powerpc/mm: Update _PAGE_KERNEL_RO

PS3 had used a PPP bit hack to implement a read only mapping in the
kernel area. Since we are bolting the ioremap area, it used the pte
flags _PAGE_PRESENT | _PAGE_USER to get a PPP value of 0x3 there by
resulting in a read only mapping. This means the area can be accessed by
user space, but kernel will never return such an address to user space.

But we can do better by implementing a read only kernel mapping using
PPP bits 0b110.

This also allows us to do read only kernel mapping for radix in later
patches.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 11 insertions, 6 deletions

diff --git a/arch/powerpc/mm/hash_utils_64.c b/arch/powerpc/mm/hash_utils_64.c
index dc0f6a00ccbd..38ed869c119e 100644
--- a/arch/powerpc/mm/hash_utils_64.c
+++ b/arch/powerpc/mm/hash_utils_64.c
@@ -167,14 +167,19 @@ unsigned long htab_convert_pte_flags(unsigned long pteflags)
         if ((pteflags & _PAGE_EXEC) == 0)
                 rflags |= HPTE_R_N;
         /*
-         * PP bits:
+         * PPP bits:
          * Linux uses slb key 0 for kernel and 1 for user.
-         * kernel areas are mapped with PP=00
-         * and there is no kernel RO (_PAGE_KERNEL_RO).
-         * User area is mapped with PP=0x2 for read/write
-         * or PP=0x3 for read-only (including writeable but clean pages).
+         * kernel RW areas are mapped with PPP=0b000
+         * User area is mapped with PPP=0b010 for read/write
+         * or PPP=0b011 for read-only (including writeable but clean pages).
          */
-        if (!(pteflags & _PAGE_PRIVILEGED)) {
+        if (pteflags & _PAGE_PRIVILEGED) {
+                /*
+                 * Kernel read only mapped with ppp bits 0b110
+                 */
+                if (!(pteflags & _PAGE_WRITE))
+                        rflags |= (HPTE_R_PP0 | 0x2);
+        } else {
                 if (pteflags & _PAGE_RWX)
                         rflags |= 0x2;
                 if (!((pteflags & _PAGE_WRITE) && (pteflags & _PAGE_DIRTY)))