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

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Message-ID: <20170704190300.GJ22013@1wt.eu>
Date:   Tue, 4 Jul 2017 21:03:00 +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 11:47:37AM -0700, Linus Torvalds wrote:
> Let's
> say that you are using lots of threads, so that you know your stack
> space is limited. What you do is to use MAP_FIXED a lot, and you lay
> out your stacks fairly densely (with each other, but also possibly
> with other mappings), with that PROT_NONE redzoning mapping in between
> the "dense" allocations.
> 
> So when the kernel wants to grow the stack, it finds the PROT_NONE
> redzone mapping - but there's possibly other maps right under it, so
> the stack_guard_gap still hits other mappings.
(...)

OK I didn't get that use case, that totally makes sense indeed! So
now we use PROT_NONE not as something that must be skipped to find
the unmapped area but as a hint that the application apparently
wants the stack to stop here.

Thanks for this clear explanation!

Willy