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

[Siddharth Chandrasekaran <sidcha@amazon.de>]

On Mon, May 03, 2021 at 08:53:09PM +0000, Sean Christopherson wrote:
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> 
> 
> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
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> 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>

Thanks for the review. Changed as suggested.

~ Sid.



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