linux-kernel - Re: [PATCH] mm: check VMA flags to avoid invalid PROT

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Message-ID: <1474850960.17726.48.camel@redhat.com>
Date:   Sun, 25 Sep 2016 20:49:20 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>
Cc:     Lorenzo Stoakes <lstoakes@...il.com>,
        linux-mm <linux-mm@...ck.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        tbsaunde@...aunde.org, robert@...llahan.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm: check VMA flags to avoid invalid PROT_NONE NUMA
 balancing

On Sun, 2016-09-25 at 15:50 -0700, Linus Torvalds wrote:
> On Sun, Sep 25, 2016 at 3:34 PM, Rik van Riel <riel@...hat.com>
> wrote:
> > 
> > 
> > The patch looks good to me, too.
> > 
> > Acked-by: Rik van Riel <riel@...hat.com>
> 
> Thanks, amended the commit since I hadn't pushed out yet.
> 
> Btw, the only reason this bug could happen is that we do that
> "force=1" for remote vm accesses, which turns into FOLL_FORCE, which
> in turn will turn into us allowing an access even when we technically
> shouldn't.
> 
> I'd really like to re-open the "drop FOLL_FORCE entirely" discussion,
> because the thing really is disgusting.
> 
> I realize that debuggers etc sometimes would want to punch through
> PROT_NONE protections,

Reading the code for a little bit, it looks like get_user_pages
interprets both PROT_NONE and PAGE_NUMA ptes as present, and will
simply return the page to the caller.

Furthermore, if a page in a PROT_NONE VMA is actually not present,
it should be faulted in with PROT_NONE permissions, after which
the page is passed to the debugger.

That is, punching through PROT_NONE permissions should only happen
from outside of the process. Inside the process, PROT_NONE should
be preserved regardless of FOLL_FORCE.

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