Re: [PATCH 2/3] KVM: x86: add system attribute to retrieve full set of supported xsave states

[Paolo Bonzini <pbonzini@redhat.com>]

On 1/27/22 16:35, Sean Christopherson wrote:
> On Wed, Jan 26, 2022, Paolo Bonzini wrote:
>> +static int kvm_x86_dev_get_attr(struct kvm_device_attr *attr)
>> +{
>> +	if (attr->group)
>> +		return -ENXIO;
>> +
>> +	switch (attr->attr) {
>> +	case KVM_X86_XCOMP_GUEST_SUPP:
>> +		if (put_user(supported_xcr0, (u64 __user *)attr->addr))
> 
> Deja vu[*].
> 
>    arch/x86/kvm/x86.c: In function ‘kvm_x86_dev_get_attr’:
>    arch/x86/kvm/x86.c:4345:46: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
>     4345 |                 if (put_user(supported_xcr0, (u64 __user *)attr->addr))
>          |                                              ^
>    arch/x86/include/asm/uaccess.h:221:31: note: in definition of macro ‘do_put_user_call’
>      221 |         register __typeof__(*(ptr)) __val_pu asm("%"_ASM_AX);           \
>          |                               ^~~
>    arch/x86/kvm/x86.c:4345:21: note: in expansion of macro ‘put_user’
>     4345 |                 if (put_user(supported_xcr0, (u64 __user *)attr->addr))
>          |                     ^~~~~~~~
>    arch/x86/kvm/x86.c:4345:46: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
>     4345 |                 if (put_user(supported_xcr0, (u64 __user *)attr->addr))
>          |                                              ^
>    arch/x86/include/asm/uaccess.h:223:21: note: in definition of macro ‘do_put_user_call’
>      223 |         __ptr_pu = (ptr);                                               \
>          |                     ^~~
>    arch/x86/kvm/x86.c:4345:21: note: in expansion of macro ‘put_user’
>     4345 |                 if (put_user(supported_xcr0, (u64 __user *)attr->addr))
>          |                     ^~~~~~~~
>    arch/x86/kvm/x86.c:4345:46: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
>     4345 |                 if (put_user(supported_xcr0, (u64 __user *)attr->addr))
>          |                                              ^
>    arch/x86/include/asm/uaccess.h:230:45: note: in definition of macro ‘do_put_user_call’
>      230 |                        [size] "i" (sizeof(*(ptr)))                      \
>          |                                             ^~~
>    arch/x86/kvm/x86.c:4345:21: note: in expansion of macro ‘put_user’
>     4345 |                 if (put_user(supported_xcr0, (u64 __user *)attr->addr))
> 
> Given that we're collectively 2 for 2 in mishandling {g,s}et_attr(), what about
> a prep pacth like so?  Compile tested only...
> 
> From: Sean Christopherson <seanjc@google.com>
> Date: Thu, 27 Jan 2022 07:31:53 -0800
> Subject: [PATCH] KVM: x86: Add a helper to retrieve userspace address from
>   kvm_device_attr
> 
> Add a helper to handle converting the u64 userspace address embedded in
> struct kvm_device_attr into a userspace pointer, it's all too easy to
> forget the intermediate "unsigned long" cast as well as the truncation
> check.
> 
> No functional change intended.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   arch/x86/kvm/x86.c | 28 +++++++++++++++++++++-------
>   1 file changed, 21 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8033eca6f3a1..67836f7c71f5 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4335,14 +4335,28 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>   	return r;
>   }
> 
> +static inline void __user *kvm_get_attr_addr(struct kvm_device_attr *attr)
> +{
> +	void __user *uaddr = (void __user*)(unsigned long)attr->addr;
> +
> +	if ((u64)(unsigned long)uaddr != attr->addr)
> +		return ERR_PTR(-EFAULT);
> +	return uaddr;
> +}
> +
>   static int kvm_x86_dev_get_attr(struct kvm_device_attr *attr)
>   {
> +	u64 __user *uaddr = kvm_get_attr_addr(attr);
> +
>   	if (attr->group)
>   		return -ENXIO;
> 
> +	if (IS_ERR(uaddr))
> +		return PTR_ERR(uaddr);
> +
>   	switch (attr->attr) {
>   	case KVM_X86_XCOMP_GUEST_SUPP:
> -		if (put_user(supported_xcr0, (u64 __user *)attr->addr))
> +		if (put_user(supported_xcr0, uaddr))
>   			return -EFAULT;
>   		return 0;
>   	default:
> @@ -5070,11 +5084,11 @@ static int kvm_arch_tsc_has_attr(struct kvm_vcpu *vcpu,
>   static int kvm_arch_tsc_get_attr(struct kvm_vcpu *vcpu,
>   				 struct kvm_device_attr *attr)
>   {
> -	u64 __user *uaddr = (u64 __user *)(unsigned long)attr->addr;
> +	u64 __user *uaddr = kvm_get_attr_addr(attr);
>   	int r;
> 
> -	if ((u64)(unsigned long)uaddr != attr->addr)
> -		return -EFAULT;
> +	if (IS_ERR(uaddr))
> +		return PTR_ERR(uaddr);
> 
>   	switch (attr->attr) {
>   	case KVM_VCPU_TSC_OFFSET:
> @@ -5093,12 +5107,12 @@ static int kvm_arch_tsc_get_attr(struct kvm_vcpu *vcpu,
>   static int kvm_arch_tsc_set_attr(struct kvm_vcpu *vcpu,
>   				 struct kvm_device_attr *attr)
>   {
> -	u64 __user *uaddr = (u64 __user *)(unsigned long)attr->addr;
> +	u64 __user *uaddr = kvm_get_attr_addr(attr);
>   	struct kvm *kvm = vcpu->kvm;
>   	int r;
> 
> -	if ((u64)(unsigned long)uaddr != attr->addr)
> -		return -EFAULT;
> +	if (IS_ERR(uaddr))
> +		return PTR_ERR(uaddr);
> 
>   	switch (attr->attr) {
>   	case KVM_VCPU_TSC_OFFSET: {
> --
> 

Nice, I applied it.

Paolo