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