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

Sean Christopherson
2021-05-03T20:53:17Z

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)
> --