[PATCH v2] hw/display/xlnx_dp: fix an out-of-bounds read in xlnx_dp_read

[Qiang Liu <cyruscyliu@gmail.com>]

xlnx_dp_read allows an out-of-bounds read at its default branch because
of an improper index.

According to
https://www.xilinx.com/html_docs/registers/ug1087/ug1087-zynq-ultrascale-registers.html
(DP Module), registers 0x3A4/0x3A4/0x3AC are allowed.

DP_INT_MASK     0x000003A4      32      mixed   0xFFFFF03F      Interrupt Mask Register for intrN.
DP_INT_EN       0x000003A8      32      mixed   0x00000000      Interrupt Enable Register.
DP_INT_DS       0x000003AC      32      mixed   0x00000000      Interrupt Disable Register.

In xlnx_dp_write, when the offset is 0x3A8 and 0x3AC, the virtual device
will write s->core_registers[0x3A4
>> 2]. That is to say, the maxize of s->core_registers could be ((0x3A4
>> 2) + 1). However, the current size of s->core_registers is (0x3AF >>
>> 2), that is ((0x3A4 >> 2) + 2), which is out of the range.
In xlxn_dp_read, the access to offset 0x3A8 or 0x3AC will be directed to
the offset 0x3A8 (incorrect functionality) or 0x3AC (out-of-bounds read)
rather than 0x3A4.

This patch enforces the read access to offset 0x3A8 and 0x3AC to 0x3A4,
but does not adjust the size of s->core_registers to avoid breaking
migration.

Fixes: 58ac482a66de ("introduce xlnx-dp")
Signed-off-by: Qiang Liu <cyruscyliu@gmail.com>
---
v2:
  - not change DP_CORE_REG_ARRAY_SIZE
  - add a qtest reproducer
  - update the code style

I have a question about the QTest reproducer. Before patching xlnx-dp,
(0x3ac >> 2) will exceed the right bound of s->core_registers.  However,
this is allowed by the assertion. There is no warning and this
reproducer will pass. Is the reprodocer OK?

 hw/display/xlnx_dp.c            |  6 +++++-
 tests/qtest/fuzz-xlnx-dp-test.c | 33 +++++++++++++++++++++++++++++++++
 tests/qtest/meson.build         |  1 +
 3 files changed, 39 insertions(+), 1 deletion(-)
 create mode 100644 tests/qtest/fuzz-xlnx-dp-test.c

diff --git a/hw/display/xlnx_dp.c b/hw/display/xlnx_dp.c
index 7bcbb13..747df6e 100644
--- a/hw/display/xlnx_dp.c
+++ b/hw/display/xlnx_dp.c
@@ -714,7 +714,11 @@ static uint64_t xlnx_dp_read(void *opaque, hwaddr offset, unsigned size)
         break;
     default:
         assert(offset <= (0x3AC >> 2));
-        ret = s->core_registers[offset];
+        if (offset == (0x3A8 >> 2) || offset == (0x3AC >> 2)) {
+            ret = s->core_registers[DP_INT_MASK];
+        } else {
+            ret = s->core_registers[offset];
+        }
         break;
     }
 
diff --git a/tests/qtest/fuzz-xlnx-dp-test.c b/tests/qtest/fuzz-xlnx-dp-test.c
new file mode 100644
index 0000000..69eb6c0
--- /dev/null
+++ b/tests/qtest/fuzz-xlnx-dp-test.c
@@ -0,0 +1,33 @@
+/*
+ * QTest fuzzer-generated testcase for xlnx-dp display device
+ *
+ * Copyright (c) 2021 Qiang Liu <cyruscyliu@gmail.com>
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#include "qemu/osdep.h"
+#include "libqos/libqtest.h"
+
+/*
+ * This used to trigger the out-of-bounds read in xlnx_dp_read
+ */
+static void test_fuzz_xlnx_dp_0x3ac(void)
+{
+    QTestState *s = qtest_init("-M xlnx-zcu102 -display none ");
+    qtest_readl(s, 0xfd4a03ac);
+    qtest_quit(s);
+}
+
+int main(int argc, char **argv)
+{
+    const char *arch = qtest_get_arch();
+
+    g_test_init(&argc, &argv, NULL);
+
+   if (strcmp(arch, "aarch64") == 0) {
+        qtest_add_func("fuzz/test_fuzz_xlnx_dp/3ac", test_fuzz_xlnx_dp_0x3ac);
+   }
+
+   return g_test_run();
+}
diff --git a/tests/qtest/meson.build b/tests/qtest/meson.build
index 83ad237..6fd6b0e 100644
--- a/tests/qtest/meson.build
+++ b/tests/qtest/meson.build
@@ -185,6 +185,7 @@ qtests_aarch64 = \
    'numa-test',
    'boot-serial-test',
    'xlnx-can-test',
+   'fuzz-xlnx-dp-test',
    'migration-test']
 
 qtests_s390x = \
-- 
2.7.4