ARM: mm: introduce present, faulting entries for PAGE_NONE

PROT_NONE mappings apply the page protection attributes defined by _P000
which translate to PAGE_NONE for ARM. These attributes specify an XN,
RDONLY pte that is inaccessible to userspace. However, on kernels
configured without support for domains, such a pte *is* accessible to
the kernel and can be read via get_user, allowing tasks to read
PROT_NONE pages via syscalls such as read/write over a pipe.

This patch introduces a new software pte flag, L_PTE_NONE, that is set
to identify faulting, present entries.

Signed-off-by: Will Deacon <will.deacon@arm.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/arch/arm/mm/proc-macros.S b/arch/arm/mm/proc-macros.S
index b29a2265af01..eb6aa73bc8b7 100644
--- a/arch/arm/mm/proc-macros.S
+++ b/arch/arm/mm/proc-macros.S
@@ -167,6 +167,10 @@
         tst     r1, #L_PTE_YOUNG
         tstne   r1, #L_PTE_PRESENT
         moveq   r3, #0
+#ifndef CONFIG_CPU_USE_DOMAINS
+        tstne   r1, #L_PTE_NONE
+        movne   r3, #0
+#endif
 
         str     r3, [r0]
         mcr     p15, 0, r0, c7, c10, 1          @ flush_pte