Re: [PATCH] KVM: x86/mmu: Protect noncoherent DMA zaps with SRCU

[Sean Christopherson <seanjc@google.com>]

On Mon, Aug 24, 2026, Kai Huang wrote:
> On Mon, 2026-08-24 at 10:49 -0700, Sean Christopherson wrote:
> > Alternatively, what if we have kvm_zap_gfn_range() acquire SRCU?  Nesting "locks"
> > is a-ok, and two of the three users of kvm_zap_gfn_range() have had this bug (see
> > commit 074c00800719 ("KVM: x86: Use SRCU to protect zap in
> > __kvm_set_or_clear_apicv_inhibit()").
> 
> kvm_vcpu_srcu_read_lock() doesn't allow nesting lock of KVM's SRCU.

No, kvm_vcpu_srcu_read_lock() doesn't allow nested usage of itself.  Commit
2031f2876896 ("KVM: Add helpers to wrap vcpu->srcu_idx and yell if it's abused")
added the protection because we had multiple bugs where KVM would clobber
vcpu->srcu_idx and cause a deadlock by leaking a grace period reference.

> Doesn't seem to be a problem for this particular case, though, because
> sev_handle_rmp_fault() calls kvm_zap_gfn_range() after
> kvm_vcpu_srcu_read_lock() is called.

kvm_vcpu_srcu_read_lock() can nested inside an existing kvm->srcu critical section,
just so long as the existing critical section isn't tracking its grace period via
vcpu->____srcu_idx.