linux-kernel - Re: [PATCH v15 00/17] arm64: untag user pointers passed to the kernel

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Message-ID: <20190531161954.GA3568@arrakis.emea.arm.com>
Date:   Fri, 31 May 2019 17:19:55 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-rdma@...r.kernel.org, linux-media@...r.kernel.org,
        kvm@...r.kernel.org,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Yishai Hadas <yishaih@...lanox.com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Alexander Deucher <Alexander.Deucher@....com>,
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        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Jens Wiklander <jens.wiklander@...aro.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Leon Romanovsky <leon@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
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        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
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        Dave Martin <Dave.Martin@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Szabolcs Nagy <Szabolcs.Nagy@....com>,
        Elliott Hughes <enh@...gle.com>,
        Khalid Aziz <khalid.aziz@...cle.com>
Subject: Re: [PATCH v15 00/17] arm64: untag user pointers passed to the kernel

On Fri, May 31, 2019 at 04:29:10PM +0200, Andrey Konovalov wrote:
> On Thu, May 30, 2019 at 7:15 PM Catalin Marinas <catalin.marinas@....com> wrote:
> > On Tue, May 28, 2019 at 04:14:45PM +0200, Andrey Konovalov wrote:
> > > Thanks for a lot of valuable input! I've read through all the replies
> > > and got somewhat lost. What are the changes I need to do to this
> > > series?
> > >
> > > 1. Should I move untagging for memory syscalls back to the generic
> > > code so other arches would make use of it as well, or should I keep
> > > the arm64 specific memory syscalls wrappers and address the comments
> > > on that patch?
> >
> > Keep them generic again but make sure we get agreement with Khalid on
> > the actual ABI implications for sparc.
> 
> OK, will do. I find it hard to understand what the ABI implications
> are. I'll post the next version without untagging in brk, mmap,
> munmap, mremap (for new_address), mmap_pgoff, remap_file_pages, shmat
> and shmdt.

It's more about not relaxing the ABI to accept non-zero top-byte unless
we have a use-case for it. For mmap() etc., I don't think that's needed
but if you think otherwise, please raise it.

> > > 2. Should I make untagging opt-in and controlled by a command line argument?
> >
> > Opt-in, yes, but per task rather than kernel command line option.
> > prctl() is a possibility of opting in.
> 
> OK. Should I store a flag somewhere in task_struct? Should it be
> inheritable on clone?

A TIF flag would do but I'd say leave it out for now (default opted in)
until we figure out the best way to do this (can be a patch on top of
this series).

Thanks.

-- 
Catalin