Re: [PATCH 2/3] KVM: x86/mmu: Add lockdep assert to enforce safe usage of kvm_unmap_gfn_range()

[Yan Zhao]

On Thu, Oct 10, 2024 at 09:14:41AM -0700, Sean Christopherson wrote:
> On Thu, Oct 10, 2024, Yan Zhao wrote:
> > On Wed, Oct 09, 2024 at 12:23:44PM -0700, Sean Christopherson wrote:
> > > Add a lockdep assertion in kvm_unmap_gfn_range() to ensure that either
> > > mmu_invalidate_in_progress is elevated, or that the range is being zapped
> > > due to memslot removal (loosely detected by slots_lock being held).
> > > Zapping SPTEs without mmu_invalidate_{in_progress,seq} protection is unsafe
> > > as KVM's page fault path snapshots state before acquiring mmu_lock, and
> > > thus can create SPTEs with stale information if vCPUs aren't forced to
> > > retry faults (due to seeing an in-progress or past MMU invalidation).
> > > 
> > > Memslot removal is a special case, as the memslot is retrieved outside of
> > > mmu_invalidate_seq, i.e. doesn't use the "standard" protections, and
> > > instead relies on SRCU synchronization to ensure any in-flight page faults
> > > are fully resolved before zapping SPTEs.
> > > 
> > > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > > ---
> > >  arch/x86/kvm/mmu/mmu.c | 10 ++++++++++
> > >  1 file changed, 10 insertions(+)
> > > 
> > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > index 09494d01c38e..c6716fd3666f 100644
> > > --- a/arch/x86/kvm/mmu/mmu.c
> > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > @@ -1556,6 +1556,16 @@ bool kvm_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range)
> > >  {
> > >  	bool flush = false;
> > >  
> > > +	/*
> > > +	 * To prevent races with vCPUs faulting in a gfn using stale data,
> > > +	 * zapping a gfn range must be protected by mmu_invalidate_in_progress
> > > +	 * (and mmu_invalidate_seq).  The only exception is memslot deletion,
> > > +	 * in which case SRCU synchronization ensures SPTEs a zapped after all
> > > +	 * vCPUs have unlocked SRCU and are guaranteed to see the invalid slot.
> > > +	 */
> > > +	lockdep_assert_once(kvm->mmu_invalidate_in_progress ||
> > > +			    lockdep_is_held(&kvm->slots_lock));
> > > +
> > Is the detection of slots_lock too loose?
> 
> Yes, but I can't think of an easy way to tighten it.  My original thought was to
> require range->slot to be invalid, but KVM (correctly) passes in the old, valid
> memslot to kvm_arch_flush_shadow_memslot().
> 
> The goal with the assert is to detect as many bugs as possible, without adding
> too much complexity, and also to document the rules for using kvm_unmap_gfn_range().
> 
> Actually, we can tighten the check, by verifying that the slot being unmapped is
> valid, but that the slot that KVM sees is invalid.  I'm not sure I love it though,
> as it's absurdly specific.
Right. It doesn't reflect the wait in kvm_swap_active_memslots() for the old
slot.

  CPU 0                  CPU 1
1. fault on old begins
                       2. swap to new
		       3. zap old
4. fault on old ends

Without CPU 1 waiting for 1&4 complete between 2&3, stale data is still
possible.

So, the detection in kvm_memslot_is_being_invalidated() only indicates the
caller is from kvm_arch_flush_shadow_memslot() with current code.

Given that, how do you feel about passing in a "bool is_flush_slot" to indicate
the caller and asserting?

> (untested)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index c6716fd3666f..12b87b746b59 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -1552,6 +1552,17 @@ static bool __kvm_rmap_zap_gfn_range(struct kvm *kvm,
>                                  start, end - 1, can_yield, true, flush);
>  }
>  
> +static kvm_memslot_is_being_invalidated(const struct kvm_memory_slot *old)
> +{
> +       const struct kvm_memory_slot *new;
> +
> +       if (old->flags & KVM_MEMSLOT_INVALID)
> +               return false;
> +
> +       new = id_to_memslot(__kvm_memslots(kvm, old->as_id), old->id);
> +       return new && new->flags & KVM_MEMSLOT_INVALID;
> +}
> +
>  bool kvm_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range)
>  {
>         bool flush = false;
> @@ -1564,7 +1575,8 @@ bool kvm_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range)
>          * vCPUs have unlocked SRCU and are guaranteed to see the invalid slot.
>          */
>         lockdep_assert_once(kvm->mmu_invalidate_in_progress ||
> -                           lockdep_is_held(&kvm->slots_lock));
> +                           (lockdep_is_held(&kvm->slots_lock) &&
> +                            kvm_memslot_is_being_invalidated(range->slot));
>  
>         if (kvm_memslots_have_rmaps(kvm))
>                 flush = __kvm_rmap_zap_gfn_range(kvm, range->slot,
> 
> 
> > If a caller just holds slots_lock without calling
> > "synchronize_srcu_expedited(&kvm->srcu)" as that in kvm_swap_active_memslots()
> > to ensure the old slot is retired, stale data may still be encountered. 
> > 
> > >  	if (kvm_memslots_have_rmaps(kvm))
> > >  		flush = __kvm_rmap_zap_gfn_range(kvm, range->slot,
> > >  						 range->start, range->end,
> > > -- 
> > > 2.47.0.rc1.288.g06298d1525-goog
> > >