esp: restrict non-DMA transfer length to that of available data

esp: restrict non-DMA transfer length to that of available data

In the case where a SCSI layer transfer is incorrectly terminated, it is
possible for a TI command to cause a SCSI buffer overflow due to the
expected transfer data length being less than the available data in the
FIFO. When this occurs the unsigned async_len variable underflows and
becomes a large offset which writes past the end of the allocated SCSI
buffer.

Restrict the non-DMA transfer length to be the smallest of the expected
transfer length and the available FIFO data to ensure that it is no longer
possible for the SCSI buffer overflow to occur.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1810
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20230913204410.65650-3-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>