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

From: Yu-cheng Yu <yu-cheng.yu@intel.com>
To: Jann Horn <jannh@google.com>, Andy Lutomirski <luto@amacapital.net>
Cc: the arch/x86 maintainers <x86@kernel.org>,
	"H . Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	kernel list <linux-kernel@vger.kernel.org>,
	linux-doc@vger.kernel.org, Linux-MM <linux-mm@kvack.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Balbir Singh <bsingharora@gmail.com>,
	Cyrill Gorcunov <gorcunov@gmail.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Eugene Syromiatnikov <esyr@redhat.com>,
	Florian Weimer <fweimer@redhat.com>,
	hjl.tools@gmail.com, Jonathan Corbet <corbet@lwn.net>,
	Kees Cook <keescook@chromium.org>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Nadav Amit <nadav.amit@gmail.com>,
	Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
	Peter Zijlstra <peterz@infradead.org>,
	rdunlap@infradead.org, ravi.v.shankar@intel.com,
	vedvyas.shanbhogue@intel.com,
	Daniel Micay <danielmicay@gmail.com>
Subject: Re: [PATCH v5 07/27] mm/mmap: Create a guard area between VMAs
Date: Thu, 11 Oct 2018 13:49:00 -0700	[thread overview]
Message-ID: <c61b4c393f938ee6e97f84d97ed76867ae75cb02.camel@intel.com> (raw)
In-Reply-To: <CAG48ez3R7XL8MX_sjff1FFYuARX_58wA_=ACbv2im-XJKR8tvA@mail.gmail.com>

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@intel.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