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

[Andrey Konovalov]

On Tue, Jun 26, 2018 at 7:29 PM, Catalin Marinas
<catalin.marinas@xxxxxxx> 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@xxxxxxxxxx> 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.

Thanks!