eCryptfs: Sanitize write counts of /dev/ecryptfs

A malicious count value specified when writing to /dev/ecryptfs may
result in a a very large kernel memory allocation.

This patch peeks at the specified packet payload size, adds that to the
size of the packet headers and compares the result with the write count
value. The resulting maximum memory allocation size is approximately 532
bytes.

Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Reported-by: Sasha Levin <levinsasha928@gmail.com>
Cc: <stable@vger.kernel.org>

fs/ecryptfs/miscdev.c

@@ -409,11 +409,47 @@ ecryptfs_miscdev_write(struct file *file, const char __user *buf,
 	ssize_t sz = 0;
 	char *data;
 	uid_t euid = current_euid();
+	unsigned char packet_size_peek[3];
 	int rc;
 
-	if (count == 0)
+	if (count == 0) {
 		goto out;
+	} else if (count == (1 + 4)) {
+		/* Likely a harmless MSG_HELO or MSG_QUIT - no packet length */
+		goto memdup;
+	} else if (count < (1 + 4 + 1)
+		   || count > (1 + 4 + 2 + sizeof(struct ecryptfs_message) + 4
+			       + ECRYPTFS_MAX_ENCRYPTED_KEY_BYTES)) {
+		printk(KERN_WARNING "%s: Acceptable packet size range is "
+		       "[%d-%lu], but amount of data written is [%zu].",
+		       __func__, (1 + 4 + 1),
+		       (1 + 4 + 2 + sizeof(struct ecryptfs_message) + 4
+			+ ECRYPTFS_MAX_ENCRYPTED_KEY_BYTES), count);
+		return -EINVAL;
+	}
+
+	if (copy_from_user(packet_size_peek, (buf + 1 + 4),
+			   sizeof(packet_size_peek))) {
+		printk(KERN_WARNING "%s: Error while inspecting packet size\n",
+		       __func__);
+		return -EFAULT;
+	}
+
+	rc = ecryptfs_parse_packet_length(packet_size_peek, &packet_size,
+					  &packet_size_length);
+	if (rc) {
+		printk(KERN_WARNING "%s: Error parsing packet length; "
+		       "rc = [%d]\n", __func__, rc);
+		return rc;
+	}
+
+	if ((1 + 4 + packet_size_length + packet_size) != count) {
+		printk(KERN_WARNING "%s: Invalid packet size [%zu]\n", __func__,
+		       packet_size);
+		return -EINVAL;
+	}
 
+memdup:
 	data = memdup_user(buf, count);
 	if (IS_ERR(data)) {
 		printk(KERN_ERR "%s: memdup_user returned error [%ld]\n",
@@ -435,23 +471,7 @@ ecryptfs_miscdev_write(struct file *file, const char __user *buf,
 		}
 		memcpy(&counter_nbo, &data[i], 4);
 		seq = be32_to_cpu(counter_nbo);
-		i += 4;
-		rc = ecryptfs_parse_packet_length(&data[i], &packet_size,
-						  &packet_size_length);
-		if (rc) {
-			printk(KERN_WARNING "%s: Error parsing packet length; "
-			       "rc = [%d]\n", __func__, rc);
-			goto out_free;
-		}
-		i += packet_size_length;
-		if ((1 + 4 + packet_size_length + packet_size) != count) {
-			printk(KERN_WARNING "%s: (1 + packet_size_length([%zd])"
-			       " + packet_size([%zd]))([%zd]) != "
-			       "count([%zd]). Invalid packet format.\n",
-			       __func__, packet_size_length, packet_size,
-			       (1 + packet_size_length + packet_size), count);
-			goto out_free;
-		}
+		i += 4 + packet_size_length;
 		rc = ecryptfs_miscdev_response(&data[i], packet_size,
 					       euid, current_user_ns(),
 					       task_pid(current), seq);