Re: GUP guarantees wrt to userspace mappings redesign

From: Kirill A. Shutemov
Date: Mon May 02 2016 - 12:21:42 EST

On Mon, May 02, 2016 at 04:15:38PM +0200, Oleg Nesterov wrote:
> I am sure I missed the problem, but...
> On 05/02, Kirill A. Shutemov wrote:
> >
> > Quick look around:
> >
> > - 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.

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.

Kirill A. Shutemov