ide: atapi: assert that the buffer pointer is in range

A case was reported where s->io_buffer_index can be out of range.
The report skimped on the details but it seems to be triggered
by s->lba == -1 on the READ/READ CD paths (e.g. by sending an
ATAPI command with LBA = 0xFFFFFFFF).  For now paper over it
with assertions.  The first one ensures that there is no overflow
when incrementing s->io_buffer_index, the second checks for the
buffer overrun.

Note that the buffer overrun is only a read, so I am not sure
if the assertion failure is actually less harmful than the overrun.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20201201120926.56559-1-pbonzini@redhat.com
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/ide/atapi.c

@@ -276,6 +276,8 @@ void ide_atapi_cmd_reply_end(IDEState *s)
         s->packet_transfer_size -= size;
         s->elementary_transfer_size -= size;
         s->io_buffer_index += size;
+        assert(size <= s->io_buffer_total_len);
+        assert(s->io_buffer_index <= s->io_buffer_total_len);
 
         /* Some adapters process PIO data right away.  In that case, we need
          * to avoid mutual recursion between ide_transfer_start