Fix buffer overflow in XBM parser

Avoid parsing over the buffer limit, or interpreting non-hex
as hex.

This still leaves parsing of lines longer than 300 chars
unreliable

Change-Id: I1c57a7e530c4380f6f9040b2ec729ccd7dc7a5fb
Reviewed-by: Robert Loehning <robert.loehning@qt.io>
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>
(cherry picked from commit c562c1fc19629fb505acd0f6380604840b634211)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

2 files changed, 41 insertions, 1 deletions

diff --git a/src/gui/image/qxbmhandler.cpp b/src/gui/image/qxbmhandler.cpp
index f06561690c..72ce7f7ecd 100644
--- a/src/gui/image/qxbmhandler.cpp
+++ b/src/gui/image/qxbmhandler.cpp
@@ -159,7 +159,9 @@ static bool read_xbm_body(QIODevice *device, int w, int h, QImage *outImage)
     w = (w+7)/8;                                // byte width
 
     while (y < h) {                                // for all encoded bytes...
-        if (p) {                                // p = "0x.."
+        if (p && p < (buf + readBytes - 3)) {      // p = "0x.."
+            if (!isxdigit(p[2]) || !isxdigit(p[3]))
+                return false;
             *b++ = hex2byte(p+2);
             p += 2;
             if (++x == w && ++y < h) {
diff --git a/tests/auto/gui/image/qimagereader/tst_qimagereader.cpp b/tests/auto/gui/image/qimagereader/tst_qimagereader.cpp
index cf8a0d1cff..984a7c7482 100644
--- a/tests/auto/gui/image/qimagereader/tst_qimagereader.cpp
+++ b/tests/auto/gui/image/qimagereader/tst_qimagereader.cpp
@@ -175,6 +175,8 @@ private slots:
 
     void xpmBufferOverflow();
 
+    void xbmBufferHandling();
+
 private:
     QString prefix;
     QTemporaryDir m_temporaryDir;
@@ -2055,5 +2057,41 @@ void tst_QImageReader::xpmBufferOverflow()
     QImageReader(":/images/oss-fuzz-23988.xpm").read();
 }
 
+void tst_QImageReader::xbmBufferHandling()
+{
+    uint8_t original_buffer[256];
+    for (int i = 0; i < 256; ++i)
+        original_buffer[i] = i;
+
+    QImage image(original_buffer, 256, 8, QImage::Format_MonoLSB);
+    image.setColorTable({0xff000000, 0xffffffff});
+
+    QByteArray buffer;
+    {
+        QBuffer buf(&buffer);
+        QImageWriter writer(&buf, "xbm");
+        writer.write(image);
+    }
+
+    QCOMPARE(QImage::fromData(buffer, "xbm"), image);
+
+    auto i = buffer.indexOf(',');
+    buffer.insert(i + 1, "                                                                                ");
+    QCOMPARE(QImage::fromData(buffer, "xbm"), image);
+    buffer.insert(i + 1, "                                                                                ");
+    QCOMPARE(QImage::fromData(buffer, "xbm"), image);
+    buffer.insert(i + 1, "                                                                              ");
+#if 0   // Lines longer than 300 chars not supported currently
+    QCOMPARE(QImage::fromData(buffer, "xbm"), image);
+#endif
+
+    i = buffer.lastIndexOf("\n ");
+    buffer.truncate(i + 1);
+    buffer.append(QByteArray(297, ' '));
+    buffer.append("0x");
+    // Only check we get no buffer overflow
+    QImage::fromData(buffer, "xbm");
+}
+
 QTEST_MAIN(tst_QImageReader)
 #include "tst_qimagereader.moc"