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
---
 hw/scsi/esp.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/hw/scsi/esp.c b/hw/scsi/esp.c
index 4218a6a960..9b11d8c573 100644
--- a/hw/scsi/esp.c
+++ b/hw/scsi/esp.c
@@ -759,7 +759,8 @@ static void esp_do_nodma(ESPState *s)
     }
 
     if (to_device) {
-        len = MIN(fifo8_num_used(&s->fifo), ESP_FIFO_SZ);
+        len = MIN(s->async_len, ESP_FIFO_SZ);
+        len = MIN(len, fifo8_num_used(&s->fifo));
         esp_fifo_pop_buf(&s->fifo, s->async_buf, len);
         s->async_buf += len;
         s->async_len -= len;
-- 
2.39.2

On 13/09/2023 22.44, Mark Cave-Ayland wrote:
> 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
> ---
>   hw/scsi/esp.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/scsi/esp.c b/hw/scsi/esp.c
> index 4218a6a960..9b11d8c573 100644
> --- a/hw/scsi/esp.c
> +++ b/hw/scsi/esp.c
> @@ -759,7 +759,8 @@ static void esp_do_nodma(ESPState *s)
>       }
>   
>       if (to_device) {
> -        len = MIN(fifo8_num_used(&s->fifo), ESP_FIFO_SZ);
> +        len = MIN(s->async_len, ESP_FIFO_SZ);
> +        len = MIN(len, fifo8_num_used(&s->fifo));
>           esp_fifo_pop_buf(&s->fifo, s->async_buf, len);
>           s->async_buf += len;
>           s->async_len -= len;

Thanks for taking care of this!

Reviewed-by: Thomas Huth <thuth@redhat.com>