...
Threads
...
and
...
tasks
...
that
...
block
...
on
...
operations
...
involving
...
network
...
or
...
file
...
I/O
...
must
...
provide
...
callers
...
with
...
an
...
explicit
...
termination
...
mechanism
...
to
...
prevent
...
...
(DoS) vulnerabilities.
...
Noncompliant
...
Code
...
Example
...
(Blocking
...
I/O,
...
Volatile
...
Flag)
...
This
...
noncompliant
...
code
...
example
...
uses
...
a
...
volatile
...
done
...
flag
...
to
...
indicate
...
it
...
whether is
...
safe
...
to
...
shut
...
down
...
the
...
thread,
...
as
...
suggested
...
in
...
...
...
...
...
...
...
...
...
.
...
However,
...
when the thread is blocked on network I/O
...
as
...
a
...
consequence
...
of
...
invoking
...
the
...
readLine() method, it cannot respond to the newly set flag until the network I/O is complete. Consequently, thread termination may be indefinitely delayed.
| Code Block | ||
|---|---|---|
| ||
// Thread-safe class }} method. {code:bgColor=#FFcccc} public final class SocketReader implements Runnable { // Thread-safe class private final Socket socket; private final BufferedReader in; private volatile boolean done = false; private final Object lock = new Object(); public SocketReader(String host, int port) throws IOException { this.socket = new Socket(host, port); this.in = new BufferedReader( new InputStreamReader(this.socket.getInputStream()) ); } // Only one thread can use the socket at a particular time @Override public void run() { try { synchronized (lock) { readData(); } } catch (IOException ie) { // Forward to handler } } public void readData() throws IOException { String string; while (!done && (string = in.readLine()) != null) { // Blocks until end of stream (null) } } public void shutdown() { done = true; } public static void main(String[] args) throws throws IOException, InterruptedException { SocketReader reader = new SocketReader("somehost", 25); Thread thread = new Thread(reader); thread.start(); Thread.sleep(1000); reader.shutdown(); // Shut Shutdowndown the thread } } {code} h2. Noncompliant Code Example |
Noncompliant Code Example (Blocking
...
I/O,
...
Interruptible)
...
This
...
noncompliant
...
code
...
example
...
is
...
similar
...
to
...
the
...
preceding
...
example but
...
uses
...
thread
...
interruption
...
to
...
shut
...
down
...
the
...
thread.
...
Network
...
I/O on a java.net.Socket is unresponsive to thread interruption.
| Code Block | ||
|---|---|---|
| ||
// Thread-safe class public final class SocketReader implements Runnable { // Other methods.. is not responsive to thread interruption when a {{java.net.Socket}} is being used. The {{readData()}} and {{main()}} methods are modified as follows: {code:bgColor=#FFcccc} public final class SocketReader implements Runnable { // Thread-safe class // ... public void readData() throws IOException { String string; while (!Thread.interrupted() && (string = in.readLine()) != null) { // Blocks until end of stream (null) } } public static void main(String[] args) throws IOException, InterruptedException { SocketReader reader = new SocketReader("somehost", 25); Thread thread = new Thread(reader); thread.start(); Thread.sleep(1000); thread.interrupt(); // Interrupt the thread } } {code} h2. Compliant Solution |
Compliant Solution (Close
...
Socket
...
Connection)
...
This
...
compliant
...
solution
...
terminates the blocking network I/O by closing the socket in the shutdown()
...
method
...
.
...
The
...
readLine()
...
method
...
throws
...
a
...
SocketException
...
when
...
the
...
socket
...
is closed, consequently allowing the thread to proceed. Note that it is impossible to keep the connection alive while simultaneously halting the thread both cleanly and immediately.
| Code Block | ||
|---|---|---|
| ||
closed which lets the thread proceed. Note that there is no way to keep the connection alive if the thread is to be cleanly halted immediately. {code:bgColor=#ccccff} public final class SocketReader implements Runnable { // Other methods... public void readData() throws IOException { String string; try { while ((string = in.readLine()) != null) { // Blocks until end of stream (null) } } finally { shutdown(); } } public void shutdown() throws IOException { socket.close(); } public static void main(String[] args) throws IOException, InterruptedException { SocketReader reader = new SocketReader("somehost", 25); Thread thread = new Thread(reader); thread.start(); Thread.sleep(1000); reader.shutdown(); } } {code} |
After
...
the
...
shutdown()
...
method
...
is
...
called
...
from
...
main()
...
,
...
the
...
finally
...
block
...
in
...
readData()
...
executes
...
and
...
calls
...
shutdown()
...
again,
...
closing
...
the
...
socket
...
for
...
a
...
second
...
time.
...
However,
...
when the socket has already been closed, this second call does nothing.
When performing asynchronous I/O,
...
a
...
java.nio.channels.Selector
...
can be unblocked by invoking either its close() or its wakeup() method.
When additional operations must be performed after emerging from the blocked state, use a boolean flag to indicate pending termination. When supplementing the code with such a flag, the shutdown() method should also set the flag to false so that the thread can cleanly exit from the while loop.
Compliant Solution (Interruptible Channel)
This compliant solution uses an interruptible channel, java.nio.channels.SocketChannel
...
,
...
instead
...
of
...
a
...
Socket
...
connection.
...
If
...
the
...
thread
...
performing
...
the
...
network
...
I/O
...
is
...
interrupted
...
using
...
the
...
Thread.interrupt()
...
method
...
while
...
it
...
is
...
reading
...
the
...
data,
...
the
...
thread
...
receives
...
a
...
ClosedByInterruptException
...
,
...
and
...
the
...
channel
...
is
...
closed
...
immediately.
...
The
...
thread's
...
interrupted
...
status
...
is
...
also
...
set.
| Code Block | ||||
|---|---|---|---|---|
| =
| |||
} public final class SocketReader implements Runnable { private final SocketChannel sc; private final Object lock = new Object(); public SocketReader(String host, int port) throws IOException { sc = SocketChannel.open(new InetSocketAddress(host, port)); } @Override public void run() { ByteBuffer buf = ByteBuffer.allocate(1024); try { synchronized (lock) { while (!Thread.interrupted()) { sc.read(buf); // ... } } } catch (IOException ie) { // Forward to handler } } public static void main(String[] args) throws IOException, InterruptedException { SocketReader reader = new SocketReader("somehost", 25); Thread thread = new Thread(reader); thread.start(); Thread.sleep(1000); thread.interrupt(); } } {code} |
This
...
technique
...
interrupts
...
the
...
current
...
thread.
...
However,
...
it
...
stops
...
the
...
thread
...
only because
...
the
...
code
...
polls
...
the
...
thread's
...
interrupted
...
status
...
with
...
the
...
Thread.interrupted()
...
method
...
and
...
terminates
...
the
...
thread
...
when
...
it
...
is
...
interrupted.
...
Using
...
a
...
SocketChannel
...
ensures
...
that
...
the
...
condition
...
in
...
the
...
while
...
loop
...
is
...
tested
...
as
...
soon
...
as
...
an
...
interruption
...
is
...
received,
...
even though the
...
read
...
is normally a
...
blocking
...
operation.
...
Similarly,
...
invoking
...
the
...
interrupt()
...
method of
...
a
...
thread
...
blocked on a java.nio.channels.Selector
...
also
...
causes
...
that
...
thread
...
to
...
awaken.
...
Noncompliant Code Example (Database
...
Connection)
...
This
...
noncompliant
...
code
...
example
...
shows
...
a
...
...
DBConnector
...
class
...
that
...
creates
...
one JDBC connection per thread. Each connection belongs to one thread and is not shared by other threads. This is a common use case because JDBC connections are intended to be single-threaded.
| Code Block | ||
|---|---|---|
| ||
Java Database Connectivity (JDBC) connection per thread. Each connection belongs to one thread and is not shared by other threads. This is a common use case because JDBC connections are not meant to be shared by multiple threads. {code:bgColor=#FFcccc} public final class DBConnector implements Runnable { private final String query; DBConnector(String query) { this.query = query; } @Override public void run() { Connection connection; try { // Username and password are hard coded for brevity connection = DriverManager.getConnection( "jdbc:driver:name", "username", "password" ); Statement stmt = connection.createStatement(); ResultSet rs = stmt.executeQuery(query); // ... } catch (SQLException e) { // Forward to handler } // ... } public static void main(String[] args) throws InterruptedException { DBConnector connector = new DBConnector("suitable query"); Thread thread = new Thread(connector); thread.start(); Thread.sleep(5000); thread.interrupt(); } } {code} Database |
Database connections,
...
like
...
sockets,
...
lack inherent interruptibility. Consequently,
...
this
...
design fails to support the client's attempts to cancel a task by closing the resource when the corresponding thread is blocked on a long-running query, such as a join.
Compliant Solution (Statement.cancel()
...
)
...
This
...
compliant
...
solution
...
uses
...
a
...
ThreadLocal
...
wrapper
...
around
...
the
...
connection
...
so
...
that
...
a
...
thread
...
calling
...
the
...
initialValue()
...
method
...
obtains
...
a
...
unique
...
connection
...
instance.
...
This approach allows provision of a cancelStatement() so that other threads or clients can interrupt a long-running query when required. The cancelStatement() method invokes the Statement.cancel()
...
method.
| Code Block | ||||
|---|---|---|---|---|
| =
| |||
} public final class DBConnector implements Runnable { private final String query; private volatile Statement stmt; DBConnector(String query) { this.query = query; } if(getConnection() != null) { try { stmt = getConnection().createStatement();private static final ThreadLocal<Connection> connectionHolder = } catch (SQLException e) { // Forward to handler } } } private static final ThreadLocal<Connection> connectionHolder = new ThreadLocal<Connection>() { Connection connection = null; @Override public Connection initialValue() { try { // ... connection = DriverManager.getConnection( "jdbc:driver:name", "username", "password" ); } catch (SQLException e) { // Forward to handler } return connection; } }; public Connection getConnection() { return connectionHolder.get(); } public boolean cancelStatement() { // Allows client to cancel statement if(stmt Statement tmpStmt = stmt; if (tmpStmt != null) { try { stmttmpStmt.cancel(); return true; } catch (SQLException e) { // Forward to handler } } return false; } @Override public void run() { try { if(stmt == null || (stmt.getConnection() != getConnection(null))) { throwstmt new IllegalStateException= getConnection().createStatement(); } if (stmt ResultSet== rsnull =|| (stmt.executeQuerygetConnection(query); != getConnection())) { throw new IllegalStateException(); } ResultSet rs = stmt.executeQuery(query); // ... } catch (SQLException e) { // Forward to handler } // ... } public static void main(String[] args) throws InterruptedException { DBConnector connector = new DBConnector("suitable query"); Thread thread = new Thread(connector); thread.start(); Thread.sleep(5000); connector.cancelStatement(); } } {code} The {{ |
The Statement.cancel()
...
method
...
cancels
...
the
...
query,
...
provided
...
the
...
database
...
management
...
system
...
(DBMS)
...
and
...
driver
...
both
...
support
...
cancellation.
...
It
...
is
...
impossible to cancel the query if either the DBMS or the driver fail to support cancellation.
According to the Java API, Interface Statement documentation [API 2014]
By default, only one
ResultSetobject perStatementobject can be open at the same time. As a result, if the reading of oneResultSetobject is interleaved with the reading of another, each must have been generated by differentStatementobjects.
This compliant solution ensures that only one ResultSet is associated with the Statement belonging to an instance, and, consequently, only one thread can access the query results.
Risk Assessment
Failure to provide facilities for thread termination can cause nonresponsiveness and DoS.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
THI04-J | Low | Probable | Medium | P4 | L3 |
Bibliography
[API 2014] | |
Section 24.3, "Stopping a Thread" | |
Chapter 7, "Cancellation and Shutdown" | |
Section 2.4, "Two Approaches to Stopping a Thread" | |
Java Thread Primitive Deprecation | |
[JPL 2006] | Section 14.12.1, "Don't Stop" |
...