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

[Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>]

On 27/06/2018 16:05, Andrey Konovalov wrote:
> On Tue, Jun 26, 2018 at 7:29 PM, Catalin Marinas
> <catalin.marinas@arm.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@google.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!
>