[PATCH] KVM: x86: Use SRCU to protect zap in __kvm_set_or_clear_apicv_inhibit

[PATCH] KVM: x86: Use SRCU to protect zap in __kvm_set_or_clear_apicv_inhibit

[Ben Gardon]

kvm_zap_gfn_range must be called in an SRCU read-critical section, but
there is no SRCU annotation in __kvm_set_or_clear_apicv_inhibit.
Add the needed SRCU annotation.

Tested: ran tools/testing/selftests/kvm/x86_64/debug_regs on a DBG
	build. This patch causes the suspicious RCU warning to disappear.
	Note that the warning is hit in __kvm_zap_rmaps, so
	kvm_memslots_have_rmaps must return true in order for this to
	repro (i.e. the TDP MMU must be off or nesting in use.)

Fixes: 36222b117e36 ("KVM: x86: don't disable APICv memslot when inhibited")
Signed-off-by: Ben Gardon <bgardon@google.com>
---
 arch/x86/kvm/x86.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index cd9eb13e2ed7..6d853e5f683d 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10091,7 +10091,10 @@ void __kvm_set_or_clear_apicv_inhibit(struct kvm *kvm,
 		kvm->arch.apicv_inhibit_reasons = new;
 		if (new) {
 			unsigned long gfn = gpa_to_gfn(APIC_DEFAULT_PHYS_BASE);
+			int idx = srcu_read_lock(&kvm->srcu);
+
 			kvm_zap_gfn_range(kvm, gfn, gfn+1);
+			srcu_read_unlock(&kvm->srcu, idx);
 		}
 	} else {
 		kvm->arch.apicv_inhibit_reasons = new;
-- 
2.38.1.431.g37b22c650d-goog

Re: [PATCH] KVM: x86: Use SRCU to protect zap in __kvm_set_or_clear_apicv_inhibit

[Sean Christopherson]

On Wed, Nov 02, 2022, Ben Gardon wrote:
> kvm_zap_gfn_range must be called in an SRCU read-critical section, but

Please add parantheses when referencing functions, i.e. kvm_zap_gfn_range().

> there is no SRCU annotation in __kvm_set_or_clear_apicv_inhibit.

__kvm_set_or_clear_apicv_inhibit()

> Add the needed SRCU annotation.

It's not an annotation, acquiring SRCU is very much functional code.

> Tested: ran tools/testing/selftests/kvm/x86_64/debug_regs on a DBG
> 	build. This patch causes the suspicious RCU warning to disappear.
> 	Note that the warning is hit in __kvm_zap_rmaps, so
> 	kvm_memslots_have_rmaps must return true in order for this to
> 	repro (i.e. the TDP MMU must be off or nesting in use.)

Please provide the stack trace or at least a verbal description of what paths
can reach __kvm_set_or_clear_apicv_inhibit() without holding SRCU, i.e. explain
why this bug isn't being hit left and right.

E.g.

  Unconditionally take KVM's SRCU lock in __kvm_set_or_clear_apicv_inhibit()
  when zapping virtual APIC SPTEs.  SRCU must be held when zapping SPTEs in
  shadow MMUs to protect the gfn=>memslot translation (the TDP MMU walks all
  roots and so doesn't dereference memslots).

  In most cases, the inhibits are updated during KVM_RUN and so SRCU is
  already held, but other ioctls() can also modify inhibits and don't
  acquire SRCU, e.g. KVM_SET_GUEST_DEBUG and KVM_SET_LAPIC.  Acquire SRCU
  unconditionally to avoid playing whack-a-mole, as nesting SRCU locks is
  safe and this is not a hot path.

> Fixes: 36222b117e36 ("KVM: x86: don't disable APICv memslot when inhibited")

Reported-by?  IIRC this originated in a syzkaller report?

Re: [PATCH] KVM: x86: Use SRCU to protect zap in __kvm_set_or_clear_apicv_inhibit

[Ben Gardon]

On Wed, Nov 2, 2022 at 12:47 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Wed, Nov 02, 2022, Ben Gardon wrote:
> > kvm_zap_gfn_range must be called in an SRCU read-critical section, but
>
> Please add parantheses when referencing functions, i.e. kvm_zap_gfn_range().
>
> > there is no SRCU annotation in __kvm_set_or_clear_apicv_inhibit.
>
> __kvm_set_or_clear_apicv_inhibit()
>
> > Add the needed SRCU annotation.
>
> It's not an annotation, acquiring SRCU is very much functional code.

Right, totally true. Will correct.

>
> > Tested: ran tools/testing/selftests/kvm/x86_64/debug_regs on a DBG
> >       build. This patch causes the suspicious RCU warning to disappear.
> >       Note that the warning is hit in __kvm_zap_rmaps, so
> >       kvm_memslots_have_rmaps must return true in order for this to
> >       repro (i.e. the TDP MMU must be off or nesting in use.)
>
> Please provide the stack trace or at least a verbal description of what paths
> can reach __kvm_set_or_clear_apicv_inhibit() without holding SRCU, i.e. explain
> why this bug isn't being hit left and right.
>
> E.g.
>
>   Unconditionally take KVM's SRCU lock in __kvm_set_or_clear_apicv_inhibit()
>   when zapping virtual APIC SPTEs.  SRCU must be held when zapping SPTEs in
>   shadow MMUs to protect the gfn=>memslot translation (the TDP MMU walks all
>   roots and so doesn't dereference memslots).
>
>   In most cases, the inhibits are updated during KVM_RUN and so SRCU is
>   already held, but other ioctls() can also modify inhibits and don't
>   acquire SRCU, e.g. KVM_SET_GUEST_DEBUG and KVM_SET_LAPIC.  Acquire SRCU
>   unconditionally to avoid playing whack-a-mole, as nesting SRCU locks is
>   safe and this is not a hot path.
>
> > Fixes: 36222b117e36 ("KVM: x86: don't disable APICv memslot when inhibited")
>
> Reported-by?  IIRC this originated in a syzkaller report?

This was found on an non-upstream Google kernel by Greg Thelen, but a
great point. I'll credit him in v2.