Re: [PATCH v1 1/2] x86/kvm/fpu: Mask guest fpstate->xfeatures with guest_supported_xcr0

[Paolo Bonzini]

On 2/5/22 09:16, 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().

Hi Leonardo, is this an issue when patch 2 is applied?  With this patch, 
we have to reason about the effect of calling KVM_SET_CPUID2 twice calls 
back to back.  I think an "&=" would be wrong in that case.

On the other hand, with patch 2 the change is only in the KVM_SET_XSAVE 
output, which is much more self-contained.

Thanks,

Paolo

> 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.
>
> 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->xfeatures only hold
> values compatible to guest_supported_xcr0. This way, on 5 the only flags
> saved by xsave(s) will be the ones compatible to guest requirements,
> and thus there will be no issue during migration.
>
> Signed-off-by: Leonardo Bras <leobras@xxxxxxxxxx>
> ---
>   arch/x86/kvm/cpuid.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 28be02adc669..8ce481cc0f9b 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -296,6 +296,9 @@ 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);
>   
> +	/* Mask out features unsupported by guest */
> +	vcpu->arch.guest_fpu.fpstate->xfeatures &= vcpu->arch.guest_supported_xcr0;
> +
>   	kvm_update_pv_runtime(vcpu);
>   
>   	vcpu->arch.maxphyaddr = cpuid_query_maxphyaddr(vcpu);