[PATCH for-4.5 v10 15/19] xen/arm: Temporarily disable mem_access for hypervisor access

[Tamas K Lengyel <tklengyel@sec.in.tum.de>]

The guestcopy helpers use the MMU to verify that the given guest has read/write
access to a given page during hypercalls. As we may have custom mem_access
permissions set on these pages, we temporarily disable them to allow Xen to
finish the hypercalls. This is permissible as mem_access events are only
reported for events when the guest directly accesses protected memory on x86
as well.

Signed-off-by: Tamas K Lengyel <tklengyel@sec.in.tum.de>
---
 xen/arch/arm/guestcopy.c | 61 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 61 insertions(+)

diff --git a/xen/arch/arm/guestcopy.c b/xen/arch/arm/guestcopy.c
index 0173597..4aa041f 100644
--- a/xen/arch/arm/guestcopy.c
+++ b/xen/arch/arm/guestcopy.c
@@ -6,6 +6,43 @@
 
 #include <asm/mm.h>
 #include <asm/guest_access.h>
+#include <asm/p2m.h>
+
+/*
+ * Temporarily disable mem_access permission restrictions.
+ * Note: In the future, events generated by the hypervisor accessing
+ * protected memory regions could be added here.
+ */
+static long temp_disable_mem_access(vaddr_t gva, unsigned long *gfn,
+                                    xenmem_access_t *xma)
+{
+    long rc;
+    paddr_t gpa;
+
+    rc = gva_to_ipa((vaddr_t) gva, &gpa);
+    if ( rc < 0 )
+        return rc;
+
+    *gfn = paddr_to_pfn(gpa);
+
+    rc = p2m_get_mem_access(current->domain, *gfn, xma);
+    if ( rc < 0 )
+        return rc;
+
+    if ( *xma != XENMEM_access_rwx )
+        rc = p2m_set_mem_access(current->domain, *gfn, 1, 0, ~0,
+                                XENMEM_access_rwx);
+
+    return rc;
+}
+
+/* Re-enable mem_access for this page (if in use). */
+static inline
+void temp_reenable_mem_access(unsigned long gfn, xenmem_access_t xma)
+{
+    if ( xma != XENMEM_access_rwx )
+        p2m_set_mem_access(current->domain, gfn, 1, 0, ~0, xma);
+}
 
 static unsigned long raw_copy_to_guest_helper(void *to, const void *from,
                                               unsigned len, int flush_dcache)
@@ -18,6 +55,11 @@ static unsigned long raw_copy_to_guest_helper(void *to, const void *from,
         void *p;
         unsigned size = min(len, (unsigned)PAGE_SIZE - offset);
         struct page_info *page;
+        unsigned long gfn;
+        xenmem_access_t xma;
+
+        if ( temp_disable_mem_access((vaddr_t) to, &gfn, &xma) < 0 )
+            return len;
 
         page = get_page_from_gva(current->domain, (vaddr_t) to, GV2M_WRITE);
         if ( page == NULL )
@@ -34,6 +76,9 @@ static unsigned long raw_copy_to_guest_helper(void *to, const void *from,
         len -= size;
         from += size;
         to += size;
+
+        temp_reenable_mem_access(gfn, xma);
+
         /*
          * After the first iteration, guest virtual address is correctly
          * aligned to PAGE_SIZE.
@@ -65,6 +110,11 @@ unsigned long raw_clear_guest(void *to, unsigned len)
         void *p;
         unsigned size = min(len, (unsigned)PAGE_SIZE - offset);
         struct page_info *page;
+        unsigned long gfn;
+        xenmem_access_t xma;
+
+        if ( temp_disable_mem_access((vaddr_t) to, &gfn, &xma) < 0 )
+            return len;
 
         page = get_page_from_gva(current->domain, (vaddr_t) to, GV2M_WRITE);
         if ( page == NULL )
@@ -78,6 +128,9 @@ unsigned long raw_clear_guest(void *to, unsigned len)
         put_page(page);
         len -= size;
         to += size;
+
+        temp_reenable_mem_access(gfn, xma);
+
         /*
          * After the first iteration, guest virtual address is correctly
          * aligned to PAGE_SIZE.
@@ -97,6 +150,11 @@ unsigned long raw_copy_from_guest(void *to, const void __user *from, unsigned le
         void *p;
         unsigned size = min(len, (unsigned)(PAGE_SIZE - offset));
         struct page_info *page;
+        unsigned long gfn;
+        xenmem_access_t xma;
+
+        if ( temp_disable_mem_access((vaddr_t) from, &gfn, &xma) < 0 )
+            return len;
 
         page = get_page_from_gva(current->domain, (vaddr_t) from, GV2M_READ);
         if ( page == NULL )
@@ -112,6 +170,9 @@ unsigned long raw_copy_from_guest(void *to, const void __user *from, unsigned le
         len -= size;
         from += size;
         to += size;
+
+        temp_reenable_mem_access(gfn, xma);
+
         /*
          * After the first iteration, guest virtual address is correctly
          * aligned to PAGE_SIZE.
-- 
2.1.0