Re: [PATCH RFC] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

David Hildenbrand
2022-12-07T20:12:01Z

>> For example, libvhost-user.c in QEMU uses for ordinary postcopy:
>>
>> /*
>> * In postcopy we're using PROT_NONE here to catch anyone
>> * accessing it before we userfault.
>> */
>> mmap_addr = mmap(0, dev_region->size + dev_region->mmap_offset,
>> PROT_NONE, MAP_SHARED | MAP_NORESERVE,
>> vmsg->fds[0], 0);
>
> I assume this is for missing mode only. More on wr-protect mode below.
>
> Personally I don't see immediately on whether this is needed. If the
> process itself is trusted then it should be under control of anyone who
> will be accessing the pages.. If the other threads are not trusted, then
> there's no way to stop anyone from mprotect(RW) after mprotect(NONE)
> anyway..

I think there is a difference between code that can read/write memory
(e.g., rings/buffers in libvhost-user.c, where I think this was added to
detect such early access) and code that can execute arbitrary mprotect()
to voluntarily break the system. I think that's the whole reason
libvhost-user.c went that direction.

>
> So I may not really get the gut of it.
>
> Another way to make sure no one access it is right after receiving the
> memory range from QEMU (VhostUserMemoryRegion), if VuDev.postcopy_listening
> is set, then we register the range with UFFD missing immediately. After
> all if postcopy_listening is set it means we passed the advise phase
> already (VHOST_USER_POSTCOPY_ADVISE). Any potential access will be blocked
> until QEMU starts to read on that uffd.
>
>>
>> I'd imagine, when using uffd-wp (VM snapshotting with shmem?) one might use
>> PROT_READ instead before the write-protection is properly set. Because read
>> access would be fine in the meantime.
>
> It'll be different for wr-protect IIUC, because unlike missing protections,
> we don't worry about writes happening before UFFDIO_WRITEPROTECT.
>
> IMHO the solo thing the vhost-user proc needs to do is one
> UFFDIO_WRITEPROTECT for each of the range when QEMU tells it to, then it'll
> be fine. Pre-writes are fine.
>
> Sorry I probably went a bit off-topic. I just want to make sure I don't
> miss any real use case of having mprotect being useful for uffd-wp being
> there, because that used to be a grey area for me.
>
>>
>> But I'm just pulling use cases out of my magic hat ;) Nothing stops user
>> space from doing things that are not clearly forbidden (well, even then
>> users might complain, but that's a different story).
>
> Yes, I think those are always fine but the user just cannot assume it'll
> work as they assumed how it will work.
>
> If "doing things that are not clearly forbidden" triggers a host warning or
> crash that's a bug, OTOH if the outcome is limited to the process itself
> then from kernel pov I think we're good. I used to even thought about
> forbid mprotect() on uffd-wp but I'm not sure whether it's good idea either.
>
> Let's see whether I missed something above, if so I'll rethink.

Let's not get distracted too much. As a reminder, I wrote that test case
to showcase that other kernel code behaves just like the migration code
does. It was the long hanging fruit to make a point, I'm happy to
exclude it for now.

Now, my 2 cents on the whole topic regarding "supported", "not
supported" etc:

(1) If something is not supported we should bail out or at least warn
the user. I'm pretty sure there are other uffd-wp dummy users like
me. Skimming over the man userfaultfd page nothing in particular
regarding PROT_WRITE, mprotect(), ... maybe I looked at the wrong
page.
(2) If something is easy to support, support it instead of having all
these surprises for users and having to document them and warn the
user. Makes all these discussions regarding what's supported, what's
a valid use case, etc ... much easier.
(3) Inconsistency confuses users. If something used to work for anon,
in an ideal world, we make shmem behave in a similar, non-surprising
way.
(4) There are much smarter people like me with much more advanced
magical hats. I'm pretty sure they will come up with use cases that
I am not even able to anticipate right now.
(5) Users will make any imaginable mistake possible and point at the
doc, that nothing speaks against it and that the system didn't bail
out.

Again, just my 2 cents. Maybe the dos and don'ts of userfaultfd-wp are
properly documented already and we just don't bail out.

--
Thanks,

David / dhildenb