mm: add arch hook to validate mmap() prot flags

Add a hook to permit architectures to perform validation on the prot
flags passed to mmap(), like arch_validate_prot() does for mprotect().
This will be used by arm64 to reject PROT_WRITE+PROT_EXEC mappings on
configurations that run with WXN enabled.
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240214122845.2033971-87-ardb+git@google.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

include/linux/mman.h

@@ -124,6 +124,21 @@ static inline bool arch_validate_flags(unsigned long flags)
 #define arch_validate_flags arch_validate_flags
 #endif
 
+#ifndef arch_validate_mmap_prot
+/*
+ * This is called from mmap(), which ignores unknown prot bits so the default
+ * is to accept anything.
+ *
+ * Returns true if the prot flags are valid
+ */
+static inline bool arch_validate_mmap_prot(unsigned long prot,
+					   unsigned long addr)
+{
+	return true;
+}
+#define arch_validate_mmap_prot arch_validate_mmap_prot
+#endif
+
 /*
  * Optimisation macro.  It is equivalent to:
  *      (x & bit1) ? bit2 : 0

mm/mmap.c

@@ -1229,6 +1229,9 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
 		if (!(file && path_noexec(&file->f_path)))
 			prot |= PROT_EXEC;
 
+	if (!arch_validate_mmap_prot(prot, addr))
+		return -EACCES;
+
 	/* force arch specific MAP_FIXED handling in get_unmapped_area */
 	if (flags & MAP_FIXED_NOREPLACE)
 		flags |= MAP_FIXED;