x86/mm: widen locked region in xenmem_add_to_physmap_one()

For pages which can be made part of the P2M by the guest, but which can
also later be de-allocated (grant table v2 status pages being the
present example), it is imperative that they be mapped at no more than a
single GFN. We therefore need to make sure that of two parallel
XENMAPSPACE_grant_table requests for the same status page one completes
before the second checks at which other GFN the underlying MFN is
presently mapped.

Pull ahead the respective get_gfn() and push down the respective
put_gfn(). This leverages that gfn_lock() really aliases p2m_lock(), but
the function makes this assumption already anyway: In the
XENMAPSPACE_gmfn case lock nesting constraints for both involved GFNs
would otherwise need to be enforced to avoid ABBA deadlocks.

This is CVE-2021-28697 / XSA-379.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
master commit: f147422bf9
master date: 2021-08-25 14:17:56 +0200

xen/arch/x86/mm.c

@@ -4768,8 +4768,20 @@ int xenmem_add_to_physmap_one(
         goto put_both;
     }
 
-    /* Remove previously mapped page if it was present. */
+    /*
+     * Note that we're (ab)using GFN locking (to really be locking of the
+     * entire P2M) here in (at least) two ways: Finer grained locking would
+     * expose lock order violations in the XENMAPSPACE_gmfn case (due to the
+     * earlier get_gfn_unshare() above). Plus at the very least for the grant
+     * table v2 status page case we need to guarantee that the same page can
+     * only appear at a single GFN. While this is a property we want in
+     * general, for pages which can subsequently be freed this imperative:
+     * Upon freeing we wouldn't be able to find other mappings in the P2M
+     * (unless we did a brute force search).
+     */
     prev_mfn = get_gfn(d, gfn_x(gpfn), &p2mt);
+
+    /* Remove previously mapped page if it was present. */
     if ( p2mt == p2m_mmio_direct )
         rc = -EPERM;
     else if ( mfn_valid(prev_mfn) )
@@ -4781,27 +4793,21 @@ int xenmem_add_to_physmap_one(
             /* Normal domain memory is freed, to avoid leaking memory. */
             rc = guest_remove_page(d, gfn_x(gpfn));
     }
-    /* In the XENMAPSPACE_gmfn case we still hold a ref on the old page. */
-    put_gfn(d, gfn_x(gpfn));
-
-    if ( rc )
-        goto put_both;
 
     /* Unmap from old location, if any. */
     old_gpfn = get_gpfn_from_mfn(mfn_x(mfn));
     ASSERT(!SHARED_M2P(old_gpfn));
     if ( space == XENMAPSPACE_gmfn && old_gpfn != gfn )
-    {
         rc = -EXDEV;
-        goto put_both;
-    }
-    if ( old_gpfn != INVALID_M2P_ENTRY )
+    else if ( !rc && old_gpfn != INVALID_M2P_ENTRY )
         rc = guest_physmap_remove_page(d, _gfn(old_gpfn), mfn, PAGE_ORDER_4K);
 
     /* Map at new location. */
     if ( !rc )
         rc = guest_physmap_add_page(d, gpfn, mfn, PAGE_ORDER_4K);
 
+    put_gfn(d, gfn_x(gpfn));
+
  put_both:
     /*
      * In the XENMAPSPACE_gmfn case, we took a ref of the gfn at the top.