diff --git a/src/tests/userprog/Make.tests b/src/tests/userprog/Make.tests
index 02c73fe..542ea0b 100644
--- a/src/tests/userprog/Make.tests
+++ b/src/tests/userprog/Make.tests
@@ -9,14 +9,14 @@ sc-bad-arg sc-bad-num sc-boundary sc-boundary-2 halt exit create-normal		\
 create-empty create-null create-bad-ptr create-long create-exists	\
 create-bound open-normal open-missing open-boundary open-empty		\
 open-null open-bad-ptr open-twice close-normal close-twice close-stdin	\
-close-stdout close-bad-fd read-normal read-bad-ptr read-boundary	\
-read-zero read-stdout read-bad-fd write-normal write-bad-ptr		\
+close-stdout close-bad-fd read-normal read-bad-ptr read-bad-buf read-boundary	\
+read-zero read-stdout read-bad-fd write-normal write-bad-ptr write-bad-buf	\
 write-boundary write-zero write-stdin write-bad-fd exec-once exec-arg	\
 exec-large-arg exec-multiple exec-missing exec-over-arg exec-over-args \
 exec-bad-ptr wait-simple wait-twice wait-killed wait-load-kill \
 wait-bad-pid wait-bad-child multi-recurse multi-child-fd rox-simple	\
 rox-child rox-multichild bad-read bad-write bad-read2 bad-write2        \
-bad-jump bad-jump2 bad-maths)
+bad-jump bad-jump2 bad-maths overflow-stack)
 
 tests/userprog_PROGS = $(tests/userprog_TESTS) $(addprefix \
 tests/userprog/,child-simple child-args child-bad child-close child-rox exec-exit)
@@ -36,6 +36,7 @@ tests/userprog/bad-read2_SRC = tests/userprog/bad-read2.c tests/main.c
 tests/userprog/bad-write2_SRC = tests/userprog/bad-write2.c tests/main.c
 tests/userprog/bad-jump2_SRC = tests/userprog/bad-jump2.c tests/main.c
 tests/userprog/bad-maths_SRC = tests/userprog/bad-maths.c tests/main.c
+tests/userprog/overflow-stack_SRC = tests/userprog/overflow-stack.c tests/main.c
 tests/userprog/sc-boundary_SRC = tests/userprog/sc-boundary.c	\
 tests/userprog/boundary.c tests/main.c
 tests/userprog/sc-boundary-2_SRC = tests/userprog/sc-boundary-2.c	\
@@ -66,6 +67,7 @@ tests/userprog/close-stdout_SRC = tests/userprog/close-stdout.c tests/main.c
 tests/userprog/close-bad-fd_SRC = tests/userprog/close-bad-fd.c tests/main.c
 tests/userprog/read-normal_SRC = tests/userprog/read-normal.c tests/main.c
 tests/userprog/read-bad-ptr_SRC = tests/userprog/read-bad-ptr.c tests/main.c
+tests/userprog/read-bad-buf_SRC = tests/userprog/read-bad-buf.c tests/main.c
 tests/userprog/read-boundary_SRC = tests/userprog/read-boundary.c	\
 tests/userprog/boundary.c tests/main.c
 tests/userprog/read-zero_SRC = tests/userprog/read-zero.c tests/main.c
@@ -73,6 +75,7 @@ tests/userprog/read-stdout_SRC = tests/userprog/read-stdout.c tests/main.c
 tests/userprog/read-bad-fd_SRC = tests/userprog/read-bad-fd.c tests/main.c
 tests/userprog/write-normal_SRC = tests/userprog/write-normal.c tests/main.c
 tests/userprog/write-bad-ptr_SRC = tests/userprog/write-bad-ptr.c tests/main.c
+tests/userprog/write-bad-buf_SRC = tests/userprog/write-bad-buf.c tests/main.c
 tests/userprog/write-boundary_SRC = tests/userprog/write-boundary.c	\
 tests/userprog/boundary.c tests/main.c
 tests/userprog/write-zero_SRC = tests/userprog/write-zero.c tests/main.c
@@ -122,10 +125,12 @@ tests/userprog/close-normal_PUTFILES += tests/userprog/sample.txt
 tests/userprog/close-twice_PUTFILES += tests/userprog/sample.txt
 tests/userprog/read-normal_PUTFILES += tests/userprog/sample.txt
 tests/userprog/read-bad-ptr_PUTFILES += tests/userprog/sample.txt
+tests/userprog/read-bad-buf_PUTFILES += tests/userprog/sample.txt
 tests/userprog/read-boundary_PUTFILES += tests/userprog/sample.txt
 tests/userprog/read-zero_PUTFILES += tests/userprog/sample.txt
 tests/userprog/write-normal_PUTFILES += tests/userprog/sample.txt
 tests/userprog/write-bad-ptr_PUTFILES += tests/userprog/sample.txt
+tests/userprog/write-bad-buf_PUTFILES += tests/userprog/sample.txt
 tests/userprog/write-boundary_PUTFILES += tests/userprog/sample.txt
 tests/userprog/write-zero_PUTFILES += tests/userprog/sample.txt
 tests/userprog/multi-child-fd_PUTFILES += tests/userprog/sample.txt
diff --git a/src/tests/userprog/Rubric.robustnessCR b/src/tests/userprog/Rubric.robustnessCR
index 9784817..256caed 100644
--- a/src/tests/userprog/Rubric.robustnessCR
+++ b/src/tests/userprog/Rubric.robustnessCR
@@ -1,5 +1,9 @@
-Full robustness of argument passing code:
-- Test user stack overflow robustness of "exec" system calls.
+Full robustness of argument passing and syscall handling code:
+- Test user stack overflow robustness of "exec" system calls and user code.
 5	exec-over-arg
 5	exec-over-args
+5	overflow-stack
 
+- Test syscall user provided buffer validity checks.
+5	read-bad-buf
+5	write-bad-buf
diff --git a/src/tests/userprog/exec-bad-ptr.ck b/src/tests/userprog/exec-bad-ptr.ck
index 63f5f78..7f2604c 100644
--- a/src/tests/userprog/exec-bad-ptr.ck
+++ b/src/tests/userprog/exec-bad-ptr.ck
@@ -2,11 +2,7 @@
 use strict;
 use warnings;
 use tests::tests;
-check_expected ([<<'EOF', <<'EOF']);
-(exec-bad-ptr) begin
-(exec-bad-ptr) end
-exec-bad-ptr: exit(0)
-EOF
+check_expected ([<<'EOF']);
 (exec-bad-ptr) begin
 exec-bad-ptr: exit(-1)
 EOF
diff --git a/src/tests/userprog/open-bad-ptr.ck b/src/tests/userprog/open-bad-ptr.ck
index 45349e2..fa686d0 100644
--- a/src/tests/userprog/open-bad-ptr.ck
+++ b/src/tests/userprog/open-bad-ptr.ck
@@ -2,11 +2,7 @@
 use strict;
 use warnings;
 use tests::tests;
-check_expected ([<<'EOF', <<'EOF']);
-(open-bad-ptr) begin
-(open-bad-ptr) end
-open-bad-ptr: exit(0)
-EOF
+check_expected ([<<'EOF']);
 (open-bad-ptr) begin
 open-bad-ptr: exit(-1)
 EOF
diff --git a/src/tests/userprog/overflow-stack.c b/src/tests/userprog/overflow-stack.c
new file mode 100644
index 0000000..d7c9c85
--- /dev/null
+++ b/src/tests/userprog/overflow-stack.c
@@ -0,0 +1,17 @@
+/* Attempt to overflow the user stack by allocating a 4kB buffer and writing into it.
+   The process must be terminated with -1 exit code until stack growth has been implemented in Task 3
+*/
+
+#include <string.h>
+#include <syscall.h>
+#include "tests/lib.h"
+#include "tests/main.h"
+
+void
+test_main (void)
+{
+  char stack_obj[4096];
+  memset (stack_obj, 'a', sizeof stack_obj);
+  memset (stack_obj+10, '\0', 1);
+  msg ("buffer: %s", stack_obj);
+}
diff --git a/src/tests/userprog/overflow-stack.ck b/src/tests/userprog/overflow-stack.ck
new file mode 100644
index 0000000..8998230
--- /dev/null
+++ b/src/tests/userprog/overflow-stack.ck
@@ -0,0 +1,14 @@
+# -*- perl -*-
+use strict;
+use warnings;
+use tests::tests;
+check_expected (IGNORE_USER_FAULTS => 1, [<<'EOF',<<'EOF']);
+(overflow-stack) begin
+overflow-stack: exit(-1)
+EOF
+(overflow-stack) begin
+(overflow-stack) buffer: aaaaaaaaaa
+(overflow-stack) end
+overflow-stack: exit(0)
+EOF
+pass;
diff --git a/src/tests/userprog/read-bad-buf.c b/src/tests/userprog/read-bad-buf.c
new file mode 100644
index 0000000..d2dc613
--- /dev/null
+++ b/src/tests/userprog/read-bad-buf.c
@@ -0,0 +1,17 @@
+/* Passes a buffer to the read system call that starts in valid memory, but runs into kernel space.
+   The process must be terminated with -1 exit code.
+*/
+
+#include <syscall.h>
+#include "tests/lib.h"
+#include "tests/main.h"
+
+void
+test_main (void)
+{
+  int handle;
+  CHECK ((handle = open ("sample.txt")) > 1, "open \"sample.txt\"");
+
+  read (handle, (char *) 0xbfffffe0, 100);
+  fail ("should not have survived read()");
+}
diff --git a/src/tests/userprog/read-bad-buf.ck b/src/tests/userprog/read-bad-buf.ck
new file mode 100644
index 0000000..703b633
--- /dev/null
+++ b/src/tests/userprog/read-bad-buf.ck
@@ -0,0 +1,10 @@
+# -*- perl -*-
+use strict;
+use warnings;
+use tests::tests;
+check_expected (IGNORE_KERNEL_FAULTS => 1, [<<'EOF']);
+(read-bad-buf) begin
+(read-bad-buf) open "sample.txt"
+read-bad-buf: exit(-1)
+EOF
+pass;
diff --git a/src/tests/userprog/read-bad-ptr.ck b/src/tests/userprog/read-bad-ptr.ck
index d10accf..abd67f7 100644
--- a/src/tests/userprog/read-bad-ptr.ck
+++ b/src/tests/userprog/read-bad-ptr.ck
@@ -2,12 +2,7 @@
 use strict;
 use warnings;
 use tests::tests;
-check_expected ([<<'EOF', <<'EOF']);
-(read-bad-ptr) begin
-(read-bad-ptr) open "sample.txt"
-(read-bad-ptr) end
-read-bad-ptr: exit(0)
-EOF
+check_expected ([<<'EOF']);
 (read-bad-ptr) begin
 (read-bad-ptr) open "sample.txt"
 read-bad-ptr: exit(-1)
diff --git a/src/tests/userprog/write-bad-buf.c b/src/tests/userprog/write-bad-buf.c
new file mode 100644
index 0000000..589499c
--- /dev/null
+++ b/src/tests/userprog/write-bad-buf.c
@@ -0,0 +1,17 @@
+/* Passes a buffer to the write system call that starts in valid memory, but runs into kernel space.
+   The process must be terminated with -1 exit code.
+*/
+
+#include <syscall.h>
+#include "tests/lib.h"
+#include "tests/main.h"
+
+void
+test_main (void)
+{
+  int handle;
+  CHECK ((handle = open ("sample.txt")) > 1, "open \"sample.txt\"");
+
+  write (handle, (char *) 0xbffffff0, 32);
+  fail ("should have exited with -1");
+}
diff --git a/src/tests/userprog/write-bad-buf.ck b/src/tests/userprog/write-bad-buf.ck
new file mode 100644
index 0000000..b0678cb
--- /dev/null
+++ b/src/tests/userprog/write-bad-buf.ck
@@ -0,0 +1,10 @@
+# -*- perl -*-
+use strict;
+use warnings;
+use tests::tests;
+check_expected (IGNORE_KERNEL_FAULTS => 1, [<<'EOF']);
+(write-bad-buf) begin
+(write-bad-buf) open "sample.txt"
+write-bad-buf: exit(-1)
+EOF
+pass;
diff --git a/src/tests/userprog/write-bad-ptr.ck b/src/tests/userprog/write-bad-ptr.ck
index ad9f399..3014879 100644
--- a/src/tests/userprog/write-bad-ptr.ck
+++ b/src/tests/userprog/write-bad-ptr.ck
@@ -2,12 +2,7 @@
 use strict;
 use warnings;
 use tests::tests;
-check_expected ([<<'EOF', <<'EOF']);
-(write-bad-ptr) begin
-(write-bad-ptr) open "sample.txt"
-(write-bad-ptr) end
-write-bad-ptr: exit(0)
-EOF
+check_expected ([<<'EOF']);
 (write-bad-ptr) begin
 (write-bad-ptr) open "sample.txt"
 write-bad-ptr: exit(-1)
diff --git a/src/threads/synch.c b/src/threads/synch.c
index c8bbd6d..65c5bde 100644
--- a/src/threads/synch.c
+++ b/src/threads/synch.c
@@ -212,6 +212,7 @@ donate_priority (struct thread *donee) {
   ASSERT (intr_get_level () == INTR_OFF);
 
   struct thread *donor = thread_current ();
+  list_remove (&donor->donor_elem);
   list_push_back (&donee->donors_list, &donor->donor_elem);
   
   while (donee != NULL)
@@ -260,6 +261,7 @@ lock_acquire (struct lock *lock)
   ASSERT (!lock_held_by_current_thread (lock));
 
   struct thread *t = thread_current ();
+  ASSERT (t->waiting_lock == NULL);
 
   enum intr_level old_level = intr_disable ();
   if (lock->holder != NULL)
@@ -341,7 +343,6 @@ lock_release (struct lock *lock)
      released, transfer the remaining orphaned donors to its donor list. */
   if (max_donor != NULL)
     {
-      list_remove (&max_donor->donor_elem);
       while (!list_empty (&orphan_list))
         list_push_back (&max_donor->donors_list, list_pop_front (&orphan_list));
     }
diff --git a/src/threads/thread.c b/src/threads/thread.c
index fd239ee..74ac41e 100644
--- a/src/threads/thread.c
+++ b/src/threads/thread.c
@@ -373,7 +373,9 @@ thread_exit (void)
      and schedule another process.  That process will destroy us
      when it calls thread_schedule_tail(). */
   intr_disable ();
-  list_remove (&thread_current()->allelem);
+  struct thread *t = thread_current ();
+  list_remove (&t->allelem);
+  list_remove (&t->donor_elem);
   thread_current ()->status = THREAD_DYING;
   schedule ();
   NOT_REACHED ();
@@ -679,6 +681,7 @@ init_thread (struct thread *t, const char *name, int nice, int priority,
   t->base_priority
       = thread_mlfqs ? calculate_bsd_priority (recent_cpu, nice) : priority;
   list_init (&t->donors_list);
+  list_push_back (&t->donors_list, &t->donor_elem);
   t->waiting_lock = NULL;
 
   t->nice = nice;
diff --git a/src/userprog/process.c b/src/userprog/process.c
index 3d12bd1..e1200bc 100644
--- a/src/userprog/process.c
+++ b/src/userprog/process.c
@@ -8,6 +8,7 @@
 #include <string.h>
 #include "userprog/gdt.h"
 #include "userprog/pagedir.h"
+#include "userprog/syscall.h"
 #include "userprog/tss.h"
 #include "filesys/directory.h"
 #include "filesys/file.h"
@@ -46,6 +47,9 @@ struct process_start_data
                           tokens while maintaining state. */
     char file_name[FNAME_MAX_LEN + 1]; /* Name of the file of the process to
                                       be started. */
+    bool success; /* Indicates whether the process was successfully loaded. */
+    struct semaphore loaded; /* Semaphore used to signal that the process has
+                                been loaded. */
   };
 
 static thread_func start_process NO_RETURN;
@@ -66,6 +70,8 @@ process_execute (const char *cmd)
     {
       return TID_ERROR;
     }
+  sema_init (&data->loaded, 0);
+  data->success = false;
 
   /* Make a copy of command.
      Otherwise there's a race between the caller and load(). */
@@ -81,8 +87,14 @@ process_execute (const char *cmd)
      of the process. */
   char *file_name = strtok_r (cmd_copy, " ", &data->cmd_saveptr);
 
-/* Validates that the current file to be executed is a valid file */
-if (filesys_open (file_name) == NULL)
+  /* NOTE: Currently, the file being executed is closed in load () and then
+          reopened here. Because load is an exported public function, this
+          might be necessary. */
+  lock_acquire (&filesys_lock);
+  /* Validates that the current file to be executed is a valid file */
+  bool valid_file = filesys_open (file_name) != NULL;
+  lock_release (&filesys_lock);
+  if (!valid_file)
     return TID_ERROR;
 
   /* Create a new thread to execute the command, by initializing
@@ -93,7 +105,14 @@ if (filesys_open (file_name) == NULL)
 
   tid = thread_create (file_name, PRI_DEFAULT, start_process, data);
   if (tid == TID_ERROR)
-    palloc_free_page (cmd_copy); 
+    palloc_free_page (cmd_copy);
+  else
+    {
+      sema_down (&data->loaded);
+      if (!data->success)
+        tid = TID_ERROR;
+    }
+  free (data);
   return tid;
 }
 
@@ -120,6 +139,20 @@ start_process (void *proc_start_data)
   if_.gs = if_.fs = if_.es = if_.ds = if_.ss = SEL_UDSEG;
   if_.cs = SEL_UCSEG;
   if_.eflags = FLAG_IF | FLAG_MBS;
+  
+  lock_acquire (&filesys_lock);
+  
+  /* Prevent writing to the file being executed. */
+  struct file *exec_file = filesys_open (data->file_name);
+  if (exec_file == NULL)
+    {
+      lock_release (&filesys_lock);
+      goto fail;
+    }
+  thread_current ()->exec_file = exec_file;
+  file_deny_write (exec_file);
+  lock_release (&filesys_lock);
+
   success = load (data->file_name, &if_.eip, &if_.esp);
 
   /* If load failed, quit. */
@@ -137,17 +170,11 @@ start_process (void *proc_start_data)
   /* If stack initialization failed, free resources and quit. */
   if (!success)
     {
-      process_exit ();
       goto fail;
     }
 
-  /* NOTE: Currently, the file being executed is closed in load () and then 
-           reopened here. Because load is an exported public function, this
-           might be necessary. */
-  struct file *exec_file = filesys_open (data->file_name);
-  thread_current ()->exec_file = exec_file;
-  file_deny_write (exec_file);
-
+  data->success = true;
+  sema_up (&data->loaded);
   /* Start the user process by simulating a return from an
      interrupt, implemented by intr_exit (in
      threads/intr-stubs.S).  Because intr_exit takes all of its
@@ -157,9 +184,10 @@ start_process (void *proc_start_data)
   asm volatile ("movl %0, %%esp; jmp intr_exit" : : "g" (&if_) : "memory");
   NOT_REACHED ();
 
-/* If starting the process failed, free its common resources and exit. */
- fail:
-  free (data);
+  /* If starting the process failed, exit. */
+fail:
+  data->success = false;
+  sema_up (&data->loaded);
   thread_exit ();
 }
 
@@ -316,7 +344,17 @@ process_exit (void)
   uint32_t *pd;
 
   printf ("%s: exit(%d)\n", cur->name, cur->exit_status);
-  file_close (cur->exec_file);
+
+  /* Clean up all open files */
+  hash_destroy (&cur->open_files, fd_cleanup);
+
+  /* Close the executable file. */
+  if (cur->exec_file != NULL)
+    {
+      lock_acquire (&filesys_lock);
+      file_close (cur->exec_file);
+      lock_release (&filesys_lock);
+    }
 
   /* Update process result. */
   if (cur->result != NULL)
@@ -341,10 +379,11 @@ process_exit (void)
   /* Free child process results or signal parent's death. */
   struct list_elem *e;
   for (e = list_begin (&cur->child_results);
-       e != list_end (&cur->child_results); e = list_next (e))
+       e != list_end (&cur->child_results);)
     {
       struct process_result *result
           = list_entry (e, struct process_result, elem);
+      struct list_elem *next = list_next (e);
       lock_acquire (&result->lock);
       /* Child has died (and was not waited for). Free the result. */
       if (sema_try_down (&result->sema))
@@ -358,6 +397,7 @@ process_exit (void)
           sema_up (&result->sema);
           lock_release (&result->lock);
         }
+      e = next;
     }
 
   /* Destroy the current process's page directory and switch back
@@ -476,6 +516,7 @@ load (const char *file_name, void (**eip) (void), void **esp)
   off_t file_ofs;
   bool success = false;
   int i;
+  lock_acquire (&filesys_lock);
 
   /* Allocate and activate page directory. */
   t->pagedir = pagedir_create ();
@@ -575,6 +616,7 @@ load (const char *file_name, void (**eip) (void), void **esp)
  done:
   /* We arrive here whether the load is successful or not. */
   file_close (file);
+  lock_release (&filesys_lock);
   return success;
 }
 
diff --git a/src/userprog/syscall.c b/src/userprog/syscall.c
index ccb02f3..29231f3 100644
--- a/src/userprog/syscall.c
+++ b/src/userprog/syscall.c
@@ -13,7 +13,9 @@
 #include <stdio.h>
 #include <syscall-nr.h>
 
-static struct lock filesys_lock;
+#define MAX_SYSCALL_ARGS 3
+#define EXIT_FAILURE -1
+
 static unsigned fd_counter = MIN_USER_FD;
 
 struct open_file
@@ -46,18 +48,19 @@ static unsigned syscall_tell (int fd);
 static void syscall_close (int fd);
 
 static struct open_file *fd_get_file (int fd);
-static void *validate_user_pointer (const void *ptr, size_t size);
+static void validate_user_pointer (const void *start, size_t size);
+static void validate_user_string (const char *str);
 
 /* A struct defining a syscall_function pointer along with its arity. */
-typedef struct
+struct syscall_arguments
   {
     syscall_function function;    /* Function pointer. */
     int arity;                    /* Number of arguments of the function. */
-  } syscall_arguments;
+  };
 
 /* A look-up table mapping numbers to system call functions with their number of
    arguments. */
-static const syscall_arguments syscall_lookup[] =
+static const struct syscall_arguments syscall_lookup[] =
 {
   [SYS_HALT] = {(syscall_function) syscall_halt, 0},
   [SYS_EXIT] = {(syscall_function) syscall_exit, 1},
@@ -77,8 +80,7 @@ static const syscall_arguments syscall_lookup[] =
 /* The number of syscall functions (i.e, number of elements) within the
    syscall_lookup table. */
 static const int LOOKUP_SIZE
-  = sizeof (syscall_lookup) / sizeof (syscall_arguments);
-
+  = sizeof (syscall_lookup) / sizeof (struct syscall_arguments);
 
 /* Initialises the syscall handling system, as well as a global lock to
    synchronise all file access between processes. */
@@ -89,28 +91,29 @@ syscall_init (void)
   lock_init (&filesys_lock);
 }
 
-/* Function that takes a interrupt frame containing a syscall and its args.
+/* Function that takes an interrupt frame containing a syscall and its args.
    Validates the arguments and pointers before calling the relevant
    high-level system call function, storing its output (if any) in f->eax */
 static void
 syscall_handler (struct intr_frame *f)
 {
   /* First, read the system call number from the stack. */
-  validate_user_pointer (f->esp, 1);
-  unsigned syscall_number = *(int *) f->esp;
+  validate_user_pointer (f->esp, sizeof (uintptr_t));
+  uintptr_t syscall_number = *(int *) f->esp;
 
   /* Ensures the number corresponds to a system call that can be handled. */
   if (syscall_number >= LOOKUP_SIZE)
-    thread_exit ();
+    syscall_exit (EXIT_FAILURE);
 
-  syscall_arguments syscall = syscall_lookup[syscall_number];
+  struct syscall_arguments syscall = syscall_lookup[syscall_number];
 
   /* Next, read and copy the arguments from the stack pointer. */
   validate_user_pointer (f->esp + sizeof (uintptr_t),
                          syscall.arity * sizeof (uintptr_t));
-  uintptr_t args[3] = {0};
-  for (int i=0; i < syscall.arity; i++)
-      args[i] = *(uintptr_t *) (f->esp + sizeof (uintptr_t) * (i + 1));
+
+  uintptr_t args[MAX_SYSCALL_ARGS] = {0};
+  for (int i = 0; i < syscall.arity && i < MAX_SYSCALL_ARGS; i++)
+    args[i] = *(uintptr_t *) (f->esp + sizeof (uintptr_t) * (i + 1));
 
   /* Call the function that handles this system call with the arguments. When
      there is a return value it is stored in f->eax. */
@@ -135,19 +138,13 @@ syscall_exit (int status)
 }
 
 /* Executes a given command with the relevant args, by calling process_execute.
-   Acquires the filesystem lock as process_execute accesses the file system.
-   Returns PID for the process that is running the CMD_LINE
- */
+   Returns PID for the process that is running the CMD_LINE. */
 static pid_t
 syscall_exec (const char *cmd_line)
 {
-  validate_user_pointer (cmd_line, 1);
+  validate_user_string (cmd_line);
 
-  lock_acquire (&filesys_lock);
-  pid_t pid = process_execute(cmd_line);
-  lock_release (&filesys_lock);
-
-  return pid;
+  return process_execute (cmd_line); /* Returns the PID of the new process */
 }
 
 /* Handles the syscall of wait. Effectively a wrapper for process_wait as the
@@ -155,16 +152,16 @@ syscall_exec (const char *cmd_line)
 static int
 syscall_wait (pid_t pid)
 {
-  return process_wait (pid);
+  return process_wait (pid); /* Returns the exit status of the waited process */
 }
 
 /* Handles the syscall for file creation. First validates the user file
    pointer. Acquires the file system lock to prevent synchronisation issues,
    and then uses FILESYS_CREATE to create the file, returning the same status */
 static bool
-syscall_create (const char *file UNUSED, unsigned initial_size UNUSED)
+syscall_create (const char *file, unsigned initial_size)
 {
-  validate_user_pointer (file, 1);
+  validate_user_string (file);
 
   lock_acquire (&filesys_lock);
   bool status = filesys_create (file, initial_size);
@@ -179,7 +176,7 @@ syscall_create (const char *file UNUSED, unsigned initial_size UNUSED)
 static bool
 syscall_remove (const char *file)
 {
-  validate_user_pointer (file, 1);
+  validate_user_string (file);
 
   lock_acquire (&filesys_lock);
   bool status = filesys_remove (file);
@@ -195,20 +192,23 @@ syscall_remove (const char *file)
 static int
 syscall_open (const char *file)
 {
-  validate_user_pointer (file, 1);
+  validate_user_string (file);
 
   lock_acquire (&filesys_lock);
   struct file *ptr = filesys_open (file);
   lock_release (&filesys_lock);
   if (ptr == NULL)
-    return -1;
+    return EXIT_FAILURE;
 
   /* Allocate space for a struct representing a mapping from an FD to a struct
      file. */
   struct open_file *file_info
     = (struct open_file*) malloc (sizeof (struct open_file));
   if (file_info == NULL)
-    return -1;
+    {
+      file_close (ptr);
+      return EXIT_FAILURE;
+    }
 
   /* Populate the above struct, with a unique FD and the current open file */
   file_info->fd = fd_counter++;
@@ -229,7 +229,7 @@ syscall_filesize (int fd)
 {
   struct open_file *file_info = fd_get_file (fd);
   if (file_info == NULL)
-      return -1;
+      return EXIT_FAILURE;
 
   lock_acquire (&filesys_lock);
   int bytes = file_length (file_info->file);
@@ -248,7 +248,7 @@ syscall_read (int fd, void *buffer, unsigned size)
   /* Only console (fd = 0) or other files, not including STDOUT, (fd > 1) are
      allowed. */
   if (fd < 0 || fd == STDOUT_FILENO)
-    return -1;
+    return EXIT_FAILURE;
 
   validate_user_pointer (buffer, size);
 
@@ -266,7 +266,7 @@ syscall_read (int fd, void *buffer, unsigned size)
       /* Reading from a file. */
       struct open_file *file_info = fd_get_file (fd);
       if (file_info == NULL)
-        return -1;
+        return EXIT_FAILURE;
 
       lock_acquire (&filesys_lock);
       int bytes_written = file_read (file_info->file, buffer, size);
@@ -381,6 +381,20 @@ fd_less (const struct hash_elem *a_, const struct hash_elem *b_,
   return a->fd < b->fd;
 }
 
+/* Function to clean up an open file entry. Closes the file and frees the
+   associated memory. */
+void
+fd_cleanup (struct hash_elem *e, void *aux UNUSED)
+{
+  struct open_file *file_info = hash_entry (e, struct open_file, elem);
+
+  lock_acquire (&filesys_lock);
+  file_close (file_info->file);
+  lock_release (&filesys_lock);
+
+  free (file_info);
+}
+
 /* Gets a file from its descriptor (FD number). If there is no file with the fd
    FD it returns NULL. */
 static struct open_file *
@@ -400,18 +414,57 @@ fd_get_file (int fd)
   return hash_entry (e, struct open_file, elem);
 }
 
-/* Validates if a block of memory starting at PTR and of size SIZE bytes is
-   fully contained within user virtual memory. Kills the thread (by calling
-   thread_exit) if the memory is invalid. Otherwise, returns the PTR given.
-   If the size is 0, the function does no checks and returns PTR.*/
-static void *
-validate_user_pointer (const void *ptr, size_t size)
+/* Validates if a block of memory starting at START and of size SIZE bytes is
+   fully contained within user virtual memory. Kills the thread (by exiting with
+   failure) if the memory is invalid. Otherwise, returns (nothing) normally.
+   If the size is 0, the function does no checks and returns the given ptr. */
+static void
+validate_user_pointer (const void *start, size_t size)
 {
-  if (size > 0 && (ptr == NULL ||
-                   !is_user_vaddr (ptr) ||
-                   !is_user_vaddr (ptr + size - 1) ||
-                   pagedir_get_page (thread_current()->pagedir, ptr) == NULL))
-    thread_exit ();
+  if (size == 0)
+    return;
 
-  return (void *) ptr;
+  const void *end = start + size - 1;
+
+  if (start == NULL || !is_user_vaddr (start) || !is_user_vaddr (end))
+    syscall_exit (EXIT_FAILURE);
+
+  /* We now need to check if the entire memory block is mapped to physical
+     memory by the page table. */
+  for (const void *ptr = start; ptr <= end; ptr += PGSIZE)
+    if (pagedir_get_page (thread_current ()->pagedir, ptr) == NULL)
+      syscall_exit (EXIT_FAILURE);
+}
+
+/* Validates if a string is fully contained within user virtual memory. Kills
+   the thread (by exiting with failure) if the memory is invalid. Otherwise,
+   returns (nothing) normally. */
+static void
+validate_user_string (const char *str)
+{
+  if (str == NULL || !is_user_vaddr (str))
+    syscall_exit (EXIT_FAILURE);
+
+  size_t offset = (uintptr_t) str % PGSIZE;
+
+  /* We move page by page, checking if the page is mapped to physical memory. */
+  for (;;)
+  {
+    void *page = pg_round_down (str);
+
+    if (!is_user_vaddr(page) ||
+        pagedir_get_page (thread_current ()->pagedir, page) == NULL)
+      syscall_exit (EXIT_FAILURE);
+
+    while (offset < PGSIZE)
+      {
+        if (*str == '\0')
+          return; /* We reached the end of the string without issues. */
+
+        str++;
+        offset++;
+      }
+
+    offset = 0; /* Next page will start at the beginning. */
+  }
 }
diff --git a/src/userprog/syscall.h b/src/userprog/syscall.h
index 0f9288b..681a8fb 100644
--- a/src/userprog/syscall.h
+++ b/src/userprog/syscall.h
@@ -2,14 +2,18 @@
 #define USERPROG_SYSCALL_H
 
 #include <hash.h>
+#include "threads/synch.h"
 
 #define MIN_USER_FD 2
 
 typedef int pid_t;
 
+struct lock filesys_lock;
+
 void syscall_init (void);
 
 unsigned fd_hash (const struct hash_elem *element, void *aux);
 bool fd_less (const struct hash_elem *a, const struct hash_elem *b, void *aux);
+void fd_cleanup (struct hash_elem *e, void *aux);
 
 #endif /* userprog/syscall.h */