linux-kernel - Re: [PATCH v5 04/19] KVM:x86: Refresh CPUID on write to guest MSR_IA32

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]

Message-ID: <0655c963-78e5-62c9-50af-20d9de8a1001@intel.com>
Date:   Wed, 9 Aug 2023 16:56:34 +0800
From:   "Yang, Weijiang" <weijiang.yang@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     <pbonzini@...hat.com>, <peterz@...radead.org>,
        <john.allen@....com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <rick.p.edgecombe@...el.com>,
        <chao.gao@...el.com>, <binbin.wu@...ux.intel.com>,
        Zhang Yi Z <yi.z.zhang@...ux.intel.com>
Subject: Re: [PATCH v5 04/19] KVM:x86: Refresh CPUID on write to guest
 MSR_IA32_XSS

On 8/5/2023 2:27 AM, Sean Christopherson wrote:
> On Thu, Aug 03, 2023, Yang Weijiang wrote:
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 0b9033551d8c..5d6d6fa33e5b 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -3780,10 +3780,12 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>   		 * IA32_XSS[bit 8]. Guests have to use RDMSR/WRMSR rather than
>>   		 * XSAVES/XRSTORS to save/restore PT MSRs.
>>   		 */
>> -		if (data & ~kvm_caps.supported_xss)
>> +		if (data & ~vcpu->arch.guest_supported_xss)
> Hmm, this is arguably wrong for userspace-initiated writes, as it would prevent
> userspace from restoring MSRs before CPUID.
>
> And it would make the handling of MSR_IA32_XSS writes inconsistent just within
> this case statement.  The initial "can this MSR be written at all" check would
> *not* honor guest CPUID for host writes, but then the per-bit check *would* honor
> guest CPUID for host writes.
>
> But if we exempt host writes, then we'll end up with another mess, as exempting
> host writes for MSR_KVM_GUEST_SSP would let the guest coerce KVM into writing an
> illegal value by modifying SMRAM while in SMM.
>
> Blech.
>
> If we can get away with it, i.e. not break userspace, I think my preference is
> to enforce guest CPUID for host accesses to XSS, XFD, XFD_ERR, etc.  I'm 99%
> certain we can make that change, because there are many, many MSRs that do NOT
> exempt host writes, i.e. the only way this would be a breaking change is if
> userspace is writing things like XSS before KVM_SET_CPUID2, but other MSRs after
> KVM_SET_CPUID2.
>
> I'm pretty sure I've advocated for the exact opposite in the past, i.e. argued
> that KVM's ABI is to not enforce ordering between KVM_SET_CPUID2 and KVM_SET_MSR.
> But this is becoming untenable, juggling the dependencies in KVM is complex and
> is going to result in a nasty bug at some point.
>
> For this series, lets just tighten the rules for XSS, i.e. drop the host_initated
> exemption.  And in a parallel/separate series, try to do a wholesale cleanup of
> all the cases that essentially allow userspace to do KVM_SET_MSR before KVM_SET_CPUID2.
OK, will do it for this series and investigate for other MSRs.
Thanks!