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

arch/powerpc/include/asm/book3s/64/pgtable.h

@@ -151,6 +151,8 @@
 #define PAGE_COPY_X	__pgprot(_PAGE_BASE | _PAGE_READ | _PAGE_EXEC)
 #define PAGE_READONLY	__pgprot(_PAGE_BASE | _PAGE_READ)
 #define PAGE_READONLY_X	__pgprot(_PAGE_BASE | _PAGE_READ | _PAGE_EXEC)
+/* Radix only, Hash uses PAGE_READONLY_X + execute-only pkey instead */
+#define PAGE_EXECONLY	__pgprot(_PAGE_BASE | _PAGE_EXEC)
 
 /* Permission masks used for kernel mappings */
 #define PAGE_KERNEL	__pgprot(_PAGE_BASE | _PAGE_KERNEL_RW)

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;

arch/powerpc/mm/fault.c

@@ -270,7 +270,11 @@ static bool access_error(bool is_write, bool is_exec, struct vm_area_struct *vma
 		return false;
 	}
 
-	if (unlikely(!vma_is_accessible(vma)))
+	/*
+	 * Check for a read fault.  This could be caused by a read on an
+	 * inaccessible page (i.e. PROT_NONE), or a Radix MMU execute-only page.
+	 */
+	if (unlikely(!(vma->vm_flags & VM_READ)))
 		return true;
 	/*
 	 * We should ideally do the vma pkey access check here. But in the