[PATCH RFCv2 08/15] xen/arm32: mm: Check if the virtual address is shared before updating it

[Julien Grall <julien@xxxxxxx>]

From: Julien Grall <jgrall@xxxxxxxxxx>

Only the first 2GB of the virtual address space is shared between all
the page-tables on Arm32.

There is a long outstanding TODO in xen_pt_update() stating that the
function is can only work with shared mapping. Nobody has ever called
the function with private mapping, however as we add more callers
there is a risk to mess things up.

Introduce a new define to mark the ened of the shared mappings and use
it in xen_pt_update() to verify if the address is correct.

Note that on Arm64, all the mappings are shared. Some compiler may
complain about an always true check, so the new define is not introduced
for arm64 and the code is protected with an #ifdef.

Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>

---
    Changes in v2:
        - New patch
---
 xen/arch/arm/mm.c            | 11 +++++++++--
 xen/include/asm-arm/config.h |  4 ++++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
index 8fac24d80086..5c17cafff847 100644
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -1275,11 +1275,18 @@ static int xen_pt_update(unsigned long virt,
      * 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 = virt_to_mfn(THIS_CPU_PGTABLE);
 
+#ifdef SHARED_VIRT_END
+    if ( virt > SHARED_VIRT_END ||
+         (SHARED_VIRT_END - virt) < nr_mfns )
+    {
+        mm_printk("Trying to map outside of the shared area.\n");
+        return -EINVAL;
+    }
+#endif
+
     /*
      * The hardware was configured to forbid mapping both writeable and
      * executable.
diff --git a/xen/include/asm-arm/config.h b/xen/include/asm-arm/config.h
index c7b77912013e..85d4a510ce8a 100644
--- a/xen/include/asm-arm/config.h
+++ b/xen/include/asm-arm/config.h
@@ -137,6 +137,10 @@
 
 #define XENHEAP_VIRT_START     _AT(vaddr_t,0x40000000)
 #define XENHEAP_VIRT_END       _AT(vaddr_t,0x7fffffff)
+
+/* The first 2GB is always shared between all the page-tables. */
+#define SHARED_VIRT_END        _AT(vaddr_t, 0x7fffffff)
+
 #define DOMHEAP_VIRT_START     _AT(vaddr_t,0x80000000)
 #define DOMHEAP_VIRT_END       _AT(vaddr_t,0xffffffff)
 
-- 
2.17.1