Andrew> This code was conceived before my time and I don't recall seeing much
> Andrew> discussion, so this is all guesswork..
>
> Andrew> I'd say that the high_memory test _is_ superfluous and that
> Andrew> if anyone cared, we would remove it and establish a
> Andrew> temporary pte against the address if it was outside the
> Andrew> direct-mapped area. But nobody cares enough to have done
> Andrew> anything about it.
>
> What about memory-mapped device registers? Isn't all memory
> physically contiguous on x86 and that's why the "p >> __pa(high_memory)" test saves you from that?

We _want_ to be able to read mmio ranges via /dev/mem, don't we? I guess
it has never come up because everyone uses kmem.

> >> On ia64, a read to non-existent physical memory causes the processor
> >> to time out and take a machine check. I'm not sure it's even possible
> >> to recover from that.
>
> Andrew> ick. That would be very poor form.
>
> Reasonable people can disagree on that.

nah ;)

> One philosophy states that if
> your kernel touches random addresses, it's better to signal a visible
> error (machine-check) than to risk silent data corruption.

An access to an illegal address should generate a fault, period. This puts
the processing into the hands of software. If software chooses to silently
ignore the fault (ie: "silent data corruption") then it is poorly designed.

If the hardware doesn't give the system programmer a choice then the
hardware is poorly designed, surely?