Re: [PATCH v2 1/5] KVM: do not allow mapping valid but non-refcounted pages

On 25.06.21 09:36, David Stevens wrote:

From: Nicholas Piggin npiggin@gmail.com

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@gmail.com

I guess this would be the small fix for stable? Do we want to add that cc?

Reviewed-by: Christian Borntraeger borntraeger@de.ibm.com

---

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 3dcc2abbfc60..f7445c3bcd90 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -2175,6 +2175,13 @@ 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,

@@ -2224,13 +2231,21 @@ 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; }

  /*