[PATCH v2 2/2] powerpc/mm: Support execute-only memory on the Radix MMU

[Russell Currey]

The Hash MMU already supports XOM (i.e. mmap with PROT_EXEC only)
through the execute-only pkey.  A PROT_EXEC-only mapping will actually
map to RX, and then the pkey will be applied on top of it.

Radix doesn't have pkeys, but it does have execute permissions built-in
to the MMU, so all we have to do to support XOM is expose it.

That's not possible with protection_map being const, so make it RO after
init instead.

Signed-off-by: Russell Currey <ruscur at russell.cc>
---
v2: Make protection_map __ro_after_init and set it in an initcall
(v1 didn't work, I tested before rebasing on Anshuman's patches)

basic test: https://raw.githubusercontent.com/ruscur/junkcode/main/mmap_test.c

 arch/powerpc/include/asm/book3s/64/radix.h |  3 +++
 arch/powerpc/include/asm/pgtable.h         |  1 -
 arch/powerpc/mm/fault.c                    | 10 ++++++++++
 arch/powerpc/mm/pgtable.c                  | 16 +++++++++++++++-
 4 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/book3s/64/radix.h b/arch/powerpc/include/asm/book3s/64/radix.h
index 686001eda936..bf316b773d73 100644
--- a/arch/powerpc/include/asm/book3s/64/radix.h
+++ b/arch/powerpc/include/asm/book3s/64/radix.h
@@ -19,6 +19,9 @@
 #include <asm/cpu_has_feature.h>
 #endif
 
+/* Execute-only page protections, Hash can use RX + execute-only pkey */
+#define PAGE_EXECONLY	__pgprot(_PAGE_BASE | _PAGE_EXEC)
+
 /* An empty PTE can still have a R or C writeback */
 #define RADIX_PTE_NONE_MASK		(_PAGE_DIRTY | _PAGE_ACCESSED)
 
diff --git a/arch/powerpc/include/asm/pgtable.h b/arch/powerpc/include/asm/pgtable.h
index 33f4bf8d22b0..3cbb6de20f9d 100644
--- a/arch/powerpc/include/asm/pgtable.h
+++ b/arch/powerpc/include/asm/pgtable.h
@@ -60,7 +60,6 @@ extern void paging_init(void);
 void poking_init(void);
 
 extern unsigned long ioremap_bot;
-extern const pgprot_t protection_map[16];
 
 /*
  * kern_addr_valid is intended to indicate whether an address is a valid
diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index 014005428687..887c0cc45ca6 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -270,6 +270,16 @@ static bool access_error(bool is_write, bool is_exec, struct vm_area_struct *vma
 		return false;
 	}
 
+	if (unlikely(!(vma->vm_flags & VM_READ))) {
+		/*
+		 * If we're on Radix, then this could be a read attempt on
+		 * execute-only memory.  On other MMUs, an "exec-only" page
+		 * will be given RX flags, so this might be redundant.
+		 */
+		if (radix_enabled())
+			return true;
+	}
+
 	if (unlikely(!vma_is_accessible(vma)))
 		return true;
 	/*
diff --git a/arch/powerpc/mm/pgtable.c b/arch/powerpc/mm/pgtable.c
index 0b2bbde5fb65..6e1a6a999c3c 100644
--- a/arch/powerpc/mm/pgtable.c
+++ b/arch/powerpc/mm/pgtable.c
@@ -475,7 +475,7 @@ pte_t *__find_linux_pte(pgd_t *pgdir, unsigned long ea,
 EXPORT_SYMBOL_GPL(__find_linux_pte);
 
 /* Note due to the way vm flags are laid out, the bits are XWR */
-const pgprot_t protection_map[16] = {
+static pgprot_t protection_map[16] __ro_after_init = {
 	[VM_NONE]					= PAGE_NONE,
 	[VM_READ]					= PAGE_READONLY,
 	[VM_WRITE]					= PAGE_COPY,
@@ -494,6 +494,20 @@ const pgprot_t protection_map[16] = {
 	[VM_SHARED | VM_EXEC | VM_WRITE | VM_READ]	= PAGE_SHARED_X
 };
 
+#ifdef CONFIG_PPC_RADIX_MMU
+static int __init radix_update_protection_map(void)
+{
+	if (early_radix_enabled()) {
+		/* Radix directly supports execute-only page protections */
+		protection_map[VM_EXEC] = PAGE_EXECONLY;
+		protection_map[VM_EXEC | VM_SHARED] = PAGE_EXECONLY;
+	}
+
+	return 0;
+}
+arch_initcall(radix_update_protection_map);
+#endif /* CONFIG_PPC_RADIX_MMU */
+
 #ifdef CONFIG_PPC_BOOK3S_64
 pgprot_t vm_get_page_prot(unsigned long vm_flags)
 {
-- 
2.37.1