Re: [PATCH 0/6] KVM: Remove uses of struct page from x86 and arm64 MMU

[Nicholas Piggin]

Excerpts from Paolo Bonzini's message of June 24, 2021 10:41 pm:
> On 24/06/21 13:42, Nicholas Piggin wrote:
>> Excerpts from Nicholas Piggin's message of June 24, 2021 8:34 pm:
>>> Excerpts from David Stevens's message of June 24, 2021 1:57 pm:
>>>> KVM supports mapping VM_IO and VM_PFNMAP memory into the guest by using
>>>> follow_pte in gfn_to_pfn. However, the resolved pfns may not have
>>>> assoicated struct pages, so they should not be passed to pfn_to_page.
>>>> This series removes such calls from the x86 and arm64 secondary MMU. To
>>>> do this, this series modifies gfn_to_pfn to return a struct page in
>>>> addition to a pfn, if the hva was resolved by gup. This allows the
>>>> caller to call put_page only when necessated by gup.
>>>>
>>>> This series provides a helper function that unwraps the new return type
>>>> of gfn_to_pfn to provide behavior identical to the old behavior. As I
>>>> have no hardware to test powerpc/mips changes, the function is used
>>>> there for minimally invasive changes. Additionally, as gfn_to_page and
>>>> gfn_to_pfn_cache are not integrated with mmu notifier, they cannot be
>>>> easily changed over to only use pfns.
>>>>
>>>> This addresses CVE-2021-22543 on x86 and arm64.
>>>
>>> Does this fix the problem? (untested I don't have a POC setup at hand,
>>> but at least in concept)
>> 
>> This one actually compiles at least. Unfortunately I don't have much
>> time in the near future to test, and I only just found out about this
>> CVE a few hours ago.
> 
> And it also works (the reproducer gets an infinite stream of userspace 
> exits and especially does not crash).  We can still go for David's 
> solution later since MMU notifiers are able to deal with this pages, but 
> it's a very nice patch for stable kernels.

Oh nice, thanks for testing. How's this?

Thanks,
Nick

---

KVM: Fix page ref underflow for regions with valid but non-refcounted pages

It's possible to create a region which maps valid but non-refcounted
pages (e.g., tail pages of non-compound higher order allocations). These
host pages can then be returned by gfn_to_page, gfn_to_pfn, etc., family
of APIs, which take a reference to the page, which takes it from 0 to 1.
When the reference is dropped, this will free the page incorrectly.

Fix this by only taking a reference on the page if it was non-zero,
which indicates it is participating in normal refcounting (and can be
released with put_page).

Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>
---
 virt/kvm/kvm_main.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 6a6bc7af0e28..46fb042837d2 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2055,6 +2055,13 @@ static bool vma_is_valid(struct vm_area_struct *vma, bool write_fault)
 	return true;
 }
 
+static int kvm_try_get_pfn(kvm_pfn_t pfn)
+{
+	if (kvm_is_reserved_pfn(pfn))
+		return 1;
+	return get_page_unless_zero(pfn_to_page(pfn));
+}
+
 static int hva_to_pfn_remapped(struct vm_area_struct *vma,
 			       unsigned long addr, bool *async,
 			       bool write_fault, bool *writable,
@@ -2104,13 +2111,21 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
 	 * Whoever called remap_pfn_range is also going to call e.g.
 	 * unmap_mapping_range before the underlying pages are freed,
 	 * causing a call to our MMU notifier.
+	 *
+	 * Certain IO or PFNMAP mappings can be backed with valid
+	 * struct pages, but be allocated without refcounting e.g.,
+	 * tail pages of non-compound higher order allocations, which
+	 * would then underflow the refcount when the caller does the
+	 * required put_page. Don't allow those pages here.
 	 */ 
-	kvm_get_pfn(pfn);
+	if (!kvm_try_get_pfn(pfn))
+		r = -EFAULT;
 
 out:
 	pte_unmap_unlock(ptep, ptl);
 	*p_pfn = pfn;
-	return 0;
+
+	return r;
 }
 
 /*
-- 
2.23.0