Re: [PATCH] KVM: Never clear KVM_REQ_VM_DEAD from a vCPU's requests

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Unhandled KVM_REQ_VM_DEAD across multiple architectures (s390, RISC-V, PowerPC, LoongArch) causes dead VMs to either improperly resume guest execution or get stuck in an infinite kernel loop.
--

--- Patch [1]: [PATCH] KVM: Never clear KVM_REQ_VM_DEAD from a vCPU's requests ---
commit c8b0ec7047292dc9b84167da2e2a4d9abf600813
Author: Sean Christopherson <[email protected]>

KVM: Never clear KVM_REQ_VM_DEAD from a vCPU's requests

This commit modifies KVM_REQ_VM_DEAD checks to use kvm_test_request()
instead of kvm_check_request(), ensuring the flag is never cleared. It
also adds a build-time assertion to guard against clearing it via standard
APIs.

> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 03bfc92864b6e..cf7fe835c4ad2 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -2324,13 +2324,18 @@ static inline bool kvm_test_request(int req, struct kvm_vcpu *vcpu)
>  	return test_bit(req & KVM_REQUEST_MASK, (void *)&vcpu->requests);
>  }
>  
> -static inline void kvm_clear_request(int req, struct kvm_vcpu *vcpu)
> +static __always_inline void kvm_clear_request(int req, struct kvm_vcpu *vcpu)
>  {
> +	BUILD_BUG_ON(req == KVM_REQ_VM_DEAD);
> +
>  	clear_bit(req & KVM_REQUEST_MASK, (void *)&vcpu->requests);
>  }
>  
> -static inline bool kvm_check_request(int req, struct kvm_vcpu *vcpu)
> +static __always_inline bool kvm_check_request(int req, struct kvm_vcpu *vcpu)
>  {
> +	/* Once a VM is dead, it needs to stay dead. */
> +	BUILD_BUG_ON(req == KVM_REQ_VM_DEAD);
> +

[Severity: High]
This is a pre-existing issue, but while tightening the KVM_REQ_VM_DEAD API,
should this request be explicitly handled on other architectures?

Currently, s390, RISC-V, PowerPC, and LoongArch do not check for
KVM_REQ_VM_DEAD in their request handling loops. Because the flag is never
handled, kvm_request_pending() remains permanently true when a VM is marked
dead (e.g., via KVM_BUG_ON() in common pfncache or MMIO paths).

Could this cause the vCPU run loops to infinitely spin inside the kernel
without returning to userspace? For example:
- arch/riscv/kvm/vcpu.c:kvm_riscv_check_vcpu_requests()
- arch/powerpc/kvm/powerpc.c:kvm_arch_vcpu_ioctl_run() calling
  kvmppc_core_check_requests()
- arch/loongarch/kvm/vcpu.c:kvm_check_requests() inside kvm_pre_enter_guest()

Additionally, for s390:
- arch/s390/kvm/kvm-s390.c:kvm_s390_handle_requests() returns 0 when no
  other requests are handled. Can this cause the dead VM to improperly
  resume guest execution via sie64a() with a broken host state?

>  	if (kvm_test_request(req, vcpu)) {
>  		kvm_clear_request(req, vcpu);
>

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=1