[PATCH v6 13/38] arm64/mm: Map pages for guarded control stack

[Mark Brown]

Map pages flagged as being part of a GCS as such rather than using the
full set of generic VM flags.

This is done using a conditional rather than extending the size of
protection_map since that would make for a very sparse array.

Signed-off-by: Mark Brown <broonie at kernel.org>
---
 arch/arm64/include/asm/mman.h |  9 +++++++++
 arch/arm64/mm/mmap.c          | 13 ++++++++++++-
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/mman.h b/arch/arm64/include/asm/mman.h
index c21849ffdd88..6d3fe6433a62 100644
--- a/arch/arm64/include/asm/mman.h
+++ b/arch/arm64/include/asm/mman.h
@@ -61,6 +61,15 @@ static inline bool arch_validate_flags(unsigned long vm_flags)
 			return false;
 	}
 
+	if (system_supports_gcs() && (vm_flags & VM_SHADOW_STACK)) {
+		/*
+		 * An executable GCS isn't a good idea, and the mm
+		 * core can't cope with a shared GCS.
+		 */
+		if (vm_flags & (VM_EXEC | VM_ARM64_BTI | VM_SHARED))
+			return false;
+	}
+
 	return true;
 
 }
diff --git a/arch/arm64/mm/mmap.c b/arch/arm64/mm/mmap.c
index 8f5b7ce857ed..e6fc7ef83ea1 100644
--- a/arch/arm64/mm/mmap.c
+++ b/arch/arm64/mm/mmap.c
@@ -79,9 +79,20 @@ arch_initcall(adjust_protection_map);
 
 pgprot_t vm_get_page_prot(unsigned long vm_flags)
 {
-	pteval_t prot = pgprot_val(protection_map[vm_flags &
+	pteval_t prot;
+
+	/* If this is a GCS then only interpret VM_WRITE. */
+	if (system_supports_gcs() && (vm_flags & VM_SHADOW_STACK)) {
+		if (vm_flags & VM_WRITE)
+			prot = _PAGE_GCS;
+		else
+			prot = _PAGE_GCS_RO;
+	} else {
+		prot = pgprot_val(protection_map[vm_flags &
 				   (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)]);
+	}
 
+	/* VM_ARM64_BTI on a GCS is rejected in arch_valdiate_flags() */
 	if (vm_flags & VM_ARM64_BTI)
 		prot |= PTE_GP;
 

-- 
2.30.2