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

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Message-ID: <0cef1643-a523-98e7-95e2-9ec595137642@arm.com>
Date:   Wed, 27 Jun 2018 16:08:09 +0100
From:   Ramana Radhakrishnan <ramana.radhakrishnan@....com>
To:     Andrey Konovalov <andreyknvl@...gle.com>,
        Catalin Marinas <Catalin.Marinas@....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-doc@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-kselftest@...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>,
        Evgeniy Stepanov <eugenis@...gle.com>, nd <nd@....com>
Subject: Re: [PATCH v4 0/7] arm64: untag user pointers passed to the kernel

On 27/06/2018 16:05, Andrey Konovalov wrote:
> On Tue, Jun 26, 2018 at 7:29 PM, Catalin Marinas
> <catalin.marinas@....com> wrote:
>> 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?
> 
> 
> OK, got it, I'll try to figure out a way to find these conversions.


This sounds like the kind of thing we should be able to get sparse to do
already, no ? It's been many years since I last looked at it but I
thought sparse was the tool of choice in the kernel to do this kind of
checking.

regards
Ramana



> 
> Thanks!
>