Re: [PATCH 3/4] x86/irq: KVM: Harden posted interrupt (un)registration paths

[Paolo Bonzini <pbonzini@redhat.com>]

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;
>   }
>