linux-kernel - Re: [PATCH] mm: larger stack guard gap, between vmas

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Date:   Wed, 5 Jul 2017 10:15:56 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Ben Hutchings <ben@...adent.org.uk>
Cc:     Michal Hocko <mhocko@...nel.org>, Willy Tarreau <w@....eu>,
        Hugh Dickins <hughd@...gle.com>,
        Oleg Nesterov <oleg@...hat.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...hat.com>,
        Larry Woodman <lwoodman@...hat.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Tony Luck <tony.luck@...el.com>,
        "James E.J. Bottomley" <jejb@...isc-linux.org>,
        Helge Diller <deller@....de>,
        James Hogan <james.hogan@...tec.com>,
        Laura Abbott <labbott@...hat.com>, Greg KH <greg@...ah.com>,
        "security@...nel.org" <security@...nel.org>,
        Qualys Security Advisory <qsa@...lys.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ximin Luo <infinity0@...ian.org>
Subject: Re: [PATCH] mm: larger stack guard gap, between vmas

On Wed, Jul 5, 2017 at 9:58 AM, Ben Hutchings <ben@...adent.org.uk> wrote:
>
> I ended up with the following two patches, which seem to deal with
> both the Java and Rust regressions.  These don't touch the
> stack-grows-up paths at all because Rust doesn't run on those
> architectures and the Java weirdness is i386-specific.
>
> They definitely need longer commit messages and comments, but aside
> from that do these look reasonable?

I thin kthey both look reasonable, but I think we might still want to
massage things a bit (cutting down the quoting to a minimum, hopefully
leaving enough context to still make sense):

> Subject: [1/2] mmap: Skip a single VM_NONE mapping when checking the stack gap
>
>         prev = vma->vm_prev;
> +       if (prev && !(prev->vm_flags & (VM_READ | VM_WRITE | VM_EXEC)))
> +               prev = prev->vm_prev;
>         if (prev && prev->vm_end > gap_addr) {

Do we just want to ignore the user-supplied guard mapping, or do we
want to say "if the user does a guard mapping, we use that *instead*
of our stack gap"?

IOW, instead of "prev = prev->vm_prev;" and continuing, maybe we want
to just return "ok".

> Subject: [2/2] mmap: Avoid mapping anywhere within the full stack extent  if finite

This is good thinking, but no, I don't think the "if finite" is right.

I've seen people use "really big values" as replacement for
RLIM_INIFITY, for various reasons.

We've had huge confusion about RLIM_INFINITY over the years - look for
things like COMPAT_RLIM_OLD_INFINITY to see the kinds of confusions
we've had.

Some people just use MAX_LONG etc, which is *not* the same as
RLIM_INFINITY, but in practice ends up doing the same thing. Yadda
yadda.

So I'm personally leery of checking and depending on "exactly
RLIM_INIFITY", because I've seen it go wrong so many times.

And I think your second patch breaks that "use a really large value to
approximate infinity" case that definitely has existed as a pattern.

              Linus