Re: GUP guarantees wrt to userspace mappings redesign

From: Oleg Nesterov
Date: Mon May 02 2016 - 13:24:19 EST

On 05/02, Kirill A. Shutemov wrote:
> On Mon, May 02, 2016 at 04:15:38PM +0200, Oleg Nesterov wrote:
> > >
> > > - I don't see any check page_count() around __replace_page() in uprobes,
> > > so it can easily replace pinned page.
> >
> > Why it should? even if it races with get_user_pages_fast()... this doesn't
> > differ from the case when an application writes to MAP_PRIVATE non-anonymous
> > region, no?
> < I know nothing about uprobes or ptrace in general >
> I think the difference is that the write is initiated by the process
> itself, but IIUC __replace_page() can be initiated by other process, so
> it's out of control of the application.

Yes. Just like gdb can insert a breakpoint into the read-only executable vma.

> So we have pages pinned by a driver and the driver expects the pinned
> pages to be mapped into userspace, then __replace_page() kicks in and put
> different page there -- driver's expectation is broken.

Yes... but I don't understand the problem space. I mean, I do not know why
this driver should expect this, how it can be broken, etc.

I do not even understand why "initiated by other process" can make any
difference... Unless this driver somehow controls all threads which could
have this page mapped.