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

[Ben Gardon]

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!

>                 if (shared)
>                         cond_resched_rwlock_read(&kvm->mmu_lock);
>                 else
> --
> 2.34.0.rc2.393.gf8c9666880-goog
>