linux-kernel - Re: [PATCH v16 02/16] arm64: untag user pointers in access_ok and __uaccess_mask

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Message-ID: <20190610185329.xhjawzfy4uddrkrj@mbp>
Date:   Mon, 10 Jun 2019 19:53:30 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andrey Konovalov <andreyknvl@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
        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,
        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>,
        Christian Koenig <Christian.Koenig@....com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Jens Wiklander <jens.wiklander@...aro.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Leon Romanovsky <leon@...nel.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Dave Martin <Dave.Martin@....com>,
        Khalid Aziz <khalid.aziz@...cle.com>, enh <enh@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christoph Hellwig <hch@...radead.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        Evgeniy Stepanov <eugenis@...gle.com>,
        Lee Smith <Lee.Smith@....com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Robin Murphy <robin.murphy@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Szabolcs Nagy <Szabolcs.Nagy@....com>
Subject: Re: [PATCH v16 02/16] arm64: untag user pointers in access_ok and
 __uaccess_mask_ptr

On Mon, Jun 10, 2019 at 11:07:03AM -0700, Kees Cook wrote:
> On Mon, Jun 10, 2019 at 06:53:27PM +0100, Catalin Marinas wrote:
> > On Mon, Jun 03, 2019 at 06:55:04PM +0200, Andrey Konovalov wrote:
> > > diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> > > index e5d5f31c6d36..9164ecb5feca 100644
> > > --- a/arch/arm64/include/asm/uaccess.h
> > > +++ b/arch/arm64/include/asm/uaccess.h
> > > @@ -94,7 +94,7 @@ static inline unsigned long __range_ok(const void __user *addr, unsigned long si
> > >  	return ret;
> > >  }
> > >  
> > > -#define access_ok(addr, size)	__range_ok(addr, size)
> > > +#define access_ok(addr, size)	__range_ok(untagged_addr(addr), size)
[...]
> > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> > index 3767fb21a5b8..fd191c5b92aa 100644
> > --- a/arch/arm64/kernel/process.c
> > +++ b/arch/arm64/kernel/process.c
> > @@ -552,3 +552,18 @@ void arch_setup_new_exec(void)
> >  
> >  	ptrauth_thread_init_user(current);
> >  }
> > +
> > +/*
> > + * Enable the relaxed ABI allowing tagged user addresses into the kernel.
> > + */
> > +int untagged_uaddr_set_mode(unsigned long arg)
> > +{
> > +	if (is_compat_task())
> > +		return -ENOTSUPP;
> > +	if (arg)
> > +		return -EINVAL;
> > +
> > +	set_thread_flag(TIF_UNTAGGED_UADDR);
> > +
> > +	return 0;
> > +}
> 
> I think this should be paired with a flag clearing in copy_thread(),
> yes? (i.e. each binary needs to opt in)

It indeed needs clearing though not in copy_thread() as that's used on
clone/fork but rather in flush_thread(), called on the execve() path.

And a note to myself: I think PR_UNTAGGED_ADDR (not UADDR) looks better
in a uapi header, the user doesn't differentiate between uaddr and
kaddr.

-- 
Catalin