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

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Message-Id: <CA5C22D9-BC3E-4B69-8DD9-4D3B75E40BD5@amacapital.net>
Date:   Mon, 28 Oct 2019 14:44:23 -0600
From:   Andy Lutomirski <luto@...capital.net>
To:     Mike Rapoport <rppt@...nel.org>
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 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.