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SubjectRe: [PATCH 3/4] x86/irq: KVM: Harden posted interrupt (un)registration paths
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On 09/10/21 02:11, Sean Christopherson wrote:
> Split the register and unregister paths for the posted interrupt wakeup
> handler, and WARN on conditions that are blatant bugs, e.g. attempting to
> overwrite an existing handler, unregistering the wrong handler, etc...
> This is very much a "low hanging fruit" hardening, e.g. a broken module
> could foul things up by doing concurrent registration from multiple CPUs.
>
> Drop the use of a dummy handler so that the rejection logic can use a
> simple NULL check. There is zero benefit to blindly calling into a dummy
> handler.
>
> Note, the registration path doesn't require synchronization, as it's the
> caller's responsibility to not generate interrupts it cares about until
> after its handler is registered, i.e. there can't be a relevant in-flight
> interrupt.
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Seems a bit overengineered, considering there are just two callers. Doing

diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
index 20773d315308..766ffe3ba313 100644
--- a/arch/x86/kernel/irq.c
+++ b/arch/x86/kernel/irq.c
@@ -291,9 +291,10 @@ void kvm_set_posted_intr_wakeup_handler(void (*handler)(void))
{
if (handler)
kvm_posted_intr_wakeup_handler = handler;
- else
+ else {
kvm_posted_intr_wakeup_handler = dummy_handler;
- synchronize_rcu();
+ synchronize_rcu();
+ }
}
EXPORT_SYMBOL_GPL(kvm_set_posted_intr_wakeup_handler);

on top of path 1 ensures that the synchronization is only done on the
unregistration path.

Paolo

> --
> arch/x86/include/asm/irq.h | 3 ++-
> arch/x86/kernel/irq.c | 29 ++++++++++++++++++++---------
> arch/x86/kvm/vmx/vmx.c | 4 ++--
> 3 files changed, 24 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/include/asm/irq.h b/arch/x86/include/asm/irq.h
> index 768aa234cbb4..c79014c2443d 100644
> --- a/arch/x86/include/asm/irq.h
> +++ b/arch/x86/include/asm/irq.h
> @@ -30,7 +30,8 @@ struct irq_desc;
> extern void fixup_irqs(void);
>
> #ifdef CONFIG_HAVE_KVM
> -extern void kvm_set_posted_intr_wakeup_handler(void (*handler)(void));
> +extern void kvm_register_posted_intr_wakeup_handler(void (*handler)(void));
> +extern void kvm_unregister_posted_intr_wakeup_handler(void (*handler)(void));
> #endif
>
> extern void (*x86_platform_ipi_callback)(void);
> diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
> index 20773d315308..97f452cc84be 100644
> --- a/arch/x86/kernel/irq.c
> +++ b/arch/x86/kernel/irq.c
> @@ -284,18 +284,26 @@ DEFINE_IDTENTRY_SYSVEC(sysvec_x86_platform_ipi)
> #endif
>
> #ifdef CONFIG_HAVE_KVM
> -static void dummy_handler(void) {}
> -static void (*kvm_posted_intr_wakeup_handler)(void) = dummy_handler;
> +static void (*kvm_posted_intr_wakeup_handler)(void);
>
> -void kvm_set_posted_intr_wakeup_handler(void (*handler)(void))
> +void kvm_register_posted_intr_wakeup_handler(void (*handler)(void))
> {
> - if (handler)
> - kvm_posted_intr_wakeup_handler = handler;
> - else
> - kvm_posted_intr_wakeup_handler = dummy_handler;
> + if (WARN_ON_ONCE(!handler || kvm_posted_intr_wakeup_handler))
> + return;
> +
> + WRITE_ONCE(kvm_posted_intr_wakeup_handler, handler);
> +}
> +EXPORT_SYMBOL_GPL(kvm_register_posted_intr_wakeup_handler);
> +
> +void kvm_unregister_posted_intr_wakeup_handler(void (*handler)(void))
> +{
> + if (WARN_ON_ONCE(!handler || handler != kvm_posted_intr_wakeup_handler))
> + return;
> +
> + WRITE_ONCE(kvm_posted_intr_wakeup_handler, NULL);
> synchronize_rcu();
> }
> -EXPORT_SYMBOL_GPL(kvm_set_posted_intr_wakeup_handler);
> +EXPORT_SYMBOL_GPL(kvm_unregister_posted_intr_wakeup_handler);
>
> /*
> * Handler for POSTED_INTERRUPT_VECTOR.
> @@ -311,9 +319,12 @@ DEFINE_IDTENTRY_SYSVEC_SIMPLE(sysvec_kvm_posted_intr_ipi)
> */
> DEFINE_IDTENTRY_SYSVEC(sysvec_kvm_posted_intr_wakeup_ipi)
> {
> + void (*handler)(void) = READ_ONCE(kvm_posted_intr_wakeup_handler);
> +
> ack_APIC_irq();
> inc_irq_stat(kvm_posted_intr_wakeup_ipis);
> - kvm_posted_intr_wakeup_handler();
> + if (handler)
> + handler();
> }
>
> /*
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index bfdcdb399212..9164f1870d49 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7553,7 +7553,7 @@ static void vmx_migrate_timers(struct kvm_vcpu *vcpu)
>
> static void hardware_unsetup(void)
> {
> - kvm_set_posted_intr_wakeup_handler(NULL);
> + kvm_unregister_posted_intr_wakeup_handler(pi_wakeup_handler);
>
> if (nested)
> nested_vmx_hardware_unsetup();
> @@ -7907,7 +7907,7 @@ static __init int hardware_setup(void)
> if (r)
> nested_vmx_hardware_unsetup();
>
> - kvm_set_posted_intr_wakeup_handler(pi_wakeup_handler);
> + kvm_register_posted_intr_wakeup_handler(pi_wakeup_handler);
>
> return r;
> }
>

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