linux-kernel - Re: [PATCH v10 00/12] arm64: untag user pointers passed to the kernel

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Date:   Tue, 26 Feb 2019 18:18:25 +0100
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Robin Murphy <robin.murphy@....com>,
        Kees Cook <keescook@...omium.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Shuah Khan <shuah@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.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>,
        Chintan Pandya <cpandya@...eaurora.org>,
        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 v10 00/12] arm64: untag user pointers passed to the kernel

On Fri, Feb 22, 2019 at 11:55 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 2/22/19 4:53 AM, Andrey Konovalov wrote:
> > The following testing approaches has been taken to find potential issues
> > with user pointer untagging:
> >
> > 1. Static testing (with sparse [3] and separately with a custom static
> >    analyzer based on Clang) to track casts of __user pointers to integer
> >    types to find places where untagging needs to be done.
>
> First of all, it's really cool that you took this approach.  Sounds like
> there was a lot of systematic work to fix up the sites in the existing
> codebase.
>
> But, isn't this a _bit_ fragile going forward?  Folks can't just "make
> sparse" to find issues with missing untags.

Yes, this static approach can only be used as a hint to find some
places where untagging is needed, but certainly not all.

> This seems like something
> where we would ideally add an __tagged annotation (or something) to the
> source tree and then have sparse rules that can look for missed untags.

This has been suggested before, search for __untagged here [1].
However there are many places in the kernel where a __user pointer is
casted into unsigned long and passed further. I'm not sure if it's
possible apply a __tagged/__untagged kind of attribute to non-pointer
types, is it?

[1] https://patchwork.kernel.org/patch/10581535/