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 <[email protected]>
---
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
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?
On Wed, Nov 2, 2022 at 12:47 PM Sean Christopherson <[email protected]> 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.