Re: [RFC PATCH v4 24/27] mm/mmap: Create a guard area between VMAs

From: Eugene Syromiatnikov <esyr@redhat.com>
To: Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>, X86 ML <x86@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	LKML <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>,
	Florian Weimer <fweimer@redhat.com>,
	"H. J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.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>,
	Randy Dunlap <rdunlap@infradead.org>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	"Shanbhogue, Vedvyas" <vedvyas.shanbhogue@intel.com>
Subject: Re: [RFC PATCH v4 24/27] mm/mmap: Create a guard area between VMAs
Date: Wed, 3 Oct 2018 18:32:26 +0200	[thread overview]
Message-ID: <20181003163226.GC9449@asgard.redhat.com> (raw)
Message-ID: <20181003163226.uwv46aWj9HG--cTKQBWg4ORLQAinh5x3K5aIcs-LP9s@z> (raw)
In-Reply-To: <5ddb0ad33298d1858e530fce9c9ea2788b2fac81.camel@intel.com>

On Wed, Oct 03, 2018 at 09:00:04AM -0700, Yu-cheng Yu wrote:
> On Tue, 2018-10-02 at 22:36 -0700, Andy Lutomirski wrote:
> > On Tue, Oct 2, 2018 at 9:55 PM Eugene Syromiatnikov <esyr@redhat.com> wrote:
> > > 
> > > On Fri, Sep 21, 2018 at 08:03:48AM -0700, Yu-cheng Yu wrote:
> > > > Create a guard area between VMAs, to detect memory corruption.
> > > 
> > > Do I understand correctly that with this patch a user space program
> > > no longer be able to place two mappings back to back? If it is so,
> > > it will likely break a lot of things; for example, it's a common ring
> > > buffer implementations technique, to map buffer memory twice back
> > > to back in order to avoid special handling of items wrapping its end.
> > 
> > I haven't checked what the patch actually does, but it shouldn't have
> > any affect on MAP_FIXED or the new no-replace MAP_FIXED variant.
> > 
> > --Andy
> 
> I did some mmap tests with/without MAP_FIXED, and it works as intended.
> In addition to the ring buffer, are there other test cases?

Right, after some more code reading I figured out that it indeed
shouldn't affect MAP_FIXED, thank you for confirmation.

I'm not sure, however, whether such a change that provides no ability
to configure or affect it will go well with all the supported
architectures.