File xsa240-2.patch of Package xen.6121

x86/mm: Disable PV linear pagetables by default

Allowing pagetables to point to other pagetables of the same level
(often called 'linear pagetables') has been included in Xen since its
inception.  But it is not used by the most common PV guests (Linux,
NetBSD, minios), and has been the source of a number of subtle
reference-counting bugs.

Add a command-line option to control whether PV linear pagetables are
allowed (disabled by default).

Note: Upstream defaults to false. SUSE defaults to true for NetWare.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -855,6 +855,21 @@ This option can be specified more than o
 ### ple\_window
 > `= <integer>`
 
+### pv-linear-pt
+> `= <boolean>`
+
+> Default: `true`
+
+Allow PV guests to have pagetable entries pointing to other pagetables
+of the same level (i.e., allowing L2 PTEs to point to other L2 pages).
+This technique is often called "linear pagetables", and is sometimes
+used to allow operating systems a simple way to consistently map the
+current process's pagetables into its own virtual address space.
+
+None of the most common PV operating systems (Linux, NetBSD, MiniOS)
+use this technique, but there may be custom operating systems which
+do.
+
 ### reboot
 > `= t[riple] | k[bd] | a[cpi] | p[ci] | e[fi] | n[o] [, [w]arm | [c]old]`
 
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -741,6 +741,9 @@ static void dec_linear_uses(struct page_
  *     frame if it is mapped by a different root table. This is sufficient and
  *     also necessary to allow validation of a root table mapping itself.
  */
+static bool_t __read_mostly pv_linear_pt_enable = 1;
+boolean_param("pv-linear-pt", pv_linear_pt_enable);
+
 #define define_get_linear_pagetable(level)                                  \
 static int                                                                  \
 get_##level##_linear_pagetable(                                             \
@@ -750,6 +753,12 @@ get_##level##_linear_pagetable(
     struct page_info *page;                                                 \
     unsigned long pfn;                                                      \
                                                                             \
+    if ( !pv_linear_pt_enable )                                             \
+    {                                                                       \
+        MEM_LOG("Attempt to create linear p.t. (feature disabled)\n");      \
+        return 0;                                                           \
+    }                                                                       \
+                                                                            \
     if ( (level##e_get_flags(pde) & _PAGE_RW) )                             \
     {                                                                       \
         MEM_LOG("Attempt to create linear p.t. with write perms");          \