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From:   Andrey Konovalov <andreyknvl@google.com>
Date:   Tue, 26 Feb 2019 18:18:25 +0100
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Subject: Re: [PATCH v10 00/12] arm64: untag user pointers passed to the kernel
To:     Dave Hansen <dave.hansen@intel.com>
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On Fri, Feb 22, 2019 at 11:55 PM Dave Hansen <dave.hansen@intel.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/