linux-kernel - Re: [PATCH 0/2] KVM: x86: Fix an FPU+CET splat

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Message-ID: <sr4fifgvdqmjg54waukwtfaws3vdblgsb5vtfxpp3crfwzbtzf@wzjlefmpc7vb>
Date: Fri, 31 Oct 2025 14:28:07 +0800
From: Yao Yuan <yaoyuan@...ux.alibaba.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH 0/2] KVM: x86: Fix an FPU+CET splat

On Thu, Oct 30, 2025 at 11:58:00AM +0800, Sean Christopherson wrote:
> Fix a explosion found via syzkaller+KASAN where KVM attempts to "put" an
> FPU without first having loading the FPU.  The underlying problem is the
> ugly hack for dealing with INIT being processed during MP_STATE.
>
> KVM needs to ensure the FPU state is resident in memory in order to clear
> MPX and CET state.  In most cases, INIT is emulated during KVM_RUN, and so
> KVM needs to put the FPU.  But for MP_STATE, the FPU doesn't need to be
> loaded, and so isn't.  Except when KVM predicts that the FPU will be
> unloaded.  CET enabling updated the "put" path but missed the prediction
> logic in MP_STATE.
>
> Rip out the ugly hack and instead do the obvious-in-hindsight thing of
> checking if the FPU is loaded (or not).  To retain a sanity check, e.g.
> that the FPU is loaded as expected during KVM_RUN, WARN if the FPU being
> loaded and the vCPU wanting to run aren't equal.
>
> Sean Christopherson (2):
>   KVM: x86: Unload "FPU" state on INIT if and only if its currently
>     in-use
>   KVM: x86: Harden KVM against imbalanced load/put of guest FPU state
>
>  arch/x86/kvm/x86.c | 31 +++++++++++++++++++++----------
>  1 file changed, 21 insertions(+), 10 deletions(-)
>
With my experience on KVM AMX related issue debugging before, and revisit
the kvm_load_guest_fpu() today:

Reviewed-by: Yao Yuan <yaoyuan@...ux.alibaba.com>

>
> base-commit: 4361f5aa8bfcecbab3fc8db987482b9e08115a6a
> --
> 2.51.1.930.gacf6e81ea2-goog
>