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

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Date:   Thu, 23 May 2019 11:42:57 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Mark Rutland <mark.rutland@....com>, kvm@...r.kernel.org,
        Christian Koenig <Christian.Koenig@....com>,
        Szabolcs Nagy <Szabolcs.Nagy@....com>,
        Catalin Marinas <catalin.marinas@....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 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:
> > 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.
> 
> 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.

Or that's the general idea.  Figuring out a sane set of rules that we
could actually check / enforce would require a bit of work.

(Whether the __vaddr base type is a pointer or an integer type is
probably moot, due to the restrictions we would place on the use of
these anyway.)

> > to tell static analysers the real type of pointers smuggled through
> > UAPI disguised as other types (*cough* KVM, etc.)
> 
> Yes, that would help alot, we often have to pass pointers through a
> u64 in the uAPI, and there is no static checker support to make sure
> they are run through the u64_to_user_ptr() helper.

Agreed.

Cheers
---Dave