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

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Message-ID: <20170704183927.GH22013@1wt.eu>
Date:   Tue, 4 Jul 2017 20:39:27 +0200
From:   Willy Tarreau <w@....eu>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Michal Hocko <mhocko@...nel.org>,
        Ben Hutchings <ben@...adent.org.uk>,
        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>,
        linux-distros@...openwall.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 Tue, Jul 04, 2017 at 11:37:15AM -0700, Linus Torvalds wrote:
> On Tue, Jul 4, 2017 at 10:22 AM, Michal Hocko <mhocko@...nel.org> wrote:
> >
> > Well, I've been thinking about this some more and the more I think about
> > it the less I am convinced we should try to be clever here. Why? Because
> > as soon as somebody tries to manage stacks explicitly you cannot simply
> > assume anything about the previous mapping. Say some interpret uses
> > [ mngmnt data][red zone]                 <--[- MAP_GROWSDOWN ]
> >
> > Now if we consider the red zone's (PROT_NONE) prev mapping we would fail
> > the expansion even though we haven't hit the red zone and that is
> > essentially what the Java and rust bugs are about. So we just risk yet
> > another regression.
> 
> Ack.
> 
> Let's make the initial version at least only check the first vma.
> 
> The long-term fix for this is to have the binaries do proper stack
> expansion probing anyway, and it's quite possible that people who do
> their own stack redzoning by adding a PROT_NONE thing already do that
> proper fix (eg the Java stack may simply not *have* those big crazy
> structures on it in the first place).

But what is wrong with stopping the loop as soon as the distance gets
larger than the stack_guard_gap ?

Willy