Index: lib/libc/sys/mmap.2
===================================================================
--- lib/libc/sys/mmap.2
+++ lib/libc/sys/mmap.2
@@ -372,6 +372,25 @@
 argument
 is not a valid open file descriptor.
 .It Bq Er EINVAL
+An invalid value was passed in the
+.Fa prot
+argument.
+.It Bq Er EINVAL
+Both
+.Dv MAP_PRIVATE
+and
+.Dv MAP_SHARED
+were specified.
+.It Bq Er EINVAL
+None of
+.Dv MAP_ANON ,
+.Dv MAP_PRIVATE ,
+.Dv MAP_SHARED ,
+or
+.Dv MAP_STACK
+was specified.
+At least one of these flags must be included.
+.It Bq Er EINVAL
 .Dv MAP_FIXED
 was specified and the
 .Fa addr
Index: sys/vm/vm_mmap.c
===================================================================
--- sys/vm/vm_mmap.c
+++ sys/vm/vm_mmap.c
@@ -203,17 +203,17 @@
 	struct vnode *vp;
 	vm_offset_t addr;
 	vm_size_t size, pageoff;
-	vm_prot_t cap_maxprot, prot, maxprot;
+	vm_prot_t cap_maxprot, maxprot;
 	void *handle;
 	objtype_t handle_type;
-	int align, error, flags;
+	int align, error, flags, prot;
 	off_t pos;
 	struct vmspace *vms = td->td_proc->p_vmspace;
 	cap_rights_t rights;
 
 	addr = (vm_offset_t) uap->addr;
 	size = uap->len;
-	prot = uap->prot & VM_PROT_ALL;
+	prot = uap->prot;
 	flags = uap->flags;
 	pos = uap->pos;
 
@@ -246,6 +246,12 @@
 	}
 	if ((flags & (MAP_EXCL | MAP_FIXED)) == MAP_EXCL)
 		return (EINVAL);
+	if ((flags & (MAP_ANON | MAP_SHARED | MAP_PRIVATE)) == 0 ||
+	    (flags & (MAP_SHARED | MAP_PRIVATE)) == (MAP_SHARED | MAP_PRIVATE))
+		return (EINVAL);
+	if (prot != PROT_NONE &&
+	    (prot & ~(PROT_READ | PROT_WRITE | PROT_EXEC)) != 0)
+		return (EINVAL);
 
 	/*
 	 * Align the file position to a page boundary,
@@ -415,6 +421,8 @@
 map:
 	td->td_fpop = fp;
 	maxprot &= cap_maxprot;
+
+	/* This relies on VM_PROT_* matching PROT_*. */
 	error = vm_mmap(&vms->vm_map, &addr, size, prot, maxprot,
 	    flags, handle_type, handle, pos);
 	td->td_fpop = NULL;