linux-kernel - Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user mappings

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Message-ID: <20191029093254.GE18773@rapoport-lnx>
Date:   Tue, 29 Oct 2019 10:32:55 +0100
From:   Mike Rapoport <rppt@...nel.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     linux-kernel@...r.kernel.org,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, linux-api@...r.kernel.org,
        linux-mm@...ck.org, x86@...nel.org,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user
 mappings

On Mon, Oct 28, 2019 at 02:44:23PM -0600, Andy Lutomirski wrote:
> 
> > On Oct 27, 2019, at 4:17 AM, Mike Rapoport <rppt@...nel.org> wrote:
> > 
> > From: Mike Rapoport <rppt@...ux.ibm.com>
> > 
> > Hi,
> > 
> > The patch below aims to allow applications to create mappins that have
> > pages visible only to the owning process. Such mappings could be used to
> > store secrets so that these secrets are not visible neither to other
> > processes nor to the kernel.
> > 
> > I've only tested the basic functionality, the changes should be verified
> > against THP/migration/compaction. Yet, I'd appreciate early feedback.
> 
> I’ve contemplated the concept a fair amount, and I think you should
> consider a change to the API. In particular, rather than having it be a
> MAP_ flag, make it a chardev.  You can, at least at first, allow only
> MAP_SHARED, and admins can decide who gets to use it.  It might also play
> better with the VM overall, and you won’t need a VM_ flag for it — you
> can just wire up .fault to do the right thing.

I think mmap()/mprotect()/madvise() are the natural APIs for such
interface. Switching to a chardev doesn't solve the major problem of direct
map fragmentation and defeats the ability to use exclusive memory mappings
with the existing allocators, while mprotect() and madvise() do not.

-- 
Sincerely yours,
Mike.