Date:   Thu, 17 Feb 2022 16:07:14 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Edmondson <david.edmondson@...cle.com>,
        Leonardo Bras <leobras@...hat.com>
Cc:     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>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/2] x86/kvm/fpu: Mask guest fpstate->xfeatures with
 guest_supported_xcr0

On 2/17/22 13:07, David Edmondson wrote:
> The single line summary is now out of date - there's no new masking.

Thanks for the review, I made the adjustments and pushed to master.

Paolo

> On Thursday, 2022-02-17 at 02:30:29 -03, Leonardo Bras wrote:
> 
>> During host/guest switch (like in kvm_arch_vcpu_ioctl_run()), the kernel
>> swaps the fpu between host/guest contexts, by using fpu_swap_kvm_fpstate().
>>
>> When xsave feature is available, the fpu swap is done by:
>> - xsave(s) instruction, with guest's fpstate->xfeatures as mask, is used
>>    to store the current state of the fpu registers to a buffer.
>> - xrstor(s) instruction, with (fpu_kernel_cfg.max_features &
>>    XFEATURE_MASK_FPSTATE) as mask, is used to put the buffer into fpu regs.
>>
>> For xsave(s) the mask is used to limit what parts of the fpu regs will
>> be copied to the buffer. Likewise on xrstor(s), the mask is used to
>> limit what parts of the fpu regs will be changed.
>>
>> The mask for xsave(s), the guest's fpstate->xfeatures, is defined on
>> kvm_arch_vcpu_create(), which (in summary) sets it to all features
>> supported by the cpu which are enabled on kernel config.
>>
>> This means that xsave(s) will save to guest buffer all the fpu regs
>> contents the cpu has enabled when the guest is paused, even if they
>> are not used.
>>
>> This would not be an issue, if xrstor(s) would also do that.
>>
>> xrstor(s)'s mask for host/guest swap is basically every valid feature
>> contained in kernel config, except XFEATURE_MASK_PKRU.
>> Accordingto kernel src, it is instead switched in switch_to() and
>> flush_thread().
>>
>> Then, the following happens with a host supporting PKRU starts a
>> guest that does not support it:
>> 1 - Host has XFEATURE_MASK_PKRU set. 1st switch to guest,
>> 2 - xsave(s) fpu regs to host fpustate (buffer has XFEATURE_MASK_PKRU)
>> 3 - xrstor(s) guest fpustate to fpu regs (fpu regs have XFEATURE_MASK_PKRU)
>> 4 - guest runs, then switch back to host,
>> 5 - xsave(s) fpu regs to guest fpstate (buffer now have XFEATURE_MASK_PKRU)
>> 6 - xrstor(s) host fpstate to fpu regs.
>> 7 - kvm_vcpu_ioctl_x86_get_xsave() copy guest fpstate to userspace (with
>>      XFEATURE_MASK_PKRU, which should not be supported by guest vcpu)
>>
>> On 5, even though the guest does not support PKRU, it does have the flag
>> set on guest fpstate, which is transferred to userspace via vcpu ioctl
>> KVM_GET_XSAVE.
>>
>> This becomes a problem when the user decides on migrating the above guest
>> to another machine that does not support PKRU:
>> The new host restores guest's fpu regs to as they were before (xrstor(s)),
>> but since the new host don't support PKRU, a general-protection exception
>> ocurs in xrstor(s) and that crashes the guest.
>>
>> This can be solved by making the guest's fpstate->user_xfeatures hold
>> a copy of guest_supported_xcr0. This way, on 7 the only flags copied to
>> userspace will be the ones compatible to guest requirements, and thus
>> there will be no issue during migration.
>>
>> As a bonus, it will also fail if userspace tries to set fpu features
>> that are not compatible to the guest configuration. (KVM_SET_XSAVE ioctl)
>>
>> Also, since kvm_vcpu_after_set_cpuid() now sets fpstate->user_xfeatures,
>> there is not need to set it in kvm_check_cpuid(). So, change
>> fpstate_realloc() so it does not touch fpstate->user_xfeatures if a
>> non-NULL guest_fpu is passed, which is the case when kvm_check_cpuid()
>> calls it.
>>
>> Signed-off-by: Leonardo Bras <leobras@...hat.com>
>> ---
>>   arch/x86/kernel/fpu/xstate.c | 5 ++++-
>>   arch/x86/kvm/cpuid.c         | 2 ++
>>   2 files changed, 6 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
>> index 02b3ddaf4f75..7c7824ae7862 100644
>> --- a/arch/x86/kernel/fpu/xstate.c
>> +++ b/arch/x86/kernel/fpu/xstate.c
>> @@ -1558,7 +1558,10 @@ static int fpstate_realloc(u64 xfeatures, unsigned int ksize,
>>   		fpregs_restore_userregs();
>>
>>   	newfps->xfeatures = curfps->xfeatures | xfeatures;
>> -	newfps->user_xfeatures = curfps->user_xfeatures | xfeatures;
>> +
>> +	if (!guest_fpu)
>> +		newfps->user_xfeatures = curfps->user_xfeatures | xfeatures;
>> +
>>   	newfps->xfd = curfps->xfd & ~xfeatures;
>>
>>   	/* Do the final updates within the locked region */
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 494d4d351859..71125291c578 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -296,6 +296,8 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>>   	vcpu->arch.guest_supported_xcr0 =
>>   		cpuid_get_supported_xcr0(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent);
>>
>> +	vcpu->arch.guest_fpu.fpstate->user_xfeatures = vcpu->arch.guest_supported_xcr0;
>> +
>>   	kvm_update_pv_runtime(vcpu);
>>
>>   	vcpu->arch.maxphyaddr = cpuid_query_maxphyaddr(vcpu);
> 
> dme.

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