[PATCH v4] xen/arm: validate non-common page-table ranges

[Gabriel Quintáns Souto <gabi.qs.mail@xxxxxxxxx>]

On arm32, page-tables are different on each CPUs, although they share some
common mappings.

The xen_pt_update function assumed that only said mappings would be
modified, but did not check this assumption.

Add a range check to reject modifications to non-common mappings.

Signed-off-by: Gabriel Quintáns Souto <gabi.qs.mail@xxxxxxxxx>
---
v4:
 - Rebased onto staging (v3 was diffed against v2).

v3:
 - Dropped accidentally tracked patch file from commit.

v2:
- Inverted polarity of xen_pt_range_is_non_common() as suggested.
- Moved xen_pt_range_is_common() implementation to arch-specific headers
(arm32 vs arm64) instead of keeping #ifdef CONFIG_ARM_32 in pt.c (tried
IS_ENABLED() for DCE, but ARM64 lacks definitions for
DOMHEAP_VIRT_START/TEMPORARY_AREA_ADDR, breaking the build).
- Added ASSERT_UNREACHABLE() on failure path in xen_pt_update().
---
 xen/arch/arm/include/asm/arm32/mm.h | 24 ++++++++++++++++++++++++
 xen/arch/arm/include/asm/arm64/mm.h | 10 ++++++++++
 xen/arch/arm/mmu/pt.c               | 14 +++++++-------
 3 files changed, 41 insertions(+), 7 deletions(-)

diff --git a/xen/arch/arm/include/asm/arm32/mm.h 
b/xen/arch/arm/include/asm/arm32/mm.h
index 856f2db..c7d09d3 100644
--- a/xen/arch/arm/include/asm/arm32/mm.h
+++ b/xen/arch/arm/include/asm/arm32/mm.h
@@ -16,6 +16,30 @@ static inline bool arch_mfns_in_directmap(unsigned long mfn, 
unsigned long nr)
     return false;
 }
 
+static inline bool ranges_overlap(unsigned long start1, unsigned long size1,
+                           unsigned long start2, unsigned long size2)
+{
+    if ( start1 < start2 )
+        return start2 - start1 < size1;
+
+    return start1 - start2 < size2;
+}
+
+/*
+ * On ARM32, page-tables differ between CPUs, so not all mappings are common.
+ */
+static inline bool xen_pt_range_is_common(unsigned long virt,
+                                       unsigned long size)
+{
+    unsigned long temporary_start = TEMPORARY_AREA_ADDR(0);
+    unsigned long temporary_size = XEN_PT_LEVEL_SIZE(1);
+
+    return !ranges_overlap(virt, size,
+                          DOMHEAP_VIRT_START, DOMHEAP_VIRT_SIZE) &&
+           !ranges_overlap(virt, size,
+                          temporary_start, temporary_size);
+}
+
 bool init_domheap_mappings(unsigned int cpu);
 
 static inline void arch_setup_page_tables(void)
diff --git a/xen/arch/arm/include/asm/arm64/mm.h 
b/xen/arch/arm/include/asm/arm64/mm.h
index b4f7545..a3cb56d 100644
--- a/xen/arch/arm/include/asm/arm64/mm.h
+++ b/xen/arch/arm/include/asm/arm64/mm.h
@@ -12,6 +12,16 @@ static inline bool arch_mfns_in_directmap(unsigned long mfn, 
unsigned long nr)
     return true;
 }
 
+/*
+ * On ARM64, the Xen page-tables are shared by all the CPUs,
+ * any range is therefore common.
+ */
+static inline bool xen_pt_range_is_common(unsigned long virt,
+                                          unsigned long size)
+{
+    return true;
+}
+
 void arch_setup_page_tables(void);
 
 void update_boot_mapping(bool enable);
diff --git a/xen/arch/arm/mmu/pt.c b/xen/arch/arm/mmu/pt.c
index 81fe121..42fa79a 100644
--- a/xen/arch/arm/mmu/pt.c
+++ b/xen/arch/arm/mmu/pt.c
@@ -13,6 +13,7 @@
 
 #include <asm/current.h>
 #include <asm/fixmap.h>
+#include <asm/mmu/layout.h>
 
 #ifdef NDEBUG
 static inline void
@@ -601,15 +602,14 @@ static int xen_pt_update(unsigned long virt,
     unsigned long vfn = virt >> PAGE_SHIFT;
     unsigned long left = nr_mfns;
 
-    /*
-     * For arm32, page-tables are different on each CPUs. Yet, they share
-     * some common mappings. It is assumed that only common mappings
-     * will be modified with this function.
-     *
-     * XXX: Add a check.
-     */
     const mfn_t root = maddr_to_mfn(READ_SYSREG64(TTBR0_EL2));
 
+    if ( !xen_pt_range_is_common(virt, nr_mfns * PAGE_SIZE) )
+    {
+        ASSERT_UNREACHABLE();
+        return -EINVAL;
+    }
+
     if ( flags_has_rwx(flags) )
     {
         mm_printk("Mappings should not be both Writeable and Executable.\n");
-- 
2.56.0