2 files changed, 58 insertions, 3 deletions

diff --git a/hw/block/fdc.c b/hw/block/fdc.c
index fa933cd326..21d18ac2e3 100644
--- a/hw/block/fdc.c
+++ b/hw/block/fdc.c
@@ -61,6 +61,12 @@
     } while (0)
 
 
+/* Anonymous BlockBackend for empty drive */
+static BlockBackend *blk_create_empty_drive(void)
+{
+    return blk_new(qemu_get_aio_context(), 0, BLK_PERM_ALL);
+}
+
 /********************************************************/
 /* qdev floppy bus                                      */
 
@@ -486,8 +492,7 @@ static void floppy_drive_realize(DeviceState *qdev, Error **errp)
     }
 
     if (!dev->conf.blk) {
-        /* Anonymous BlockBackend for an empty drive */
-        dev->conf.blk = blk_new(qemu_get_aio_context(), 0, BLK_PERM_ALL);
+        dev->conf.blk = blk_create_empty_drive();
         ret = blk_attach_dev(dev->conf.blk, qdev);
         assert(ret == 0);
 
@@ -1161,7 +1166,19 @@ static FDrive *get_drv(FDCtrl *fdctrl, int unit)
 
 static FDrive *get_cur_drv(FDCtrl *fdctrl)
 {
-    return get_drv(fdctrl, fdctrl->cur_drv);
+    FDrive *cur_drv = get_drv(fdctrl, fdctrl->cur_drv);
+
+    if (!cur_drv->blk) {
+        /*
+         * Kludge: empty drive line selected. Create an anonymous
+         * BlockBackend to avoid NULL deref with various BlockBackend
+         * API calls within this model (CVE-2021-20196).
+         * Due to the controller QOM model limitations, we don't
+         * attach the created to the controller device.
+         */
+        cur_drv->blk = blk_create_empty_drive();
+    }
+    return cur_drv;
 }
 
 /* Status A register : 0x00 (read-only) */
diff --git a/tests/qtest/fdc-test.c b/tests/qtest/fdc-test.c
index 26b69f7c5c..8f6eee84a4 100644
--- a/tests/qtest/fdc-test.c
+++ b/tests/qtest/fdc-test.c
@@ -32,6 +32,9 @@
 /* TODO actually test the results and get rid of this */
 #define qmp_discard_response(...) qobject_unref(qmp(__VA_ARGS__))
 
+#define DRIVE_FLOPPY_BLANK \
+    "-drive if=floppy,file=null-co://,file.read-zeroes=on,format=raw,size=1440k"
+
 #define TEST_IMAGE_SIZE 1440 * 1024
 
 #define FLOPPY_BASE 0x3f0
@@ -546,6 +549,40 @@ static void fuzz_registers(void)
     }
 }
 
+static bool qtest_check_clang_sanitizer(void)
+{
+#if defined(__SANITIZE_ADDRESS__) || __has_feature(address_sanitizer)
+    return true;
+#else
+    g_test_skip("QEMU not configured using --enable-sanitizers");
+    return false;
+#endif
+}
+static void test_cve_2021_20196(void)
+{
+    QTestState *s;
+
+    if (!qtest_check_clang_sanitizer()) {
+        return;
+    }
+
+    s = qtest_initf("-nographic -m 32M -nodefaults " DRIVE_FLOPPY_BLANK);
+
+    qtest_outw(s, 0x3f4, 0x0500);
+    qtest_outb(s, 0x3f5, 0x00);
+    qtest_outb(s, 0x3f5, 0x00);
+    qtest_outw(s, 0x3f4, 0x0000);
+    qtest_outb(s, 0x3f5, 0x00);
+    qtest_outw(s, 0x3f1, 0x0400);
+    qtest_outw(s, 0x3f4, 0x0000);
+    qtest_outw(s, 0x3f4, 0x0000);
+    qtest_outb(s, 0x3f5, 0x00);
+    qtest_outb(s, 0x3f5, 0x01);
+    qtest_outw(s, 0x3f1, 0x0500);
+    qtest_outb(s, 0x3f5, 0x00);
+    qtest_quit(s);
+}
+
 int main(int argc, char **argv)
 {
     int fd;
@@ -576,6 +613,7 @@ int main(int argc, char **argv)
     qtest_add_func("/fdc/read_no_dma_18", test_read_no_dma_18);
     qtest_add_func("/fdc/read_no_dma_19", test_read_no_dma_19);
     qtest_add_func("/fdc/fuzz-registers", fuzz_registers);
+    qtest_add_func("/fdc/fuzz/cve_2021_20196", test_cve_2021_20196);
 
     ret = g_test_run();