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

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Date:   Sun, 25 Sep 2016 15:50:21 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Rik van Riel <riel@...hat.com>, 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, 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, and I also realize that right now we only have
a read/write flag, and we have that whole issue with "what if it's
executable but not readable", which currently FOLL_FORCE makes a
non-issue.

But at the same time, FOLL_FORCE really is a major nasty thing. It
shouldn't be a security issue (we still do check VM_MAY_READ/WRITE etc
to verify that even if something isn't readable or writable we *could*
have had permissions to do this), but this bug is a prime example of
how it violates our deeply held beliefs of how VM permissions *should*
work, and it screwed up the numa case as a result.

So how about we consider getting rid of FOLL_FORCE? Addign Hugh
Dickins to the cc, because I think he argued for that many moons ago..

                  Linus