[Xen-devel] [PATCH v2 09/20] arm/mm: Introduce modify_xen_mappings

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

Which is only used by Livepatch code. The purpose behind
this call is to modify the page table entries flags.

Specifically the .ro and .nx flags. The current mechanism
puts cache attributes in the flags and the .ro and .nx are
locked down and assumed to be .ro=0, nx=1.

Livepatch needs .nx=0 and also .ro to be set to 1.

We introduce a new 'flags' where various bits determine
whether .ro and .nx bits are set or cleared. We can't use
an enum as the function prototype would diverge from x86.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
--
Cc: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Julien Grall <julien.grall@xxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
[Added as he wrote the x86 modify_xen_mappings version]

RFC: First submission.
v1: Add #defines to make it simpler to understand the bit-shifting.
v2: Add sanity checks on ro=0 and nx=0 never occuring (executable
    read/write code).
    Piggyback on "x86: change modify_xen_mappings to return error"
    to be able to return an error value.
---
 xen/arch/arm/mm.c          | 27 ++++++++++++++++++++++++---
 xen/include/asm-arm/page.h | 11 +++++++++++
 2 files changed, 35 insertions(+), 3 deletions(-)

diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
index 4e256c2..d622686 100644
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -836,6 +836,7 @@ static int create_xen_table(lpae_t *entry)
 enum xenmap_operation {
     INSERT,
     REMOVE,
+    MODIFY,
     RESERVE
 };
 
@@ -881,14 +882,28 @@ static int create_xen_entries(enum xenmap_operation op,
                 pte.pt.table = 1;
                 write_pte(&third[third_table_offset(addr)], pte);
                 break;
+            case MODIFY:
             case REMOVE:
                 if ( !third[third_table_offset(addr)].pt.valid )
                 {
-                    printk("create_xen_entries: trying to remove a 
non-existing mapping addr=%lx\n",
-                           addr);
+                    printk("create_xen_entries: trying to %s a non-existing 
mapping addr=%lx\n",
+                           op == REMOVE ? "remove" : "modify", addr);
                     return -EINVAL;
                 }
-                pte.bits = 0;
+                if ( op == REMOVE )
+                    pte.bits = 0;
+                else
+                {
+                    pte = third[third_table_offset(addr)];
+                    pte.pt.ro = PTE_RO_MASK(ai);
+                    pte.pt.xn = PTE_NX_MASK(ai);
+                    if ( !pte.pt.ro && !pte.pt.xn )
+                    {
+                        printk("create_xen_entries: Incorrect combination for 
addr=%lx\n",
+                               addr);
+                        return -EINVAL;
+                    }
+                }
                 write_pte(&third[third_table_offset(addr)], pte);
                 break;
             default:
@@ -922,6 +937,12 @@ void destroy_xen_mappings(unsigned long v, unsigned long e)
     create_xen_entries(REMOVE, v, 0, (e - v) >> PAGE_SHIFT, 0);
 }
 
+int modify_xen_mappings(unsigned long s, unsigned long e, unsigned int flags)
+{
+    ASSERT((flags & (PTE_NX | PTE_RO)) == flags);
+    return create_xen_entries(MODIFY, s, 0, (e - s) >> PAGE_SHIFT, flags);
+}
+
 enum mg { mg_clear, mg_ro, mg_rw, mg_rx };
 static void set_pte_flags_on_range(const char *p, unsigned long l, enum mg mg)
 {
diff --git a/xen/include/asm-arm/page.h b/xen/include/asm-arm/page.h
index 05d9f82..2f66740 100644
--- a/xen/include/asm-arm/page.h
+++ b/xen/include/asm-arm/page.h
@@ -66,6 +66,17 @@
 #define PAGE_HYPERVISOR_WC      (DEV_WC)
 
 /*
+ * Defines for changing the PTE .ro and .nx bits. This is only to be
+ * used with modify_xen_mappings.
+ */
+#define _PTE_NX_BIT     0U
+#define _PTE_RO_BIT     1U
+#define PTE_NX          (1U << _PTE_NX_BIT)
+#define PTE_RO          (1U << _PTE_RO_BIT)
+#define PTE_NX_MASK(x)  (((x) >> _PTE_NX_BIT) & 0x1U)
+#define PTE_RO_MASK(x)  (((x) >> _PTE_RO_BIT) & 0x1U)
+
+/*
  * Stage 2 Memory Type.
  *
  * These are valid in the MemAttr[3:0] field of an LPAE stage 2 page
-- 
2.4.11


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