RE: [PATCH] hyperv: mshv: zero VTL hypercall input page

Hi Michael,

That makes sense. Please drop this patch.

I sent a v2 for the output-page issue with the mshv_vtl prefix and
your suggested changes.

Thanks,
Yousef

On Thu, 25 Jun 2026 16:41:51 +0000, Michael Kelley <[email protected]> wrote:
> From: Yousef Alhouseen <[email protected]> Sent: Wednesday, June 24, 2026 10:57 AM
> > Subject: [PATCH] hyperv: mshv: zero VTL hypercall input page
> >
>
> Same comment here about the patch "Subject:" prefix.
>
> > mshv_vtl_hvcall_call() copies only the user-provided input size.
> >
> > It then passes the page to hv_do_hypercall().
> >
> > For short inputs, stale bytes can remain in the bounce page.
> >
> > Those bytes can be consumed by the hypervisor.
>
> It's unclear to me that there's really a problem here. In a
> CoCo VM, the host hypervisor isn't trusted, so hypercall sites
> must be careful to only expose intended data in the hypercall
> input and output pages. But this code already doesn't support
> CoCo VMs, as noted in the comment. So in the supported
> scenario, the hypervisor has access to all of guest memory. Passing
> stale bytes to the hypervisor vs. passing zeros really wouldn't matter.
> And user space can already pass stale/garbage bytes to the hypervisor
> if it wants to. This code doesn't try to validate the input data for
> whatever hypercall user space is requesting to be made.
>
> When support for CoCo VMs is added, this code will indeed
> need to make sure not to allow garbage kernel data in the
> hypercall input or output pages. But decrypting the pages
> so the hypervisor can access them should take care of that
> issue.
>
> Michael
>
> >
> > Allocate the input page zeroed, matching the output page.
> >
> > Signed-off-by: Yousef Alhouseen <[email protected]>
> > ---
> > drivers/hv/mshv_vtl_main.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/hv/mshv_vtl_main.c b/drivers/hv/mshv_vtl_main.c
> > index 0365d207c..f2633148c 100644
> > --- a/drivers/hv/mshv_vtl_main.c
> > +++ b/drivers/hv/mshv_vtl_main.c
> > @@ -1146,7 +1146,7 @@ static int mshv_vtl_hvcall_call(struct mshv_vtl_hvcall_fd *fd,
> > *
> > * TODO: Take care of this when CVM support is added.
> > */
> > - in = (void *)__get_free_page(GFP_KERNEL);
> > + in = (void *)__get_free_page(GFP_KERNEL | __GFP_ZERO);
> > out = (void *)__get_free_page(GFP_KERNEL | __GFP_ZERO);
> > if (!in || !out) {
> > ret = -ENOMEM;
> > --
> > 2.54.0
> >