From: George Dunlap <george.dunlap@citrix.com>
Date: Thu, 16 Nov 2017 11:17:15 +0000 (+0100)
Subject: x86/mm: Make PV linear pagetables optional
X-Git-Url: http://xenbits.xensource.com/gitweb?a=commitdiff_plain;h=42ea1dc425558c5b1f6ad6bf3094a50f8e1a4f59;p=xen.git

x86/mm: Make PV linear pagetables optional

Allowing pagetables to point to other pagetables of the same level
(often called 'linear pagetables') has been included in Xen since its
inception; but recently it has been the source of a number of subtle
reference-counting bugs.

It is not used by Linux or MiniOS; but it is used by NetBSD and Novell
Netware.  There are significant numbers of people who are never going
to use the feature, along with significant numbers who need the
feature.

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---

diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index 16bfb39d02..8573f4c129 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1137,6 +1137,25 @@ The following resources are available:
   L3 cache occupancy.
   * `cmt` instructs Xen to enable/disable Cache Monitoring Technology.
   * `rmid_max` indicates the max value for rmid.
+	
+### pv-linear-pt
+> `= <boolean>`
+
+> Default: `true`
+
+Only available if Xen is compiled with CONFIG\_PV\_LINEAR\_PT support
+enabled.
+
+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.
+
+Linux and MiniOS don't use this technique.  NetBSD and Novell Netware
+do; there may be other custom operating systems which do.  If you're
+certain you don't plan on having PV guests which use this feature,
+turning it off can reduce the attack surface.
 
 ### reboot
 > `= t[riple] | k[bd] | a[cpi] | p[ci] | e[fi] | n[o] [, [w]arm | [c]old]`
diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index a3e42f721a..41d13a475c 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -734,6 +734,9 @@ static void dec_linear_uses(struct page_info *pg)
  *     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 opt_pv_linear_pt = 1;
+boolean_param("pv-linear-pt", opt_pv_linear_pt);
+
 #define define_get_linear_pagetable(level)                                  \
 static int                                                                  \
 get_##level##_linear_pagetable(                                             \
@@ -743,6 +746,12 @@ get_##level##_linear_pagetable(                                             \
     struct page_info *page;                                                 \
     unsigned long pfn;                                                      \
                                                                             \
+    if ( !opt_pv_linear_pt )                                                \
+    {                                                                       \
+        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");          \