diff --git a/app/src/main/java/com/wireguard/android/util/RootShell.java b/app/src/main/java/com/wireguard/android/util/RootShell.java
index 7fbafa33..9e487a31 100644
--- a/app/src/main/java/com/wireguard/android/util/RootShell.java
+++ b/app/src/main/java/com/wireguard/android/util/RootShell.java
@@ -31,6 +31,7 @@ public class RootShell {
     private final String deviceNotRootedMessage;
     private final File localBinaryDir;
     private final File localTemporaryDir;
+    private final Object lock = new Object();
     private final String preamble;
     private Process process;
     private BufferedReader stderr;
@@ -57,16 +58,18 @@ public class RootShell {
         return false;
     }
 
-    public synchronized boolean isRunning() {
-        try {
-            // Throws an exception if the process hasn't finished yet.
-            if (process != null)
-                process.exitValue();
-        } catch (final IllegalThreadStateException ignored) {
-            // The existing process is still running.
-            return true;
+    private boolean isRunning() {
+        synchronized (lock) {
+            try {
+                // Throws an exception if the process hasn't finished yet.
+                if (process != null)
+                    process.exitValue();
+                return false;
+            } catch (final IllegalThreadStateException ignored) {
+                // The existing process is still running.
+                return true;
+            }
         }
-        return false;
     }
 
     /**
@@ -77,101 +80,108 @@ public class RootShell {
      * @param command Command to run as root.
      * @return The exit value of the command.
      */
-    public synchronized int run(final Collection<String> output, final String command)
+    public int run(final Collection<String> output, final String command)
             throws IOException, NoRootException {
-        start();
-        final String marker = UUID.randomUUID().toString();
-        final String script = "echo " + marker + "; echo " + marker + " >&2; (" + command +
-                "); ret=$?; echo " + marker + " $ret; echo " + marker + " $ret >&2\n";
-        Log.v(TAG, "executing: " + command);
-        stdin.write(script);
-        stdin.flush();
-        String line;
-        int errnoStdout = Integer.MIN_VALUE;
-        int errnoStderr = Integer.MAX_VALUE;
-        int markersSeen = 0;
-        while ((line = stdout.readLine()) != null) {
-            if (line.startsWith(marker)) {
-                ++markersSeen;
-                if (line.length() > marker.length() + 1) {
-                    errnoStdout = Integer.valueOf(line.substring(marker.length() + 1));
-                    break;
+        synchronized (lock) {
+            /* Start inside synchronized block to prevent a concurrent call to stop(). */
+            start();
+            final String marker = UUID.randomUUID().toString();
+            final String script = "echo " + marker + "; echo " + marker + " >&2; (" + command +
+                    "); ret=$?; echo " + marker + " $ret; echo " + marker + " $ret >&2\n";
+            Log.v(TAG, "executing: " + command);
+            stdin.write(script);
+            stdin.flush();
+            String line;
+            int errnoStdout = Integer.MIN_VALUE;
+            int errnoStderr = Integer.MAX_VALUE;
+            int markersSeen = 0;
+            while ((line = stdout.readLine()) != null) {
+                if (line.startsWith(marker)) {
+                    ++markersSeen;
+                    if (line.length() > marker.length() + 1) {
+                        errnoStdout = Integer.valueOf(line.substring(marker.length() + 1));
+                        break;
+                    }
+                } else if (markersSeen > 0) {
+                    if (output != null)
+                        output.add(line);
+                    Log.v(TAG, "stdout: " + line);
                 }
-            } else if (markersSeen > 0) {
-                if (output != null)
-                    output.add(line);
-                Log.v(TAG, "stdout: " + line);
             }
-        }
-        while ((line = stderr.readLine()) != null) {
-            if (line.startsWith(marker)) {
-                ++markersSeen;
-                if (line.length() > marker.length() + 1) {
-                    errnoStderr = Integer.valueOf(line.substring(marker.length() + 1));
-                    break;
+            while ((line = stderr.readLine()) != null) {
+                if (line.startsWith(marker)) {
+                    ++markersSeen;
+                    if (line.length() > marker.length() + 1) {
+                        errnoStderr = Integer.valueOf(line.substring(marker.length() + 1));
+                        break;
+                    }
+                } else if (markersSeen > 2) {
+                    Log.v(TAG, "stderr: " + line);
                 }
-            } else if (markersSeen > 2) {
-                Log.v(TAG, "stderr: " + line);
             }
+            if (markersSeen != 4)
+                throw new IOException("Expected 4 markers, received " + markersSeen);
+            if (errnoStdout != errnoStderr)
+                throw new IOException("Unable to read exit status");
+            Log.v(TAG, "exit: " + errnoStdout);
+            return errnoStdout;
         }
-        if (markersSeen != 4)
-            throw new IOException("Expected 4 markers, received " + markersSeen);
-        if (errnoStdout != errnoStderr)
-            throw new IOException("Unable to read exit status");
-        Log.v(TAG, "exit: " + errnoStdout);
-        return errnoStdout;
     }
 
-    public synchronized void start() throws IOException, NoRootException {
-        if (isRunning())
-            return;
-        if (!localBinaryDir.isDirectory() && !localBinaryDir.mkdirs())
-            throw new FileNotFoundException("Could not create local binary directory");
-        if (!localTemporaryDir.isDirectory() && !localTemporaryDir.mkdirs())
-            throw new FileNotFoundException("Could not create local temporary directory");
+    public void start() throws IOException, NoRootException {
         if (!isExecutableInPath(SU))
             throw new NoRootException(deviceNotRootedMessage);
-        try {
-            final ProcessBuilder builder = new ProcessBuilder().command(SU);
-            builder.environment().put("LC_ALL", "C");
+        synchronized (lock) {
+            if (isRunning())
+                return;
+            if (!localBinaryDir.isDirectory() && !localBinaryDir.mkdirs())
+                throw new FileNotFoundException("Could not create local binary directory");
+            if (!localTemporaryDir.isDirectory() && !localTemporaryDir.mkdirs())
+                throw new FileNotFoundException("Could not create local temporary directory");
             try {
-                process = builder.start();
-            } catch (final IOException e) {
-                // A failure at this stage means the device isn't rooted.
-                throw new NoRootException(deviceNotRootedMessage, e);
-            }
-            stdin = new OutputStreamWriter(process.getOutputStream(), StandardCharsets.UTF_8);
-            stdout = new BufferedReader(new InputStreamReader(process.getInputStream(),
-                    StandardCharsets.UTF_8));
-            stderr = new BufferedReader(new InputStreamReader(process.getErrorStream(),
-                    StandardCharsets.UTF_8));
-            stdin.write(preamble);
-            stdin.flush();
-            // Check that the shell started successfully.
-            final String uid = stdout.readLine();
-            if (!"0".equals(uid)) {
-                Log.w(TAG, "Root check did not return correct UID: " + uid);
-                throw new NoRootException(deviceNotRootedMessage);
-            }
-            if (!isRunning()) {
-                String line;
-                while ((line = stderr.readLine()) != null) {
-                    Log.w(TAG, "Root check returned an error: " + line);
-                    if (line.contains("Permission denied"))
-                        throw new NoRootException(deviceNotRootedMessage);
+                final ProcessBuilder builder = new ProcessBuilder().command(SU);
+                builder.environment().put("LC_ALL", "C");
+                try {
+                    process = builder.start();
+                } catch (final IOException e) {
+                    // A failure at this stage means the device isn't rooted.
+                    throw new NoRootException(deviceNotRootedMessage, e);
                 }
-                throw new IOException("Shell failed to start: " + process.exitValue());
+                stdin = new OutputStreamWriter(process.getOutputStream(), StandardCharsets.UTF_8);
+                stdout = new BufferedReader(new InputStreamReader(process.getInputStream(),
+                        StandardCharsets.UTF_8));
+                stderr = new BufferedReader(new InputStreamReader(process.getErrorStream(),
+                        StandardCharsets.UTF_8));
+                stdin.write(preamble);
+                stdin.flush();
+                // Check that the shell started successfully.
+                final String uid = stdout.readLine();
+                if (!"0".equals(uid)) {
+                    Log.w(TAG, "Root check did not return correct UID: " + uid);
+                    throw new NoRootException(deviceNotRootedMessage);
+                }
+                if (!isRunning()) {
+                    String line;
+                    while ((line = stderr.readLine()) != null) {
+                        Log.w(TAG, "Root check returned an error: " + line);
+                        if (line.contains("Permission denied"))
+                            throw new NoRootException(deviceNotRootedMessage);
+                    }
+                    throw new IOException("Shell failed to start: " + process.exitValue());
+                }
+            } catch (final IOException | NoRootException e) {
+                stop();
+                throw e;
             }
-        } catch (final IOException | NoRootException e) {
-            stop();
-            throw e;
         }
     }
 
-    public synchronized void stop() throws IOException {
-        if (process != null) {
-            process.destroy();
-            process = null;
+    public void stop() {
+        synchronized (lock) {
+            if (process != null) {
+                process.destroy();
+                process = null;
+            }
         }
     }