[v4,for-6.0,05/12] esp: introduce esp_fifo_pop_buf() and use it instead of fifo8_pop_buf()

The const pointer returned by fifo8_pop_buf() lies directly within the array used
to model the FIFO. Building with address sanitizers enabled shows that if the
caller expects a minimum number of bytes present then if the FIFO is nearly full,
the caller may unexpectedly access past the end of the array.

Introduce esp_fifo_pop_buf() which takes a destination buffer and performs a
memcpy() in it to guarantee that the caller cannot overwrite the FIFO array and
update all callers to use it. Similarly add underflow protection similar to
esp_fifo_push() and esp_fifo_pop() so that instead of triggering an assert()
the operation becomes a no-op.

Buglink: https://bugs.launchpad.net/qemu/+bug/1909247
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Tested-by: Alexander Bulekov <alxndr@bu.edu>
---
 hw/scsi/esp.c | 40 ++++++++++++++++++++++++++++------------
 1 file changed, 28 insertions(+), 12 deletions(-)

On 4/7/21 9:57 PM, Mark Cave-Ayland wrote:
> The const pointer returned by fifo8_pop_buf() lies directly within the array used
> to model the FIFO. Building with address sanitizers enabled shows that if the
> caller expects a minimum number of bytes present then if the FIFO is nearly full,
> the caller may unexpectedly access past the end of the array.
> 
> Introduce esp_fifo_pop_buf() which takes a destination buffer and performs a
> memcpy() in it to guarantee that the caller cannot overwrite the FIFO array and
> update all callers to use it. Similarly add underflow protection similar to
> esp_fifo_push() and esp_fifo_pop() so that instead of triggering an assert()
> the operation becomes a no-op.
> 
> Buglink: https://bugs.launchpad.net/qemu/+bug/1909247
> Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
> Tested-by: Alexander Bulekov <alxndr@bu.edu>
> ---
>  hw/scsi/esp.c | 40 ++++++++++++++++++++++++++++------------
>  1 file changed, 28 insertions(+), 12 deletions(-)

Way cleaner/safer.

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

On Wed, 7 Apr 2021 at 21:02, Mark Cave-Ayland
<mark.cave-ayland@ilande.co.uk> wrote:
>
> The const pointer returned by fifo8_pop_buf() lies directly within the array used
> to model the FIFO. Building with address sanitizers enabled shows that if the
> caller expects a minimum number of bytes present then if the FIFO is nearly full,
> the caller may unexpectedly access past the end of the array.
>
> Introduce esp_fifo_pop_buf() which takes a destination buffer and performs a
> memcpy() in it to guarantee that the caller cannot overwrite the FIFO array and
> update all callers to use it. Similarly add underflow protection similar to
> esp_fifo_push() and esp_fifo_pop() so that instead of triggering an assert()
> the operation becomes a no-op.
>
> Buglink: https://bugs.launchpad.net/qemu/+bug/1909247
> Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
> Tested-by: Alexander Bulekov <alxndr@bu.edu>
> ---
>  hw/scsi/esp.c | 40 ++++++++++++++++++++++++++++------------
>  1 file changed, 28 insertions(+), 12 deletions(-)
>
> diff --git a/hw/scsi/esp.c b/hw/scsi/esp.c
> index ff8fa73de9..1aa2caf57d 100644
> --- a/hw/scsi/esp.c
> +++ b/hw/scsi/esp.c
> @@ -117,6 +117,23 @@ static uint8_t esp_fifo_pop(Fifo8 *fifo)
>      return fifo8_pop(fifo);
>  }
>
> +static uint32_t esp_fifo_pop_buf(Fifo8 *fifo, uint8_t *dest, int maxlen)
> +{
> +    const uint8_t *buf;
> +    uint32_t n;
> +
> +    if (maxlen == 0) {
> +        return 0;
> +    }
> +
> +    buf = fifo8_pop_buf(fifo, maxlen, &n);
> +    if (dest) {
> +        memcpy(dest, buf, n);
> +    }
> +
> +    return n;
> +}
> +
>  static uint32_t esp_get_tc(ESPState *s)
>  {
>      uint32_t dmalen;
> @@ -241,11 +258,11 @@ static uint32_t get_cmd(ESPState *s, uint32_t maxlen)
>          if (dmalen == 0) {
>              return 0;
>          }
> -        memcpy(buf, fifo8_pop_buf(&s->fifo, dmalen, &n), dmalen);
> -        if (dmalen >= 3) {
> +        n = esp_fifo_pop_buf(&s->fifo, buf, dmalen);
> +        if (n >= 3) {
>              buf[0] = buf[2] >> 5;
>          }
> -        fifo8_push_all(&s->cmdfifo, buf, dmalen);
> +        fifo8_push_all(&s->cmdfifo, buf, n);
>      }
>      trace_esp_get_cmd(dmalen, target);

This bit could be tidied up further -- it currently sets
   dmalen = MIN(fifo8_num_used(&s->fifo), maxlen);
in order not to pull more bytes out of the FIFO than it has in it;
but now we are making that check in esp_fifo_pop_buf() I think you
could just do
    dmalen = esp_fifo_pop_buf(&s->fifo, buf, maxlen);
and drop the
        dmalen = MIN(fifo8_num_used(&s->fifo), maxlen);
        if (dmalen == 0) {
            return 0;
        }
part and the use of 'n' entirely.

But this code isn't wrong, so we can do that later to avoid having
to respin here.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

thanks
-- PMM