Re: [PATCH v4 1/2] x86/kvm/fpu: Mask guest fpstate->xfeatures with guest_supported_xcr0

[David Edmondson]

The single line summary is now out of date - there's no new masking.

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@xxxxxxxxxx>
> ---
>  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.
-- 
All those lines and circles, to me, a mystery.