hw/display/xlnx_dp: fix an out-of-bounds read in xlnx_dp_read

hw/display/xlnx_dp: fix an out-of-bounds read in xlnx_dp_read

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>
Acked-by: Thomas Huth <thuth@redhat.com>
Acked-by: Alexander Bulekov <alxndr@bu.edu>
Message-Id: <1628059910-12060-1-git-send-email-cyruscyliu@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>