linux-kernel - Re: [PATCH v15 05/17] arms64: untag user pointers passed to memory syscalls

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Message-ID: <20190523090427.GA44383@arrakis.emea.arm.com>
Date:   Thu, 23 May 2019 10:04:28 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Evgenii Stepanov <eugenis@...gle.com>
Cc:     Andrey Konovalov <andreyknvl@...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>,
        Kees Cook <keescook@...omium.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>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...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>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Dave Martin <Dave.Martin@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        Szabolcs Nagy <Szabolcs.Nagy@....com>
Subject: Re: [PATCH v15 05/17] arms64: untag user pointers passed to memory
 syscalls

On Wed, May 22, 2019 at 02:16:57PM -0700, Evgenii Stepanov wrote:
> On Wed, May 22, 2019 at 4:49 AM Catalin Marinas <catalin.marinas@....com> wrote:
> > On Mon, May 06, 2019 at 06:30:51PM +0200, Andrey Konovalov wrote:
> > > This patch is a part of a series that extends arm64 kernel ABI to allow to
> > > pass tagged user pointers (with the top byte set to something else other
> > > than 0x00) as syscall arguments.
> > >
> > > This patch allows tagged pointers to be passed to the following memory
> > > syscalls: brk, get_mempolicy, madvise, mbind, mincore, mlock, mlock2,
> > > mmap, mmap_pgoff, mprotect, mremap, msync, munlock, munmap,
> > > remap_file_pages, shmat and shmdt.
> > >
> > > This is done by untagging pointers passed to these syscalls in the
> > > prologues of their handlers.
> >
> > I'll go through them one by one to see if we can tighten the expected
> > ABI while having the MTE in mind.
> >
> > > diff --git a/arch/arm64/kernel/sys.c b/arch/arm64/kernel/sys.c
> > > index b44065fb1616..933bb9f3d6ec 100644
> > > --- a/arch/arm64/kernel/sys.c
> > > +++ b/arch/arm64/kernel/sys.c
> > > @@ -35,10 +35,33 @@ SYSCALL_DEFINE6(mmap, unsigned long, addr, unsigned long, len,
> > >  {
> > >       if (offset_in_page(off) != 0)
> > >               return -EINVAL;
> > > -
> > > +     addr = untagged_addr(addr);
> > >       return ksys_mmap_pgoff(addr, len, prot, flags, fd, off >> PAGE_SHIFT);
> > >  }
> >
> > If user passes a tagged pointer to mmap() and the address is honoured
> > (or MAP_FIXED is given), what is the expected return pointer? Does it
> > need to be tagged with the value from the hint?
> 
> For HWASan the most convenient would be to use the tag from the hint.
> But since in the TBI (not MTE) mode the kernel has no idea what
> meaning userspace assigns to pointer tags, perhaps it should not try
> to guess, and should return raw (zero-tagged) address instead.

Then, just to relax the ABI for hwasan, shall we simply disallow tagged
pointers on mmap() arguments? We can leave them in for
mremap(old_address), madvise().

> > With MTE, we may want to use this as a request for the default colour of
> > the mapped pages (still under discussion).
> 
> I like this - and in that case it would make sense to return the
> pointer that can be immediately dereferenced without crashing the
> process, i.e. with the matching tag.

This came up from the Android investigation work where large memory
allocations (using mmap) could be more efficiently pre-tagged by the
kernel on page fault. Not sure about the implementation details yet.

-- 
Catalin