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Date: Mon, 2 May 2016 18:22:11 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
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
Message-ID: <20160502162211.GA11678@redhat.com>
References: <20160428232127.GL11700@redhat.com> <20160429005106.GB2847@node.shutemov.name> <20160428204542.5f2053f7@ul30vt.home> <20160429070611.GA4990@node.shutemov.name> <20160429163444.GM11700@redhat.com> <20160502104119.GA23305@node.shutemov.name> <20160502111513.GA4079@gmail.com> <20160502121402.GB23305@node.shutemov.name> <20160502141538.GA5961@redhat.com> <20160502162128.GF24419@node.shutemov.name>
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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.

Oleg.