linux-kernel - Re: [PATCH v4 0/7] arm64: untag user pointers passed to the kernel

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Message-ID: <20180801174256.5mbyf33eszml4nmu@armageddon.cambridge.arm.com>
Date:   Wed, 1 Aug 2018 18:42:56 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        linux-doc@...r.kernel.org, Will Deacon <will.deacon@....com>,
        Kostya Serebryany <kcc@...gle.com>,
        linux-kselftest@...r.kernel.org,
        Chintan Pandya <cpandya@...eaurora.org>,
        Shuah Khan <shuah@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        linux-arch@...r.kernel.org, Jacob Bramley <Jacob.Bramley@....com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Evgeniy Stepanov <eugenis@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        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 v4 0/7] arm64: untag user pointers passed to the kernel

On Mon, Jul 16, 2018 at 01:25:59PM +0200, Andrey Konovalov wrote:
> On Thu, Jun 28, 2018 at 9:30 PM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
> > On Wed, Jun 27, 2018 at 5:05 PM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
> >> On Tue, Jun 26, 2018 at 7:29 PM, Catalin Marinas
> >> <catalin.marinas@....com> wrote:
> >>> While I support this work, as a maintainer I'd like to understand
> >>> whether we'd be in a continuous chase of ABI breaks with every kernel
> >>> release or we have a better way to identify potential issues. Is there
> >>> any way to statically analyse conversions from __user ptr to long for
> >>> example? Or, could we get the compiler to do this for us?
> >>
> >> OK, got it, I'll try to figure out a way to find these conversions.
> >
> > I've prototyped a checker on top of clang static analyzer (initially
> > looked at sparse, but couldn't find any documentation or examples).
> > The results are here [1], search for "warning: user pointer cast".
> > Sharing in case anybody wants to take a look, will look at them myself
> > tomorrow.
> >
> > [1] https://gist.github.com/xairy/433edd5c86456a64026247cb2fef2115
> 
> So the checker reports ~100 different places where a __user pointer
> being casted. I've looked through them and found 3 places where we
> need to add untagging. Source code lines below come from 4.18-rc2+
> (6f0d349d).
[...]
> I'll add the 3 patches with fixes to v5 of this patchset.

Thanks for investigating. You can fix those three places in your code
but I was rather looking for a way to check such casting in the future
for newly added code. While for the khwasan we can assume it's a debug
option, the tagged user pointers are ABI and we need to keep it stable.

We could we actually add some macros for explicit conversion between
__user ptr and long and silence the warning there (I guess this would
work better for sparse). We can then detect new ptr to long casts as
they appear. I just hope that's not too intrusive.

(I haven't tried the sparse patch yet, hopefully sometime this week)

-- 
Catalin