path: root/virt/kvm/kvm_main.c
diff options
authorNicholas Piggin <>2021-06-24 08:29:04 -0400
committerPaolo Bonzini <>2021-06-24 11:55:11 -0400
commitf8be156be163a052a067306417cd0ff679068c97 (patch)
tree2f291bec641f75e1f350177da6179e8316699d71 /virt/kvm/kvm_main.c
parent309505dd56854c1f9744c9a2b8aa40d897002bca (diff)
KVM: do not allow mapping valid but non-reference-counted 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 valid pages if it was non-zero, which indicates it is participating in normal refcounting (and can be released with put_page). This addresses CVE-2021-22543. Signed-off-by: Nicholas Piggin <> Tested-by: Paolo Bonzini <> Cc: Signed-off-by: Paolo Bonzini <>
Diffstat (limited to 'virt/kvm/kvm_main.c')
1 files 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;
pte_unmap_unlock(ptep, ptl);
*p_pfn = pfn;
- return 0;
+ return r;