Re: [PATCH v5 23/39] mm: Don't allow write GUPs to shadow stack memory

David Hildenbrand
2023-01-25T09:30:15Z

On 25.01.23 00:41, Edgecombe, Rick P wrote:
> On Tue, 2023-01-24 at 15:08 -0800, Kees Cook wrote:
>>> GDB support for shadow stack is queued up for whenever the kernel
>>> interface settles. I believe it just uses ptrace, and not this
>>> proc.
>>> But yea ptrace poke will still need to use FOLL_FORCE and be able
>>> to
>>> write through shadow stacks.
>>
>> I'd prefer to avoid adding more FOLL_FORCE if we can. If gdb can do
>> stack manipulations through a ptrace interface then let's leave off
>> FOLL_FORCE.
>
> Ptrace and /proc/self/mem both use FOLL_FORCE. I think ptrace will
> always need it or something like it for debugging.
>
> To jog your memory, this series doesn't change what uses FOLL_FORCE. It
> just sets the shadow stack rules to be the same as read-only memory. So
> even though shadow stack memory is sort of writable, it's a bit more
> locked down and FOLL_FORCE is required to write to it with GUP.
>
> If we just remove FOLL_FORCE from /proc/self/mem, something will
> probably break right? How do we do this? Some sort of opt-in?

I don't think removing that is an option. It's another debug interface
that has been allowing such access for ever ...

Blocking /proc/self/mem access completely for selected processes might
be the better alternative.

--
Thanks,

David / dhildenb