scsi: Reject commands if the CDB length exceeds buf_len

scsi: Reject commands if the CDB length exceeds buf_len

In scsi_req_parse_cdb(), if the CDB length implied by the command type
exceeds the initialized portion of the command buffer, reject the request.

Rejected requests are recorded by the `scsi_req_parse_bad` trace event.

On example of a bug detected by this check is SunOS's use of interleaved
DMA and non-DMA commands. This guest behavior currently causes QEMU to
parse uninitialized memory as a SCSI command, with unpredictable
outcomes.

With the new check in place:

  * QEMU consistently creates a trace event and rejects the request.

  * SunOS retries the request(s) and is able to successfully boot from
    disk.

Signed-off-by: John Millikin <john@john-millikin.com>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1127
Message-Id: <20220817053458.698416-2-john@john-millikin.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>