linux-kernel - Re: [PATCH v3 1/1] x86/kvm/fpu: Mask guest fpstate->xfeatures with guest_supported

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Mon, 14 Feb 2022 10:56:22 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Edmondson <david.edmondson@...cle.com>
Cc:     Leonardo Bras <leobras@...hat.com>,
        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>,
        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>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Yang Zhong <yang.zhong@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v3 1/1] x86/kvm/fpu: Mask guest fpstate->xfeatures with
 guest_supported_xcr0

On 2/14/22 10:43, David Edmondson wrote:
> Sorry if this is a daft question:
> 
> In what situations will there be bits set in
> vcpu->arch.guest_supported_xcr0 that are not set in
> vcpu->arch.guest_fpu.fpstate->xfeatures ?
> 
> guest_supported_xcr0 is filtered based on supported_xcr0, which I would
> expect to weed out all bits that are not set in ->xfeatures.

Good point, so we can do just

	vcpu->arch.guest_fpu.fpstate->user_xfeatures =
		vcpu->arch.guest_supported_xcr0;

On top of this patch, we can even replace vcpu->arch.guest_supported_xcr0
with vcpu->arch.guest_fpu.fpstate->user_xfeatures.  Probably with local
variables or wrapper functions though, so as to keep the code readable.
For example:

static inline u64 kvm_guest_supported_xfd()
{
	u64 guest_supported_xcr0 = vcpu->arch.guest_fpu.fpstate->user_xfeatures;

	return guest_supported_xcr0 & XFEATURE_MASK_USER_DYNAMIC;
}

Also, already in this patch fpstate_realloc should do

         newfps->user_xfeatures = curfps->user_xfeatures | xfeatures;

only if !guest_fpu.  In other words, the user_xfeatures of the guest FPU
should be controlled exclusively by KVM_SET_CPUID2.

Thanks,

Paolo