Re: [RFC][Patch v8 4/7] KVM: Disabling page poisoning to prevent corruption

[Alexander Duyck <alexander.duyck@gmail.com>]

On Thu, Feb 7, 2019 at 9:56 AM Nitesh Narayan Lal <nitesh@redhat.com> wrote:
>
>
> On 2/7/19 12:23 PM, Alexander Duyck wrote:
> > On Mon, Feb 4, 2019 at 2:11 PM Nitesh Narayan Lal <nitesh@redhat.com> wrote:
> >> This patch disables page poisoning if guest page hinting is enabled.
> >> It is required to avoid possible guest memory corruption errors.
> >> Page Poisoning is a feature in which the page is filled with a specific
> >> pattern of (0x00 or 0xaa) after arch_free_page and the same is verified
> >> before arch_alloc_page to prevent following issues:
> >>     *information leak from the freed data
> >>     *use after free bugs
> >>     *memory corruption
> >> Selection of the pattern depends on the CONFIG_PAGE_POISONING_ZERO
> >> Once the guest pages which are supposed to be freed are sent to the
> >> hypervisor it frees them. After freeing the pages in the global list
> >> following things may happen:
> >>     *Hypervisor reallocates the freed memory back to the guest
> >>     *Hypervisor frees the memory and maps a different physical memory
> >> In order to prevent any information leak hypervisor before allocating
> >> memory to the guest fills it with zeroes.
> >> The issue arises when the pattern used for Page Poisoning is 0xaa while
> >> the newly allocated page received from the hypervisor by the guest is
> >> filled with the pattern 0x00. This will result in memory corruption errors.
> >>
> >> Signed-off-by: Nitesh Narayan Lal <nitesh@redhat.com>
> > This seems kind of backwards to me. Why disable page poisoning instead
> > of just not hinting about the free pages? There shouldn't be that many
> > instances when page poisoning is enabled, and when it is it would make
> > more sense to leave it enabled rather than silently disable it.
> As I have mentioned in the cover email, I intend to reuse Wei's already
> merged work.
>
> This will enable the guest to communicate the poison value which is in
> use to the host.

That is far from being reliable given that you are having to buffer
the pages for some period of time. I really think it would be better
to just allow page poisoning to function and when you can support
applying poison to a newly allocated page then you could look at
re-enabling it.

What I am getting at is that those that care about poisoning won't
likely care about performance and I would lump the memory hinting in
with other performance features.