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 <npiggin@gmail.com>
Tested-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 6a6bc7a..46fb042 100644 (file)
--- 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;
 }
 
 /*