Re: [PATCH v3 07/11] esp: don't underflow cmdfifo in do_cmd()

[Philippe Mathieu-Daudé]

On 4/1/21 9:49 AM, Mark Cave-Ayland wrote:
> If the guest tries to execute a CDB when cmdfifo is not empty before the start
> of the message out phase then clearing the message out phase data will cause
> cmdfifo to underflow due to cmdfifo_cdb_offset being larger than the amount of
> data within.
> 
> Since this can only occur by issuing deliberately incorrect instruction
> sequences, ensure that the maximum length of esp_fifo_pop_buf() is limited to
> the size of the data within cmdfifo.
> 
> Buglink: https://bugs.launchpad.net/qemu/+bug/1909247
> Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
> ---
>  hw/scsi/esp.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/scsi/esp.c b/hw/scsi/esp.c
> index 4decbbfc29..7f49522e1d 100644
> --- a/hw/scsi/esp.c
> +++ b/hw/scsi/esp.c
> @@ -319,13 +319,15 @@ static void do_busid_cmd(ESPState *s, uint8_t busid)
>  
>  static void do_cmd(ESPState *s)
>  {
> -    uint8_t busid = fifo8_pop(&s->cmdfifo);
> +    uint8_t busid = esp_fifo_pop(&s->cmdfifo);
> +    int len;
>  
>      s->cmdfifo_cdb_offset--;
>  
>      /* Ignore extended messages for now */
>      if (s->cmdfifo_cdb_offset) {
> -        esp_fifo_pop_buf(&s->cmdfifo, NULL, s->cmdfifo_cdb_offset);
> +        len = MIN(s->cmdfifo_cdb_offset, fifo8_num_used(&s->cmdfifo));

Do we want to log(GUEST_ERRORS) this?

> +        esp_fifo_pop_buf(&s->cmdfifo, NULL, len);
>          s->cmdfifo_cdb_offset = 0;
>      }
>  
> 

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>