Re: [PATCH v2 1/2] KVM: x86/mmu: Ensure TDP MMU roots are freed after yield

[Maciej S. Szmigiero]

On 06.01.2021 20:03, Ben Gardon wrote:
> On Wed, Jan 6, 2021 at 10:59 AM Ben Gardon <bgardon@xxxxxxxxxx> wrote:
> > Many TDP MMU functions which need to perform some action on all TDP MMU
> > roots hold a reference on that root so that they can safely drop the MMU
> > lock in order to yield to other threads. However, when releasing the
> > reference on the root, there is a bug: the root will not be freed even
> > if its reference count (root_count) is reduced to 0.
> >
> > To simplify acquiring and releasing references on TDP MMU root pages, and
> > to ensure that these roots are properly freed, move the get/put operations
> > into the TDP MMU root iterator macro. Not all functions which use the macro
> > currently get and put a reference to the root, but adding this behavior is
> > harmless.
> >
> > Moving the get/put operations into the iterator macro also helps
> > simplify control flow when a root does need to be freed. Note that using
> > the list_for_each_entry_unsafe macro would not have been appropriate in
> > this situation because it could keep a reference to the next root across
> > an MMU lock release + reacquire.
> >
> > Reported-by: Maciej S. Szmigiero <maciej.szmigiero@xxxxxxxxxx>
> > Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Fixes: faaf05b00aec ("kvm: x86/mmu: Support zapping SPTEs in the TDP MMU")
> > Fixes: 063afacd8730 ("kvm: x86/mmu: Support invalidate range MMU notifier for TDP MMU")
> > Fixes: a6a0b05da9f3 ("kvm: x86/mmu: Support dirty logging for the TDP MMU")
> > Fixes: 14881998566d ("kvm: x86/mmu: Support disabling dirty logging for the tdp MMU")
> > Signed-off-by: Ben Gardon <bgardon@xxxxxxxxxx>
> > ---
(..)
> I tested v2 with Maciej's test
> (https://urldefense.com/v3/__https://gist.github.com/maciejsszmigiero/890218151c242d99f63ea0825334c6c0__;!!GqivPVa7Brio!NUh8Xbu1YkhSf49HvbyhI-svvPmJXWj9KECqaEd7ZJMKPdz-HdND1sKduH2VpwasEN8Gpg$ ,
> near the bottom of the page) on an Intel Skylake Machine and can
> confirm that v1 failed the test but v2 passes. The problem with v1 was
> that roots were being removed from the list before list_next_entry was
> called, resulting in a bad value.

I've tested the fix now and can confirm, too, that I can no longer
observe any crash.

Thanks,
Maciej