powerpc/mm: Support execute-only memory on the Radix MMU

Add support for execute-only memory (XOM) for the Radix MMU by using an
execute-only mapping, as opposed to the RX mapping used by powerpc's
other MMUs.

The Hash MMU already supports XOM through the execute-only pkey,
which is a separate mechanism shared with x86.  A PROT_EXEC-only mapping
will map to RX, and then the pkey will be applied on top of it.

mmap() and mprotect() consumers in userspace should observe the same
behaviour on Hash and Radix despite the differences in implementation.

Replacing the vma_is_accessible() check in access_error() with a read
check should be functionally equivalent for non-Radix MMUs, since it
follows write and execute checks.  For Radix, the change enables
detecting faults on execute-only mappings where vma_is_accessible() would
return true.

Signed-off-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220817050640.406017-1-ruscur@russell.cc

1 files changed, 9 insertions, 2 deletions

diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c
index 7b9966402b25..f6151a589298 100644
--- a/arch/powerpc/mm/book3s64/pgtable.c
+++ b/arch/powerpc/mm/book3s64/pgtable.c
@@ -553,8 +553,15 @@ EXPORT_SYMBOL_GPL(memremap_compat_align);
 
 pgprot_t vm_get_page_prot(unsigned long vm_flags)
 {
-	unsigned long prot = pgprot_val(protection_map[vm_flags &
-					(VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)]);
+	unsigned long prot;
+
+	/* Radix supports execute-only, but protection_map maps X -> RX */
+	if (radix_enabled() && ((vm_flags & VM_ACCESS_FLAGS) == VM_EXEC)) {
+		prot = pgprot_val(PAGE_EXECONLY);
+	} else {
+		prot = pgprot_val(protection_map[vm_flags &
+						 (VM_ACCESS_FLAGS | VM_SHARED)]);
+	}
 
 	if (vm_flags & VM_SAO)
 		prot |= _PAGE_SAO;