On Mon, Nov 29, 2021, David Stevens wrote:
> From: David Stevens <stevensd@xxxxxxxxxxxx>
>
> Remove two warnings that require ref counts for pages to be non-zero, as
> mapped pfns from follow_pfn may not have an initialized ref count.
>
> Signed-off-by: David Stevens <stevensd@xxxxxxxxxxxx>
> ---
> arch/x86/kvm/mmu/mmu.c | 7 -------
> virt/kvm/kvm_main.c | 2 +-
> 2 files changed, 1 insertion(+), 8 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 0626395ff1d9..7c4c7fededf0 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -621,13 +621,6 @@ static int mmu_spte_clear_track_bits(struct kvm *kvm, u64 *sptep)
>
> pfn = spte_to_pfn(old_spte);
>
> - /*
> - * KVM does not hold the refcount of the page used by
> - * kvm mmu, before reclaiming the page, we should
> - * unmap it from mmu first.
> - */
> - WARN_ON(!kvm_is_reserved_pfn(pfn) && !page_count(pfn_to_page(pfn)));
> -
> if (is_accessed_spte(old_spte))
> kvm_set_pfn_accessed(pfn);
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 16a8a71f20bf..d81edcb3e107 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -170,7 +170,7 @@ bool kvm_is_zone_device_pfn(kvm_pfn_t pfn)
> * the device has been pinned, e.g. by get_user_pages(). WARN if the
> * page_count() is zero to help detect bad usage of this helper.
Stale comment.
> */
> - if (!pfn_valid(pfn) || WARN_ON_ONCE(!page_count(pfn_to_page(pfn))))
> + if (!pfn_valid(pfn) || !page_count(pfn_to_page(pfn)))
Hrm, I know the whole point of this series is to support pages without an elevated
refcount, but this WARN was extremely helpful in catching several use-after-free
bugs in the TDP MMU. We talked about burying a slow check behind MMU_WARN_ON, but
that isn't very helpful because no one runs with MMU_WARN_ON, and this is also a
type of check that's most useful if it runs in production.
IIUC, this series explicitly disallows using pfns that have a struct page without
refcounting, and the issue with the WARN here is that kvm_is_zone_device_pfn() is
called by kvm_is_reserved_pfn() before ensure_pfn_ref() rejects problematic pages,
i.e. triggers false positive.
So, can't we preserve the use-after-free benefits of the check by moving it to
where KVM releases the PFN? I.e.
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index fbca2e232e94..675b835525fa 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2904,15 +2904,19 @@ EXPORT_SYMBOL_GPL(kvm_release_pfn_dirty);
void kvm_set_pfn_dirty(kvm_pfn_t pfn)
{
- if (!kvm_is_reserved_pfn(pfn) && !kvm_is_zone_device_pfn(pfn))
+ if (!kvm_is_reserved_pfn(pfn) && !kvm_is_zone_device_pfn(pfn)) {
+ WARN_ON_ONCE(!page_count(pfn_to_page(pfn)));
SetPageDirty(pfn_to_page(pfn));
+ }
}
EXPORT_SYMBOL_GPL(kvm_set_pfn_dirty);
void kvm_set_pfn_accessed(kvm_pfn_t pfn)
{
- if (!kvm_is_reserved_pfn(pfn) && !kvm_is_zone_device_pfn(pfn))
+ if (!kvm_is_reserved_pfn(pfn) && !kvm_is_zone_device_pfn(pfn)) {
+ WARN_ON_ONCE(!page_count(pfn_to_page(pfn)));
mark_page_accessed(pfn_to_page(pfn));
+ }
}
EXPORT_SYMBOL_GPL(kvm_set_pfn_accessed);
In a way, that's even better than the current check as it makes it more obvious
that the WARN is due to a use-after-free.
