Re: [PATCH 27/28] KVM: x86/mmu: Do remote TLB flush before dropping RCU in TDP MMU resched

[Sean Christopherson]

On Tue, Nov 23, 2021, Ben Gardon wrote:
> On Fri, Nov 19, 2021 at 8:51 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> >
> > When yielding in the TDP MMU iterator, service any pending TLB flush
> > before  dropping RCU protections in anticipation of using the callers RCU
> > "lock" as a proxy for vCPUs in the guest.
> >
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> 
> Reviewed-by: Ben Gardon <bgardon@xxxxxxxxxx>
> 
> > ---
> >  arch/x86/kvm/mmu/tdp_mmu.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index 79a52717916c..55c16680b927 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -732,11 +732,11 @@ static inline bool tdp_mmu_iter_cond_resched(struct kvm *kvm,
> >                 return false;
> >
> >         if (need_resched() || rwlock_needbreak(&kvm->mmu_lock)) {
> > -               rcu_read_unlock();
> > -
> >                 if (flush)
> >                         kvm_flush_remote_tlbs(kvm);
> >
> > +               rcu_read_unlock();
> > +
> 
> Just to check my understanding:
> Theoretically PT memory could be freed as soon as we release the RCU
> lock, if this is the only thread in a read critical section.In order
> to ensure that we can use RCU as a proxy for TLB flushes we need to
> flush the TLBs while still holding the RCU read lock. Without this
> change (and with the next one) we could wind up in a situation where
> we drop the RCU read lock, then the RCU callback runs and frees the
> memory, and then the guest does a lookup through the paging structure
> caches and we get a use-after-free bug. By flushing in an RCU critical
> section, we ensure that the TLBs will have been flushed by the time
> the RCU callback runs to free the memory. Clever!

Yep, exactly.