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

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Message-ID: <20180626172900.ufclp2pfrhwkxjco@armageddon.cambridge.arm.com>
Date:   Tue, 26 Jun 2018 18:29:01 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Robin Murphy <robin.murphy@....com>,
        Al Viro <viro@...iv.linux.org.uk>,
        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>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-doc@...r.kernel.org,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-arch@...r.kernel.org, linux-kselftest@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Chintan Pandya <cpandya@...eaurora.org>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Lee Smith <Lee.Smith@....com>,
        Kostya Serebryany <kcc@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Evgeniy Stepanov <eugenis@...gle.com>
Subject: Re: [PATCH v4 0/7] arm64: untag user pointers passed to the kernel

Hi Andrey,

On Tue, Jun 26, 2018 at 02:47:50PM +0200, Andrey Konovalov wrote:
> On Wed, Jun 20, 2018 at 5:24 PM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
> > arm64 has a feature called Top Byte Ignore, which allows to embed pointer
> > tags into the top byte of each pointer. Userspace programs (such as
> > HWASan, a memory debugging tool [1]) might use this feature and pass
> > tagged user pointers to the kernel through syscalls or other interfaces.
> >
> > This patch makes a few of the kernel interfaces accept tagged user
> > pointers. The kernel is already able to handle user faults with tagged
> > pointers and has the untagged_addr macro, which this patchset reuses.
> >
> > We're not trying to cover all possible ways the kernel accepts user
> > pointers in one patchset, so this one should be considered as a start.
> >
> > Thanks!
> >
> > [1] http://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html
> 
> Is there anything I should do to move forward with this?
> 
> I've received zero replies to this patch set (v3 and v4) over the last
> month.

The patches in this series look fine but my concern is that they are not
sufficient and we don't have (yet?) a way to identify where such
annotations are required. You even say in patch 6 that this is "some
initial work for supporting non-zero address tags passed to the kernel".
Unfortunately, merging (or relaxing) an ABI without a clear picture is
not really feasible.

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?

Thanks.

-- 
Catalin