diff --git a/kern/pmap.c b/kern/pmap.c
index 2c590f2..9bd5767 100644
--- a/kern/pmap.c
+++ b/kern/pmap.c
@@ -6,6 +6,7 @@
 #include <inc/string.h>
 #include <inc/assert.h>
 
+#include <kern/monitor.h>
 #include <kern/pmap.h>
 #include <kern/kclock.h>
 #include <kern/env.h>
@@ -562,6 +563,21 @@ int
 user_mem_check(struct Env *env, const void *va, size_t len, int perm)
 {
 	// LAB 3: Your code here.
+	const char* bottom = ROUNDDOWN(va, PGSIZE);
+	size_t aligned_count = ROUNDUP(len, PGSIZE) / PGSIZE;
+	pde_t mask = PTE_P | PTE_U | PTE_W;
+	pde_t perms = mask;
+
+#define VALID ((perms & (perm | PTE_P)) == (perm | PTE_P))
+#define DO_CHECK if(!VALID) { user_mem_check_addr = (uintptr_t) bottom; return -E_FAULT; }
+
+	while(aligned_count--) {
+		perms &= env->env_pgdir[PDX(bottom)] & mask;
+		DO_CHECK;
+		perms &= (*pgdir_walk(env->env_pgdir, bottom, 0)) & mask;
+		DO_CHECK;
+		bottom += PGSIZE;
+	}
 
 	return 0;
 }
diff --git a/kern/syscall.c b/kern/syscall.c
index 59adfa7..d332f11 100644
--- a/kern/syscall.c
+++ b/kern/syscall.c
@@ -19,26 +19,7 @@ sys_cputs(const char *s, size_t len)
 {
 	// Check that the user has permission to read memory [s, s+len).
 	// Destroy the environment if not.
-	const char* bottom = ROUNDDOWN(s, PGSIZE);
-	size_t aligned_count = ROUNDUP(len, PGSIZE) / PGSIZE;
-	while(aligned_count--) {
-		if (!(curenv->env_pgdir[PDX(bottom)] & PTE_P)) {
-			cprintf("[%08x] Tried to print unmapped address\n", curenv->env_id);
-			env_destroy(curenv);
-			return;
-		}
-
-		pde_t pgdir_allowed = curenv->env_pgdir[PDX(bottom)] & PTE_U;
-		pde_t pt_allowed = *pgdir_walk(curenv->env_pgdir, bottom, 0) & PTE_U;
-		if(((curenv->env_tf.tf_cs & 0b11) > 1) && !(pt_allowed & pgdir_allowed)) {
-			cprintf("[%08x] Tried to print forbidden memory\n", curenv->env_id); 
-			env_destroy(curenv);
-			return;
-		}
-
-		bottom += PGSIZE;
-	}
-
+	user_mem_assert(curenv, s, len, 0);
 	// Print the string supplied by the user.
 	cprintf("%.*s", len, s);
 }