From 9a4b34729f1bb92eea1e1efe52e6face9f0b17ae Mon Sep 17 00:00:00 2001
From: George Dunlap <george.dunlap@citrix.com>
Date: Fri, 22 Sep 2017 11:46:55 +0100
Subject: [PATCH 2/2] 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).

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
Changes since v2:
- s/_/-/; in command-line option
- Added __read_mostly
---
 docs/misc/xen-command-line.markdown | 15 +++++++++++++++
 xen/arch/x86/mm.c                   |  9 +++++++++
 2 files changed, 24 insertions(+)

diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index 73f5265fc6..061aff5edc 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1280,6 +1280,21 @@ The following resources are available:
     CDP, one COS will corespond two CBMs other than one with CAT, due to the
     sum of CBMs is fixed, that means actual `cos_max` in use will automatically
     reduce to half when CDP is enabled.
+
+### pv-linear-pt
+> `= <boolean>`
+
+> Default: `false`
+
+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] | P[ower] | e[fi] | n[o] [, [w]arm | [c]old]`
diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index e81a461b91..f748d4a221 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -799,6 +799,9 @@ 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 pv_linear_pt_enable = 0;
+boolean_param("pv-linear-pt", pv_linear_pt_enable);
+
 #define define_get_linear_pagetable(level)                                  \
 static int                                                                  \
 get_##level##_linear_pagetable(                                             \
@@ -808,6 +811,12 @@ 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)");        \
+        return 0;                                                           \
+    }                                                                       \
+                                                                            \
     if ( (level##e_get_flags(pde) & _PAGE_RW) )                             \
     {                                                                       \
         MEM_LOG("Attempt to create linear p.t. with write perms");          \
-- 
2.14.1