Re: BUG selftests/mm]

[James Houghton <jthoughton@google.com>]

On Mon, Mar 11, 2024 at 12:28 PM Peter Xu <peterx@redhat.com> wrote:
>
> On Mon, Mar 11, 2024 at 11:59:59AM -0700, Axel Rasmussen wrote:
> > I'd prefer not to require root or CAP_SYS_ADMIN or similar for
> > UFFDIO_POISON, because those control access to lots more things
> > besides, which we don't necessarily want the process using UFFD to be
> > able to do. :/

I agree; UFFDIO_POISON should not require CAP_SYS_ADMIN.

> >
> > Ratelimiting seems fairly reasonable to me. I do see the concern about
> > dropping some addresses though.
>
> Do you know how much could an admin rely on such addresses?  How frequent
> would MCE generate normally in a sane system?

I'm not sure about how much admins rely on the address themselves. +cc
Jiaqi Yan

It's possible for a sane hypervisor dealing with a buggy guest / guest
userspace to trigger lots of these pr_errs. Consider the case where a
guest userspace uses HugeTLB-1G, finds poison (which HugeTLB used to
ignore), and then ignores SIGBUS. It will keep getting MCEs /
SIGBUSes.

The sane hypervisor will use UFFDIO_POISON to prevent the guest from
re-accessing *real* poison, but we will still get the pr_err, and we
still keep injecting MCEs into the guest. We have observed scenarios
like this before.

>
> > Perhaps we can mitigate that concern by defining our own ratelimit
> > interval/burst configuration?
>
> Any details?
>
> > Another idea would be to only ratelimit it if !CONFIG_DEBUG_VM or
> > similar. Not sure if that's considered valid or not. :)
>
> This, OTOH, sounds like an overkill..
>
> I just checked again on the detail of ratelimit code, where we by default
> it has:
>
> #define DEFAULT_RATELIMIT_INTERVAL      (5 * HZ)
> #define DEFAULT_RATELIMIT_BURST         10
>
> So it allows a 10 times burst rather than 2.. IIUC it means even if
> there're continous 10 MCEs it won't get suppressed, until the 11th came, in
> 5 seconds interval.  I think it means it's possibly even less of a concern
> to directly use pr_err_ratelimited().

I'm okay with any rate limiting everyone agrees on. IMO, silencing
these pr_errs if they came from UFFDIO_POISON (or, perhaps, if they
did not come from real hardware MCE events) sounds like the most
correct thing to do, but I don't mind. Just don't make UFFDIO_POISON
require CAP_SYS_ADMIN. :)

Thanks.