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

[David Edmondson]

On Saturday, 2022-02-12 at 12:02:14 +01, Paolo Bonzini wrote:

> 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;
>
> ?

Sorry if this is a daft question:

In what situations will there be bits set in
vcpu->arch.guest_supported_xcr0 that are not set in
vcpu->arch.guest_fpu.fpstate->xfeatures ?

guest_supported_xcr0 is filtered based on supported_xcr0, which I would
expect to weed out all bits that are not set in ->xfeatures.

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

dme.
-- 
I used to get mad at my school, the teachers who taught me weren't cool.