[xen master] x86/mm: Make PV linear pagetables optional

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[xen master] x86/mm: Make PV linear pagetables optional

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commit 3285e75dea89afb0ef5b3ee39bd15194bd7cc110
Author:     George Dunlap <[hidden email]>
AuthorDate: Fri Oct 27 14:26:27 2017 +0100
Commit:     George Dunlap <[hidden email]>
CommitDate: Fri Oct 27 14:36:45 2017 +0100

    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 <[hidden email]>
    Signed-off-by: George Dunlap <[hidden email]>
    Reviewed-by: Jan Beulich <[hidden email]>
    Release-acked-by: Julien Grall <[hidden email]>
---
 docs/misc/xen-command-line.markdown | 19 +++++++++++++++++++
 xen/arch/x86/Kconfig                | 20 ++++++++++++++++++++
 xen/arch/x86/mm.c                   | 37 +++++++++++++++++++++++++++++++++++++
 3 files changed, 76 insertions(+)

diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index eb4995e..781110d 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1422,6 +1422,25 @@ 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.
 
 ### rcu-idle-timer-period-ms
 > `= <integer>`
diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig
index 64955dc..7c45829 100644
--- a/xen/arch/x86/Kconfig
+++ b/xen/arch/x86/Kconfig
@@ -37,6 +37,26 @@ 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 1ff4a9d..a20fdca 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -626,6 +626,8 @@ 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;
@@ -693,6 +695,9 @@ 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(                                             \
@@ -702,6 +707,13 @@ 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,                                            \
@@ -759,6 +771,27 @@ 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)
 {
@@ -2444,6 +2477,7 @@ 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) )
             {
                 /*
@@ -2458,6 +2492,9 @@ 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
 
             set_tlbflush_timestamp(page);
         }
--
generated by git-patchbot for /home/xen/git/xen.git#master

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