Re: [PATCH V2] mem_event: Allow emulating an instruction that caused a page fault

[Tim Deegan <tim@xen.org>]

At 14:53 +0200 on 22 Jan (1358866429), Razvan Cojocaru wrote:
> >#DF (-1, +12 = 55).  Now that's a pretty unlikely scenario (and I may
> >have got some of the details wrong) but the upshot is: a single x86
> >instruction can access enormous amounts of memory, so turning off
> >protection and single-stepping, especially if you don't trust the OS, is
> >exposing a lot more than the single frame you took the first fault on.
> 
> Thank you, Tim, for clearing that up. Now, 'touching' a page is quite 
> different from 'writing to' a page, and I'm really only interested in 
> the latter. So, in a scenario where reads are permitted by default and 
> we're only interested in writes, are we still talking about these 
> limitations? A MOVSW, for example, only needs to write to a single page, 
> even though it does touch more pages in read mode.

Ok, talking only about writes, we have the destination operand, plus all
the pagetables (for setting Accessed bits) plus any stacks and TSSes
needed in delivering faults; something like 32 pages for the full
double-fault scenario.

Tim