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
