diff --git a/kernel/src/memory/Memory.cpp b/kernel/src/memory/Memory.cpp
index 6ff02898..673b6a4e 100644
--- a/kernel/src/memory/Memory.cpp
+++ b/kernel/src/memory/Memory.cpp
@@ -45,10 +45,10 @@ uint64_t Memory::get_usable()
 
 bool Memory::is_kernel_address(uintptr_t address)
 {
-    return address >= 0xfffffffff8000000;
+    return address >= 0xffff800000000000;
 }
 
 bool Memory::is_user_address(uintptr_t address)
 {
-    return address && address < 0xfffffffff8000000;
-}
\ No newline at end of file
+    return address && address < 0x00007fffffffffff;
+}
diff --git a/kernel/src/sys/mem.cpp b/kernel/src/sys/mem.cpp
index 99a6a0b0..bc240b0f 100644
--- a/kernel/src/sys/mem.cpp
+++ b/kernel/src/sys/mem.cpp
@@ -55,9 +55,9 @@ void sys_mmap(Context* context, void* address, size_t size, int prot, int fd, of
     if (address)
     {
         kdbgln("mmap(): %ld pages at address %p, %s, fd %d", size / PAGE_SIZE, address, format_prot(prot), fd);
-        if (Memory::is_kernel_address((uintptr_t)address))
+        if (!Memory::is_user_address((uintptr_t)address))
         {
-            kwarnln("munmap() failed: attempted to unmap a kernel page");
+            kwarnln("mmap() failed: attempted to map a non-user page");
             context->rax = MAP_FAIL(ENOMEM);
             return;
         }
@@ -153,9 +153,9 @@ void sys_munmap(Context* context, void* address, size_t size)
         context->rax = -EINVAL;
         return;
     }
-    if (Memory::is_kernel_address((uintptr_t)address))
+    if (!Memory::is_user_address((uintptr_t)address))
     {
-        kwarnln("munmap() failed: attempted to unmap a kernel page");
+        kwarnln("munmap() failed: attempted to unmap a non-user page");
         context->rax = -EINVAL;
         return;
     }
@@ -202,12 +202,19 @@ void sys_mprotect(Context* context, void* address, size_t size, int prot)
         context->rax = -EINVAL;
         return;
     }
-    if (Memory::is_kernel_address((uintptr_t)address))
+    if (!Memory::is_user_address((uintptr_t)address))
     {
-        kwarnln("mprotect() failed: attempted to protect a kernel page");
+        kwarnln("mprotect() failed: attempted to protect a non-user page");
         context->rax = -EINVAL;
         return;
     }
+    // FIXME: Check for overflow when adding address + size.
+    if (!Memory::is_user_address((uintptr_t)address + size))
+    {
+	kwarnln("mprotect() failed: end of given range is out of user memory");
+	context->rax = -EINVAL;
+	return;
+    }
     uint64_t flags = VMM::get_flags((uint64_t)address);
     if (flags == (uint64_t)-1)
     {
@@ -223,4 +230,4 @@ void sys_mprotect(Context* context, void* address, size_t size, int prot)
     kdbgln("mprotect() succeeded");
     context->rax = 0;
     return;
-}
\ No newline at end of file
+}