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

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Date:   Wed, 22 May 2019 14:49:28 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>, kvm@...r.kernel.org,
        Szabolcs Nagy <Szabolcs.Nagy@....com>,
        Will Deacon <will.deacon@....com>,
        dri-devel@...ts.freedesktop.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org,
        Felix Kuehling <Felix.Kuehling@....com>,
        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, Dmitry Vyukov <dvyukov@...gle.com>,
        Evgeniy Stepanov <eugenis@...gle.com>,
        linux-media@...r.kernel.org, Kevin Brodsky <kevin.brodsky@....com>,
        Kees Cook <keescook@...omium.org>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Kostya Serebryany <kcc@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Yishai Hadas <yishaih@...lanox.com>,
        linux-kernel@...r.kernel.org,
        Jens Wiklander <jens.wiklander@...aro.org>,
        Lee Smith <Lee.Smith@....com>,
        Alexander Deucher <Alexander.Deucher@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Robin Murphy <robin.murphy@....com>,
        Christian Koenig <Christian.Koenig@....com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>
Subject: Re: [PATCH v15 00/17] arm64: untag user pointers passed to the kernel

On Tue, May 21, 2019 at 03:48:56PM -0300, Jason Gunthorpe wrote:
> On Fri, May 17, 2019 at 03:49:31PM +0100, Catalin Marinas wrote:
> 
> > The tagged pointers (whether hwasan or MTE) should ideally be a
> > transparent feature for the application writer but I don't think we can
> > solve it entirely and make it seamless for the multitude of ioctls().
> > I'd say you only opt in to such feature if you know what you are doing
> > and the user code takes care of specific cases like ioctl(), hence the
> > prctl() proposal even for the hwasan.
> 
> I'm not sure such a dire view is warrented.. 
> 
> The ioctl situation is not so bad, other than a few special cases,
> most drivers just take a 'void __user *' and pass it as an argument to
> some function that accepts a 'void __user *'. sparse et al verify
> this. 
> 
> As long as the core functions do the right thing the drivers will be
> OK.
> 
> The only place things get dicy is if someone casts to unsigned long
> (ie for vma work) but I think that reflects that our driver facing
> APIs for VMAs are compatible with static analysis (ie I have no
> earthly idea why get_user_pages() accepts an unsigned long), not that
> this is too hard.

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.

In the latter case the pointer (or other type) is a virtual address
that the call does not dereference but my do other things with.

Also

U __really(T)

to tell static analysers the real type of pointers smuggled through
UAPI disguised as other types (*cough* KVM, etc.)

We could gradually make sparse more strict about the presence of
annotations and allowed conversions, add get/put_user() variants
that demand explicit annotation, etc.

find_vma() wouldn't work with a __object pointer, for example.  A
get_user_pages_for_dereference() might be needed for __object pointers
(embodying a promise from the caller that only the object will be
dereferenced within the mapped pages).

Thoughts?

This kind of thing would need widespread buy-in in order to be viable.

Cheers
---Dave