linux-kernel - Re: [PATCH v5 07/27] mm/mmap: Create a guard area between VMAs

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Date:   Thu, 11 Oct 2018 13:49:00 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Jann Horn <jannh@...gle.com>, Andy Lutomirski <luto@...capital.net>
Cc:     the arch/x86 maintainers <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-doc@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>, hjl.tools@...il.com,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>, rdunlap@...radead.org,
        ravi.v.shankar@...el.com, vedvyas.shanbhogue@...el.com,
        Daniel Micay <danielmicay@...il.com>
Subject: Re: [PATCH v5 07/27] mm/mmap: Create a guard area between VMAs

On Thu, 2018-10-11 at 22:39 +0200, Jann Horn wrote:
> On Thu, Oct 11, 2018 at 5:20 PM Yu-cheng Yu <yu-cheng.yu@...el.com> wrote:
> > Create a guard area between VMAs to detect memory corruption.
> 
> [...]
> > +config VM_AREA_GUARD
> > +       bool "VM area guard"
> > +       default n
> > +       help
> > +         Create a guard area between VM areas so that access beyond
> > +         limit can be detected.
> > +
> >  endmenu
> 
> Sorry to bring this up so late, but Daniel Micay pointed out to me
> that, given that VMA guards will raise the number of VMAs by
> inhibiting vma_merge(), people are more likely to run into
> /proc/sys/vm/max_map_count (which limits the number of VMAs to ~65k by
> default, and can't easily be raised without risking an overflow of
> page->_mapcount on systems with over ~800GiB of RAM, see
> https://lore.kernel.org/lkml/20180208021112.GB14918@bombadil.infradead.org/
> and replies) with this change.

Can we use the VMA guard only for Shadow Stacks?

Yu-cheng