linux-kernel - Re: [PATCH v4 0/2] x86/kvm/fpu: Fix guest migration bugs that can crash guest

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Date:   Thu, 17 Feb 2022 15:52:28 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Leonardo Bras <leobras@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        David Gilbert <dgilbert@...hat.com>,
        Peter Xu <peterx@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 0/2] x86/kvm/fpu: Fix guest migration bugs that can
 crash guest

On 2/17/22 06:30, Leonardo Bras wrote:
> This patchset comes from a bug I found during qemu guest migration from a
> host with newer CPU to a host with an older version of this CPU, and thus
> having less FPU features.
> 
> When the guests were created, the one with less features is used as
> config, so migration is possible.
> 
> Patch 1 fix a bug that always happens during this migration, and is
> related to the fact that xsave saves all feature flags, but xrstor does
> not touch the PKRU flag. It also changes how fpstate->user_xfeatures
> is set, going from kvm_check_cpuid() to the later called
> kvm_vcpu_after_set_cpuid().
> 
> Patch 2 removes kvm_vcpu_arch.guest_supported_xcr0 since it now
> duplicates guest_fpu.fpstate->user_xfeatures. Some wrappers were
> introduced in order to make it easier to read the replaced version.
> 
> Patches were compile-tested, and could fix the bug found.

Queued, thanks (for 5.17 of course)!  For patch 2, I renamed the 
function to kvm_guest_supported_xcr0.

Paolo

> Please let me know of anything to improve!
> 
> Best regards,
> Leo
> 
> --
> Changes since v3:
> - Add new patch to remove the use of kvm_vcpu_arch.guest_supported_xcr0,
>    since it is now duplicating guest_fpu.fpstate->user_xfeatures.
> - On patch 1, also avoid setting user_xfeatures on kvm_check_cpuid(),
>    since it is already set in kvm_vcpu_after_set_cpuid() now.
> Changes since v2:
> - Fix building error because I forgot to EXPORT_SYMBOL(fpu_user_cfg)
> Changes since v1:
> - Instead of masking xfeatures, mask user_xfeatures instead. This will
>    only change the value sent to user, instead of the one saved in buf.
> - Above change removed the need of the patch 2/2
> - Instead of masking the current value of user_xfeatures, save on it
>    fpu_user_cfg.default_features & vcpu->arch.guest_supported_xcr0
> 
> Leonardo Bras (2):
>    x86/kvm/fpu: Mask guest fpstate->xfeatures with guest_supported_xcr0
>    x86/kvm/fpu: Remove kvm_vcpu_arch.guest_supported_xcr0
> 
>   arch/x86/include/asm/kvm_host.h |  1 -
>   arch/x86/kernel/fpu/xstate.c    |  5 ++++-
>   arch/x86/kvm/cpuid.c            |  5 ++++-
>   arch/x86/kvm/x86.c              | 20 +++++++++++++++-----
>   4 files changed, 23 insertions(+), 8 deletions(-)
>