Re: [PATCH RFC v2 4/6] x86: Disable PTI on compatibility mode

Dmitry Safonov
2018-02-16T16:25:55Z

2018-02-15 20:02 GMT+00:00 Andy Lutomirski <luto@kernel.org>:
> On Thu, Feb 15, 2018 at 4:36 PM, Nadav Amit <namit@vmware.com> wrote:
>> Based on the understanding that there should be no way for userspace to
>> address the kernel-space from compatibility mode, disable it while
>> running in compatibility mode as long as the 64-bit code segment of the
>> user is not used.
>>
>> Reenabling PTI is performed by restoring NX-bits to the userspace
>> mappings, flushing the TLBs, and notifying all the CPUs that use the
>> affected mm to disable PTI. Each core responds by removing the present
>> bit for the 64-bit code-segment, and marking that PTI is disabled on
>> that core.
>>
>
> I dislike this patch because it's conflating two things. The patch
> claims to merely disable PTI for compat tasks, whatever those are.
> But it's also introducing a much stronger concept of what a compat
> task is. The kernel currently mostly doesn't care whether a task is
> "compat" or not, and I think that most remaining code paths that do
> care are buggy and should be removed.

Yes, please, don't do a stronger concept..
Speaking from CRIU side, it C/R ia32 tasks with x86_64 binaries.

--
Dmitry