linux-kernel - Re: [PATCH v2 00/17] x86/ldt: Use a VMA based read only mapping

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Message-ID: <alpine.DEB.2.20.1712141302540.4998@nanos>
Date:   Thu, 14 Dec 2017 13:03:37 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirsky <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Borislav Petkov <bpetkov@...e.de>,
        Greg KH <gregkh@...uxfoundation.org>, keescook@...gle.com,
        hughd@...gle.com, Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        David Laight <David.Laight@...lab.com>,
        Eduardo Valentin <eduval@...zon.com>, aliguori@...zon.com,
        Will Deacon <will.deacon@....com>, linux-mm@...ck.org,
        kirill.shutemov@...ux.intel.com, dan.j.williams@...el.com
Subject: Re: [PATCH v2 00/17] x86/ldt: Use a VMA based read only mapping

On Thu, 14 Dec 2017, Peter Zijlstra wrote:
> So here's a second posting of the VMA based LDT implementation; now without
> most of the crazy.
> 
> I took out the write fault handler and the magic LAR touching code.
> 
> Additionally there are a bunch of patches that address generic vm issue.
> 
>  - gup() access control; In specific I looked at accessing !_PAGE_USER pages
>    because these patches rely on not being able to do that.
> 
>  - special mappings; A whole bunch of mmap ops don't make sense on special
>    mappings so disallow them.
> 
> Both things make sense independent of the rest of the series. Similarly, the
> patches that kill that rediculous LDT inherit on exec() are also unquestionably
> good.
> 
> So I think at least the first 6 patches are good, irrespective of the
> VMA approach.
> 
> On the whole VMA approach, Andy I know you hate it with a passion, but I really
> rather like how it ties the LDT to the process that it belongs to and it
> reduces the amount of 'special' pages in the whole PTI mapping.
> 
> I'm not the one going to make the decision on this; but I figured I at least
> post a version without the obvious crap parts of the last one.
> 
> Note: if we were to also disallow munmap() for special mappings (which I
> suppose makes perfect sense) then we could further reduce the actual LDT
> code (we'd no longer need the sm::close callback and related things).

That makes a lot of sense for the other special mapping users like VDSO and
kprobes.

Thanks,

	tglx