Re: [PATCH] KVM: x86: Hoist input checks in kvm_add_msr_filter()

[Sean Christopherson <seanjc@google.com>]

On Mon, May 03, 2021, Siddharth Chandrasekaran wrote:
> In ioctl KVM_X86_SET_MSR_FILTER, input from user space is validated
> after a memdup_user(). For invalid inputs we'd memdup and then call
> kfree unnecessarily. Hoist input validation to avoid kfree altogether.
> 
> Signed-off-by: Siddharth Chandrasekaran <sidcha@amazon.de>
> ---
>  arch/x86/kvm/x86.c | 21 ++++++---------------
>  1 file changed, 6 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ee0dc58ac3a5..15c20b31cc91 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5393,11 +5393,16 @@ static int kvm_add_msr_filter(struct kvm_x86_msr_filter *msr_filter,
>  	struct msr_bitmap_range range;
>  	unsigned long *bitmap = NULL;
>  	size_t bitmap_size;
> -	int r;
>  
>  	if (!user_range->nmsrs)
>  		return 0;
>  
> +	if (user_range->flags & ~(KVM_MSR_FILTER_READ | KVM_MSR_FILTER_WRITE))
> +		return -EINVAL;
> +
> +	if (!user_range->flags)
> +		return -EINVAL;
> +
>  	bitmap_size = BITS_TO_LONGS(user_range->nmsrs) * sizeof(long);
>  	if (!bitmap_size || bitmap_size > KVM_MSR_FILTER_MAX_BITMAP_SIZE)
>  		return -EINVAL;
> @@ -5413,24 +5418,10 @@ static int kvm_add_msr_filter(struct kvm_x86_msr_filter *msr_filter,
>  		.bitmap = bitmap,
>  	};
>  
> -	if (range.flags & ~(KVM_MSR_FILTER_READ | KVM_MSR_FILTER_WRITE)) {
> -		r = -EINVAL;
> -		goto err;
> -	}
> -
> -	if (!range.flags) {
> -		r = -EINVAL;
> -		goto err;
> -	}
> -
> -	/* Everything ok, add this range identifier. */
>  	msr_filter->ranges[msr_filter->count] = range;

Might be worth elminating the intermediate "range", too.  Doesn't affect output,
but it would make it a little more obvious that the new range is mostly coming
straight from userspace input.  E.g.

	msr_filter->ranges[msr_filter->count] = (struct msr_bitmap_range) {
		.flags = user_range->flags,
		.base = user_range->base,
		.nmsrs = user_range->nmsrs,
		.bitmap = bitmap,
	};

Either way:

Reviewed-by: Sean Christopherson <seanjc@google.com>

>  	msr_filter->count++;
>  
>  	return 0;
> -err:
> -	kfree(bitmap);
> -	return r;
>  }
>  
>  static int kvm_vm_ioctl_set_msr_filter(struct kvm *kvm, void __user *argp)
> --