linux-kernel - Re: [PATCH 0/3] KVM: x86: Fix XSAVE related bugs

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Message-ID: <YwYMInTCevZ/FYNl@work-vm>
Date:   Wed, 24 Aug 2022 12:31:46 +0100
From:   "Dr. David Alan Gilbert" <dgilbert@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Leonardo Bras <leobras@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH 0/3] KVM: x86: Fix XSAVE related bugs

* Sean Christopherson (seanjc@...gle.com) wrote:
> Patch 2 (from Dave) is the headliner and fixes a bug where KVM clear the
> FP+SSE bits in user_xfeatures when XSAVE is hidden from the guest and thus
> prevent userspace from saving/restoring FP+SSE state on XSAVE host.  This
> most visibily manifests as a failed migration (KVM_GET_XSAVE succeeds on a
> non-XSAVE host and KVM_SET_XSAVE fails on an XSAVE host), but also causes
> KVM_GET_SAVE on XSAVE hosts to effectively corrupt guest FP+SSE state.
> 
> Patch 1 fixes a mostly theoretical bug, and is also a prerequisite for
> patch 2.
> 
> Patch 3 fixes a bug found by inspection when staring at all of this.  KVM
> fails to check CR4.OSXSAVE when emulating XSETBV (the interception case
> gets away without the check because the intercept happens after hardware
> checks CR4).

Thanks for pulling those together; the set of 3 passes my same (light) smoke test.

Dave
> 
> Dr. David Alan Gilbert (1):
>   KVM: x86: Always enable legacy FP/SSE in allowed user XFEATURES
> 
> Sean Christopherson (2):
>   KVM: x86: Reinstate kvm_vcpu_arch.guest_supported_xcr0
>   KVM: x86: Inject #UD on emulated XSETBV if XSAVES isn't enabled
> 
>  arch/x86/include/asm/kvm_host.h |  1 +
>  arch/x86/kvm/cpuid.c            | 11 ++++++++---
>  arch/x86/kvm/emulate.c          |  3 +++
>  arch/x86/kvm/x86.c              | 10 +++-------
>  4 files changed, 15 insertions(+), 10 deletions(-)
> 
> 
> base-commit: 372d07084593dc7a399bf9bee815711b1fb1bcf2
> -- 
> 2.37.1.595.g718a3a8f04-goog
> 
-- 
Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK