Re: [PATCH v3 1/1] x86/kvm/fpu: Mask guest fpstate->xfeatures with guest_supported_xcr0

[Paolo Bonzini]

On 2/11/22 07:07, 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 only hold
> values compatible to 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, will also fail if userspace tries to set fpu features
> that are not compatible to the guest configuration. (KVM_SET_XSAVE ioctl)
>
> Signed-off-by: Leonardo Bras <leobras@xxxxxxxxxx>
> ---
>   arch/x86/kernel/fpu/core.c | 1 +
>   arch/x86/kvm/cpuid.c       | 4 ++++
>   2 files changed, 5 insertions(+)
>
> diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> index 8dea01ffc5c1..e83d8b1fbc83 100644
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -34,6 +34,7 @@ DEFINE_PER_CPU(u64, xfd_state);
>   /* The FPU state configuration data for kernel and user space */
>   struct fpu_state_config	fpu_kernel_cfg __ro_after_init;
>   struct fpu_state_config fpu_user_cfg __ro_after_init;
> +EXPORT_SYMBOL(fpu_user_cfg);
>   
>   /*
>    * Represents the initial FPU state. It's mostly (but not completely) zeroes,
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 494d4d351859..aecebd6bc490 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -296,6 +296,10 @@ 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->user_xfeatures =
> +		fpu_user_cfg.default_features & vcpu->arch.guest_supported_xcr0;

This is not correct, because default_features does not include the
optional features (such as AMX) that were the original reason to
go through all this mess.  What about:

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

?

Paolo

>   	kvm_update_pv_runtime(vcpu);
>   
>   	vcpu->arch.maxphyaddr = cpuid_query_maxphyaddr(vcpu);