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 add6a0cd1c5b ("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 2e2e3738af33 ("KVM:
Handle vma regions with no backing page", 2008-07-20), as they also did
not check whether the PFN was writable.

Fixes: 2e2e3738af33 ("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>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 8367d88..335a1a2 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -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;
 }