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.

Add a Kconfig option for the feature (default to 'y').  Also add a
command-line option to control whether PV linear pagetables are
allowed (default to 'true').

NB that we leave linear_pt_count in the page struct.  It's in a union,
so its presence doesn't increase the size of the data struct.
Changing the layout of the other elements based on configuration
options is asking for trouble however; so we'll just leave it there
and ASSERT that it's zero.

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 3285e75dea89afb0ef5b3ee39bd15194bd7cc110
master date: 2017-10-27 14:36:45 +0100

diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index 44d99852aa150dac79f4283a6b97ccce8adf2a1e..0202b1643d508682df4d21feb9c4d3d3f8ad9d41 100644 (file)
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1374,6 +1374,25 @@ 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: `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] | P[ower] | e[fi] | n[o] [, [w]arm | [c]old]`

diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig
index 30c2769684060b95c5ebb6777a26dc594d85c276..ee75fd51f5b32278d4b659b55922eccac8f28e64 100644 (file)
--- a/xen/arch/x86/Kconfig
+++ b/xen/arch/x86/Kconfig
@@ -35,6 +35,26 @@ source "arch/Kconfig"
 config PV
        def_bool y
 
+config PV_LINEAR_PT
+       bool "Support for PV linear pagetables"
+       depends on PV
+       default y
+       ---help---
+         Linear pagetables (also called "recursive pagetables") refers
+         to the practice of a guest operating system having pagetable
+         entries pointing to other pagetables of the same level (i.e.,
+         allowing L2 PTEs to point to other L2 pages).  Some operating
+         systems use it as 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.
+
+         If unsure, say Y.
+
 config HVM
        def_bool y
 

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 9ff098902cac9071f5e1cdf4c9e1981d3e068c37..9e491a3574e5c0d6f5268a717e4af15e7a099b30 100644 (file)
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -747,6 +747,8 @@ static void put_data_page(
         put_page(page);
 }
 
+#ifdef CONFIG_PV_LINEAR_PT
+
 static bool inc_linear_entries(struct page_info *pg)
 {
     typeof(pg->linear_pt_count) nc = read_atomic(&pg->linear_pt_count), oc;
@@ -814,6 +816,9 @@ 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 __read_mostly opt_pv_linear_pt = true;
+boolean_param("pv-linear-pt", opt_pv_linear_pt);
+
 #define define_get_linear_pagetable(level)                                  \
 static int                                                                  \
 get_##level##_linear_pagetable(                                             \
@@ -823,6 +828,13 @@ get_##level##_linear_pagetable(                                             \
     struct page_info *page;                                                 \
     unsigned long pfn;                                                      \
                                                                             \
+    if ( !opt_pv_linear_pt )                                                \
+    {                                                                       \
+        gdprintk(XENLOG_WARNING,                                            \
+                 "Attempt to create linear p.t. (feature disabled)\n");     \
+        return 0;                                                           \
+    }                                                                       \
+                                                                            \
     if ( (level##e_get_flags(pde) & _PAGE_RW) )                             \
     {                                                                       \
         gdprintk(XENLOG_WARNING,                                            \
@@ -880,6 +892,27 @@ get_##level##_linear_pagetable(                                             \
     return 1;                                                               \
 }
 
+#else /* CONFIG_PV_LINEAR_PT */
+
+#define define_get_linear_pagetable(level)                              \
+static int                                                              \
+get_##level##_linear_pagetable(                                         \
+        level##_pgentry_t pde, unsigned long pde_pfn, struct domain *d) \
+{                                                                       \
+        return 0;                                                       \
+}
+
+static void dec_linear_uses(struct page_info *pg)
+{
+    ASSERT(pg->linear_pt_count == 0);
+}
+
+static void dec_linear_entries(struct page_info *pg)
+{
+    ASSERT(pg->linear_pt_count == 0);
+}
+
+#endif /* CONFIG_PV_LINEAR_PT */
 
 bool is_iomem_page(mfn_t mfn)
 {
@@ -2614,6 +2647,7 @@ static int _put_page_type(struct page_info *page, bool preemptible,
                 break;
             }
 
+#ifdef CONFIG_PV_LINEAR_PT
             if ( ptpg && PGT_type_equal(x, ptpg->u.inuse.type_info) )
             {
                 /*
@@ -2628,6 +2662,9 @@ static int _put_page_type(struct page_info *page, bool preemptible,
                 ASSERT(ptpg->linear_pt_count > 0);
                 ptpg = NULL;
             }
+#else /* CONFIG_PV_LINEAR_PT */
+            BUG_ON(ptpg && PGT_type_equal(x, ptpg->u.inuse.type_info));
+#endif
 
             /*
              * Record TLB information for flush later. We do not stamp page