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

[Russell Currey <ruscur@russell.cc>]

The Hash MMU already supports XOM (i.e. mmap with PROT_EXEC only)
through the execute-only pkey.  A PROT_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.

Signed-off-by: Russell Currey <ruscur@russell.cc>
---
quick test: https://raw.githubusercontent.com/ruscur/junkcode/main/mmap_test.c
I can make it a selftest.

 arch/powerpc/include/asm/book3s/64/radix.h |  3 +++
 arch/powerpc/mm/book3s64/radix_pgtable.c   |  4 ++++
 arch/powerpc/mm/fault.c                    | 10 ++++++++++
 3 files changed, 17 insertions(+)

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/mm/book3s64/radix_pgtable.c b/arch/powerpc/mm/book3s64/radix_pgtable.c
index 698274109c91..2edb56169805 100644
--- a/arch/powerpc/mm/book3s64/radix_pgtable.c
+++ b/arch/powerpc/mm/book3s64/radix_pgtable.c
@@ -617,6 +617,10 @@ void __init radix__early_init_mmu(void)
 	__pmd_frag_nr = RADIX_PMD_FRAG_NR;
 	__pmd_frag_size_shift = RADIX_PMD_FRAG_SIZE_SHIFT;
 
+	/* Radix directly supports execute-only page protections */
+	protection_map[VM_EXEC] = PAGE_EXECONLY;
+	protection_map[VM_EXEC | VM_SHARED] = PAGE_EXECONLY;
+
 	radix_init_pgtable();
 
 	if (!firmware_has_feature(FW_FEATURE_LPAR)) {
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;
 	/*
-- 
2.37.1