...
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 delayed for an arbitrary duration. {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 IOException, 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 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 } } |
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 | ||
|---|---|---|
| ||
{code} h2. 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: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 | ||||
|---|---|---|---|---|
| =
| |||
} 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
...
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) { private static final ThreadLocal<Connection> connectionHolder = try { stmt = getConnection().createStatement(); } catch (SQLException e) { // Forward to handler new ThreadLocal<Connection>() }{ } Connection connection = }null; private static final@Override ThreadLocal<Connection> connectionHolder = new ThreadLocal<Connection>() { Connection connection = null; @Override public Connection 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 Statement tmpStmt = stmt; if (stmttmpStmt != null) { try { stmttmpStmt.cancel(); return true; } catch (SQLException e) { // Forward to handler } } return false; } @Override public void run() { try { if (getConnection(stmt) =!= null) { stmt = getConnection().createStatement(); } if (stmt == null || (stmt.getConnection() != 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" |
...