linux-kernel - Re: [PATCH 1/5] KVM: x86: Reject Hyper-V's SEND_IPI hypercalls if local APIC isn't in-kernel

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Message-ID: <Z4WT5VA4N7hLQnAr@google.com>
Date: Mon, 13 Jan 2025 14:29:57 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Vitaly Kuznetsov <vkuznets@...hat.com>, Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Dongjie Zou <zoudongjie@...wei.com>
Subject: Re: [PATCH 1/5] KVM: x86: Reject Hyper-V's SEND_IPI hypercalls if
 local APIC isn't in-kernel

On Mon, Jan 13, 2025, Sean Christopherson wrote:
> Advertise support for Hyper-V's SEND_IPI and SEND_IPI_EX hypercalls if and
> only if the local API is emulated/virtualized by KVM, and explicitly reject
> said hypercalls if the local APIC is emulated in userspace, i.e. don't rely
> on userspace to opt-in to KVM_CAP_HYPERV_ENFORCE_CPUID.
> 
> Rejecting SEND_IPI and SEND_IPI_EX fixes a NULL-pointer dereference if
> Hyper-V enlightenments are exposed to the guest without an in-kernel local
> APIC:
> 
>   dump_stack+0xbe/0xfd
>   __kasan_report.cold+0x34/0x84
>   kasan_report+0x3a/0x50
>   __apic_accept_irq+0x3a/0x5c0
>   kvm_hv_send_ipi.isra.0+0x34e/0x820
>   kvm_hv_hypercall+0x8d9/0x9d0
>   kvm_emulate_hypercall+0x506/0x7e0
>   __vmx_handle_exit+0x283/0xb60
>   vmx_handle_exit+0x1d/0xd0
>   vcpu_enter_guest+0x16b0/0x24c0
>   vcpu_run+0xc0/0x550
>   kvm_arch_vcpu_ioctl_run+0x170/0x6d0
>   kvm_vcpu_ioctl+0x413/0xb20
>   __se_sys_ioctl+0x111/0x160
>   do_syscal1_64+0x30/0x40
>   entry_SYSCALL_64_after_hwframe+0x67/0xd1
> 
> Note, checking the sending vCPU is sufficient, as the per-VM irqchip_mode
> can't be modified after vCPUs are created, i.e. if one vCPU has an
> in-kernel local APIC, then all vCPUs have an in-kernel local APIC.
> 
> Reported-by: Dongjie Zou <zoudongjie@...wei.com>
> Fixes: 214ff83d4473 ("KVM: x86: hyperv: implement PV IPI send hypercalls")
> Fixes: 2bc39970e932 ("x86/kvm/hyper-v: Introduce KVM_GET_SUPPORTED_HV_CPUID")
> Cc: stable@...r.kernel

Argh, I missed the ".org".

Paolo, if you end up applying this instead of me, can you fixup the email?