linux-kernel - Re: GUP guarantees wrt to userspace mappings redesign

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Date:	Mon, 2 May 2016 19:21:28 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Jerome Glisse <j.glisse@...il.com>,
	Hugh Dickins <hughd@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	kirill.shutemov@...ux.intel.com, linux-kernel@...r.kernel.org,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: GUP guarantees wrt to userspace mappings redesign

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