linux-kernel - Re: [PATCH v8 0/8] arm64: untag user pointers passed to the kernel

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Message-ID: <20181129181650.GG22027@arrakis.emea.arm.com>
Date:   Thu, 29 Nov 2018 18:16:51 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Will Deacon <will.deacon@....com>,
        Kostya Serebryany <kcc@...gle.com>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Chintan Pandya <cpandya@...eaurora.org>,
        Shuah Khan <shuah@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Evgeniy Stepanov <eugenis@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Lee Smith <Lee.Smith@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Robin Murphy <robin.murphy@....com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v8 0/8] arm64: untag user pointers passed to the kernel

Hi Andrey,

On Thu, Nov 08, 2018 at 03:48:10PM +0100, Andrey Konovalov wrote:
> On Thu, Nov 8, 2018 at 3:36 PM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
> > Changes in v8:
> > - Rebased onto 65102238 (4.20-rc1).
> > - Added a note to the cover letter on why syscall wrappers/shims that untag
> >   user pointers won't work.
> > - Added a note to the cover letter that this patchset has been merged into
> >   the Pixel 2 kernel tree.
> > - Documentation fixes, in particular added a list of syscalls that don't
> >   support tagged user pointers.
> 
> I've changed the documentation to be more specific, please take a look.
> 
> I haven't done anything about adding a way for the user to find out
> that the kernel supports this ABI extension. I don't know what would
> the the preferred way to do this, and we haven't received any comments
> on that from anybody else. Probing "on some innocuous syscall
> currently returning -EFAULT on tagged pointer arguments" works though,
> as you mentioned.

We've had some internal discussions and also talked to some people at
Plumbers. I think the best option is to introduce an AT_FLAGS bit to
describe the ABI relaxation on tagged pointers. Vincenzo is going to
propose a patch on top of this series.

> As mentioned in the cover letter, this patchset has been merged into
> the Pixel 2 kernel tree.

I just hope it's not enabled on production kernels, it would introduce
a user ABI that may differ from what ends up upstream.

-- 
Catalin