Re: [PATCH 1/2] esp.c: fix esp_cdb_ready() FIFO wraparound limit calculation

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

On 25/9/25 14:28, Mark Cave-Ayland wrote:
> The original calculation in commit 3cc70889a3 ("esp.c: prevent cmdfifo overflow
> in esp_cdb_ready()") subtracted cmdfifo_cdb_offset from fifo8_num_used() to

"substracted"

> calculate the outstanding cmdfifo length, but this is incorrect because
> fifo8_num_used() can also include wraparound data.
> 
> Instead calculate the maximum offset used by scsi_cdb_length() which is just
> the first byte after cmdfifo_cdb_offset, and then peek the entire content
> of the cmdfifo. The fifo8_peek_bufptr() result will then return the maximum
> length of remaining data up to the end of the internal cmdfifo array, which
> can then be used for the overflow check.
> 
> Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
> Fixes: 3cc70889a3 ("esp.c: prevent cmdfifo overflow in esp_cdb_ready()")

Buglink: https://issues.oss-fuzz.com/issues/439878564

> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3082
> ---
>   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 1d264c40e5..2809fcdee0 100644
> --- a/hw/scsi/esp.c
> +++ b/hw/scsi/esp.c
> @@ -447,7 +447,9 @@ static void write_response(ESPState *s)
>   
>   static bool esp_cdb_ready(ESPState *s)
>   {
> -    int len = fifo8_num_used(&s->cmdfifo) - s->cmdfifo_cdb_offset;
> +    /* scsi_cdb_length() only reads the first byte */
> +    int limit = s->cmdfifo_cdb_offset + 1;
> +    int len = fifo8_num_used(&s->cmdfifo);
>       const uint8_t *pbuf;
>       uint32_t n;
>       int cdblen;
> @@ -457,7 +459,7 @@ static bool esp_cdb_ready(ESPState *s)
>       }
>   
>       pbuf = fifo8_peek_bufptr(&s->cmdfifo, len, &n);
> -    if (n < len) {
> +    if (n < limit) {
>           /*
>            * In normal use the cmdfifo should never wrap, but include this check
>            * to prevent a malicious guest from reading past the end of the
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>