Re: [PATCH v3 12/17] esp.c: prevent cmdfifo overflow in esp_cdb_ready()

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 24/3/24 20:17, Mark Cave-Ayland wrote:
> During normal use the cmdfifo will never wrap internally and cmdfifo_cdb_offset
> will always indicate the start of the SCSI CDB. However it is possible that a
> malicious guest could issue an invalid ESP command sequence such that cmdfifo
> wraps internally and cmdfifo_cdb_offset could point beyond the end of the FIFO
> data buffer.
> 
> Add an extra check to fifo8_peek_buf() to ensure that if the cmdfifo has wrapped
> internally then esp_cdb_ready() will exit rather than allow scsi_cdb_length() to
> access data outside the cmdfifo data buffer.
> 
> Reported-by: Chuhong Yuan <hslester96@gmail.com>
> Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
> ---
>   hw/scsi/esp.c | 12 +++++++++++-
>   1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/scsi/esp.c b/hw/scsi/esp.c
> index f47abc36d6..d8db33b921 100644
> --- a/hw/scsi/esp.c
> +++ b/hw/scsi/esp.c
> @@ -429,13 +429,23 @@ static bool esp_cdb_ready(ESPState *s)
>   {
>       int len = fifo8_num_used(&s->cmdfifo) - s->cmdfifo_cdb_offset;
>       const uint8_t *pbuf;
> +    uint32_t n;
>       int cdblen;
>   
>       if (len <= 0) {
>           return false;
>       }
>   
> -    pbuf = fifo8_peek_buf(&s->cmdfifo, len, NULL);
> +    pbuf = fifo8_peek_buf(&s->cmdfifo, len, &n);
> +    if (n < len) {
> +        /*
> +         * In normal use the cmdfifo should never wrap, but include this check
> +         * to prevent a malicious guest from reading past the end of the
> +         * cmdfifo data buffer below
> +         */

Can we qemu_log_mask(LOG_GUEST_ERROR) something here?

> +        return false;
> +    }
> +
>       cdblen = scsi_cdb_length((uint8_t *)&pbuf[s->cmdfifo_cdb_offset]);
>   
>       return cdblen < 0 ? false : (len >= cdblen);