arm64/mm: Map pages for guarded control stack

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.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241001-arm64-gcs-v13-15-222b78d87eee@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: broonie

arch/arm64/include/asm/mman.h

@@ -71,6 +71,15 @@ 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. */
+		if (vm_flags & VM_EXEC)
+			return false;
+
+		/* The memory management core should prevent this */
+		VM_WARN_ON(vm_flags & VM_SHARED);
+	}
+
 	return true;
 
 }

arch/arm64/mm/mmap.c

@@ -83,8 +83,15 @@ 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;
+
+	/* Short circuit GCS to avoid bloating the table. */
+	if (system_supports_gcs() && (vm_flags & VM_SHADOW_STACK)) {
+		prot = _PAGE_GCS_RO;
+	} else {
+		prot = pgprot_val(protection_map[vm_flags &
 				   (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)]);
+	}
 
 	if (vm_flags & VM_ARM64_BTI)
 		prot |= PTE_GP;