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

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Message-ID: <20190523165708.q6ru7xg45aqfjzpr@mbp>
Date:   Thu, 23 May 2019 17:57:09 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Dave Martin <Dave.Martin@....com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        Mark Rutland <mark.rutland@....com>, kvm@...r.kernel.org,
        Christian Koenig <Christian.Koenig@....com>,
        Szabolcs Nagy <Szabolcs.Nagy@....com>,
        Will Deacon <will.deacon@....com>,
        dri-devel@...ts.freedesktop.org, linux-mm@...ck.org,
        Lee Smith <Lee.Smith@....com>, linux-kselftest@...r.kernel.org,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Leon Romanovsky <leon@...nel.org>, linux-rdma@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org,
        Evgeniy Stepanov <eugenis@...gle.com>,
        linux-media@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Felix Kuehling <Felix.Kuehling@....com>,
        linux-kernel@...r.kernel.org,
        Jens Wiklander <jens.wiklander@...aro.org>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Alexander Deucher <Alexander.Deucher@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Robin Murphy <robin.murphy@....com>,
        Yishai Hadas <yishaih@...lanox.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>
Subject: Re: [PATCH v15 00/17] arm64: untag user pointers passed to the kernel

On Thu, May 23, 2019 at 11:42:57AM +0100, Dave P Martin wrote:
> On Wed, May 22, 2019 at 09:20:52PM -0300, Jason Gunthorpe wrote:
> > On Wed, May 22, 2019 at 02:49:28PM +0100, Dave Martin wrote:
> > > If multiple people will care about this, perhaps we should try to
> > > annotate types more explicitly in SYSCALL_DEFINEx() and ABI data
> > > structures.
> > > 
> > > For example, we could have a couple of mutually exclusive modifiers
> > > 
> > > T __object *
> > > T __vaddr * (or U __vaddr)
> > > 
> > > In the first case the pointer points to an object (in the C sense)
> > > that the call may dereference but not use for any other purpose.
> > 
> > How would you use these two differently?
> > 
> > So far the kernel has worked that __user should tag any pointer that
> > is from userspace and then you can't do anything with it until you
> > transform it into a kernel something
> 
> Ultimately it would be good to disallow casting __object pointers execpt
> to compatible __object pointer types, and to make get_user etc. demand
> __object.
> 
> __vaddr pointers / addresses would be freely castable, but not to
> __object and so would not be dereferenceable even indirectly.

I think it gets too complicated and there are ambiguous cases that we
may not be able to distinguish. For example copy_from_user() may be used
to copy a user data structure into the kernel, hence __object would
work, while the same function may be used to copy opaque data to a file,
so __vaddr may be a better option (unless I misunderstood your
proposal).

We currently have T __user * and I think it's a good starting point. The
prior attempt [1] was shut down because it was just hiding the cast
using __force. We'd need to work through those cases again and rather
start changing the function prototypes to avoid unnecessary casting in
the callers (e.g. get_user_pages(void __user *) or come up with a new
type) while changing the explicit casting to a macro where it needs to
be obvious that we are converting a user pointer, potentially typed
(tagged), to an untyped address range. We may need a user_ptr_to_ulong()
macro or similar (it seems that we have a u64_to_user_ptr, wasn't aware
of it).

It may actually not be far from what you suggested but I'd keep the
current T __user * to denote possible dereference.

[1] https://lore.kernel.org/lkml/5d54526e5ff2e5ad63d0dfdd9ab17cf359afa4f2.1535629099.git.andreyknvl@google.com/

-- 
Catalin