linux-kernel - Re: [PATCH v7 0/8] arm64: untag user pointers passed to the kernel

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Date:   Thu, 18 Oct 2018 18:33:31 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Evgenii Stepanov <eugenis@...gle.com>
Cc:     Andrey Konovalov <andreyknvl@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Will Deacon <will.deacon@....com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Chintan Pandya <cpandya@...eaurora.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Shuah Khan <shuah@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Kostya Serebryany <kcc@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Lee Smith <Lee.Smith@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Robin Murphy <robin.murphy@....com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v7 0/8] arm64: untag user pointers passed to the kernel

On Wed, Oct 17, 2018 at 01:25:42PM -0700, Evgenii Stepanov wrote:
> On Wed, Oct 17, 2018 at 7:20 AM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
> > On Wed, Oct 17, 2018 at 4:06 PM, Vincenzo Frascino
> > <vincenzo.frascino@....com> wrote:
> >> I have been thinking a bit lately on how to address the problem of
> >> user tagged pointers passed to the kernel through syscalls, and
> >> IMHO probably the best way we have to catch them all and make sure
> >> that the approach is maintainable in the long term is to introduce
> >> shims that tag/untag the pointers passed to the kernel.
> >>
> >> In details, what I am proposing can live either in userspace
> >> (preferred solution so that we do not have to relax the ABI) or in
> >> kernel space and can be summarized as follows:
> >>  - A shim is specific to a syscall and is called by the libc when
> >>  it needs to invoke the respective syscall.
> >>  - It is required only if the syscall accepts pointers.
> >>  - It saves the tags of a pointers passed to the syscall in memory
> >>  (same approach if the we are passing a struct that contains
> >>  pointers to the kernel, with the difference that all the tags of
> >>  the pointers in the struct need to be saved singularly)
> >>  - Untags the pointers
> >>  - Invokes the syscall
> >>  - Retags the pointers with the tags stored in memory
> >>  - Returns
> >>
> >> What do you think?
> >
> > If I correctly understand what you are proposing, I'm not sure if that
> > would work with the countless number of different ioctl calls. For
> > example when an ioctl accepts a struct with a bunch of pointer fields.
> > In this case a shim like the one you propose can't live in userspace,
> > since libc doesn't know about the interface of all ioctls, so it can't
> > know which fields to untag. The kernel knows about those interfaces
> > (since the kernel implements them), but then we would need a custom
> > shim for each ioctl variation, which doesn't seem practical.
> 
> The current patchset handles majority of pointers in a just a few
> common places, like copy_from_user. Userspace shims will need to untag
> & retag all pointer arguments - we are looking at hundreds if not
> thousands of shims. They will also be located in a different code base
> from the syscall / ioctl implementations, which would make them
> impossible to keep up to date.

I think ioctls are a good reason not to attempt such user-space shim
layer (though it would have been much easier for the kernel ;)).

-- 
Catalin