Commit bd2fae8d authored Feb 01, 2021 by Paolo Bonzini

KVM: do not assume PTE is writable after follow_pfn



In order to convert an HVA to a PFN, KVM usually tries to use
the get_user_pages family of functinso.  This however is not
possible for VM_IO vmas; in that case, KVM instead uses follow_pfn.

In doing this however KVM loses the information on whether the
PFN is writable.  That is usually not a problem because the main
use of VM_IO vmas with KVM is for BARs in PCI device assignment,
however it is a bug.  To fix it, use follow_pte and check pte_write
while under the protection of the PTE lock.  The information can
be used to fail hva_to_pfn_remapped or passed back to the
caller via *writable.

Usage of follow_pfn was introduced in commit add6a0cd ("KVM: MMU: try to fix
up page faults before giving up", 2016-07-05); however, even older version
have the same issue, all the way back to commit 2e2e3738 ("KVM:
Handle vma regions with no backing page", 2008-07-20), as they also did
not check whether the PFN was writable.

Fixes: 2e2e3738 ("KVM: Handle vma regions with no backing page")
Reported-by: David Stevens <stevensd@google.com>
Cc: 3pvd@google.com
Cc: Jann Horn <jannh@google.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

parent 87aa9ec9

virt/kvm/kvm_main.c

+12 −3

Original line number	Diff line number	Diff line
		@@ -1904,9 +1904,11 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
		kvm_pfn_t *p_pfn)
		{
		unsigned long pfn;
		pte_t *ptep;
		spinlock_t *ptl;
		int r;

		r = follow_pfn(vma, addr, &pfn);
		r = follow_pte(vma->vm_mm, addr, NULL, &ptep, NULL, &ptl);
		if (r) {
		/*
		* get_user_pages fails for VM_IO and VM_PFNMAP vmas and does
		@@ -1921,14 +1923,19 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
		if (r)
		return r;

		r = follow_pfn(vma, addr, &pfn);
		r = follow_pte(vma->vm_mm, addr, NULL, &ptep, NULL, &ptl);
		if (r)
		return r;
		}

		if (write_fault && !pte_write(*ptep)) {
		pfn = KVM_PFN_ERR_RO_FAULT;
		goto out;
		}

		if (writable)
		*writable = true;
		writable = pte_write(ptep);
		pfn = pte_pfn(*ptep);

		/*
		* Get a reference here because callers of hva_to_pfn and
		@@ -1943,6 +1950,8 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
		*/
		kvm_get_pfn(pfn);

		out:
		pte_unmap_unlock(ptep, ptl);
		*p_pfn = pfn;
		return 0;
		}