apparmor: fix oops, validate buffer size in apparmor_setprocattr()

When proc_pid_attr_write() was changed to use memdup_user apparmor's
(interface violating) assumption that the setprocattr buffer was always
a single page was violated.

The size test is not strictly speaking needed as proc_pid_attr_write()
will reject anything larger, but for the sake of robustness we can keep
it in.

SMACK and SELinux look safe to me, but somebody else should probably
have a look just in case.

Based on original patch from Vegard Nossum <vegard.nossum@oracle.com>
modified for the case that apparmor provides null termination.

Fixes: bb646cdb12
Reported-by: Vegard Nossum <vegard.nossum@oracle.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: John Johansen <john.johansen@canonical.com>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: Eric Paris <eparis@parisplace.org>
Cc: Casey Schaufler <casey@schaufler-ca.com>
Cc: stable@kernel.org
Signed-off-by: John Johansen <john.johansen@canonical.com>
Reviewed-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>

security/apparmor/lsm.c

@@ -500,34 +500,34 @@ static int apparmor_setprocattr(struct task_struct *task, char *name,
 {
 	struct common_audit_data sa;
 	struct apparmor_audit_data aad = {0,};
-	char *command, *args = value;
+	char *command, *largs = NULL, *args = value;
 	size_t arg_size;
 	int error;
 
 	if (size == 0)
 		return -EINVAL;
-	/* args points to a PAGE_SIZE buffer, AppArmor requires that
-	 * the buffer must be null terminated or have size <= PAGE_SIZE -1
-	 * so that AppArmor can null terminate them
-	 */
-	if (args[size - 1] != '\0') {
-		if (size == PAGE_SIZE)
-			return -EINVAL;
-		args[size] = '\0';
-	}
-
 	/* task can only write its own attributes */
 	if (current != task)
 		return -EACCES;
 
-	args = value;
+	/* AppArmor requires that the buffer must be null terminated atm */
+	if (args[size - 1] != '\0') {
+		/* null terminate */
+		largs = args = kmalloc(size + 1, GFP_KERNEL);
+		if (!args)
+			return -ENOMEM;
+		memcpy(args, value, size);
+		args[size] = '\0';
+	}
+
+	error = -EINVAL;
 	args = strim(args);
 	command = strsep(&args, " ");
 	if (!args)
-		return -EINVAL;
+		goto out;
 	args = skip_spaces(args);
 	if (!*args)
-		return -EINVAL;
+		goto out;
 
 	arg_size = size - (args - (char *) value);
 	if (strcmp(name, "current") == 0) {
@@ -553,10 +553,12 @@ static int apparmor_setprocattr(struct task_struct *task, char *name,
 			goto fail;
 	} else
 		/* only support the "current" and "exec" process attributes */
-		return -EINVAL;
+		goto fail;
 
 	if (!error)
 		error = size;
+out:
+	kfree(largs);
 	return error;
 
 fail:
@@ -565,9 +567,9 @@ fail:
 	aad.profile = aa_current_profile();
 	aad.op = OP_SETPROCATTR;
 	aad.info = name;
-	aad.error = -EINVAL;
+	aad.error = error = -EINVAL;
 	aa_audit_msg(AUDIT_APPARMOR_DENIED, &sa, NULL);
-	return -EINVAL;
+	goto out;
 }
 
 static int apparmor_task_setrlimit(struct task_struct *task,