Re: [Qemu-devel] [PATCH] scsi: esp: check TI buffer index before read/write

[Peter Maydell]

On 30 May 2016 at 19:58, P J P <address@hidden> wrote:
> From: Prasad J Pandit <address@hidden>
>
> The 53C9X Fast SCSI Controller(FSC) comes with internal 16-byte
> FIFO buffers. One is used to handle commands and other is for
> information transfer. While reading/writing to these buffers
> an index into 's->ti_buf[TI_BUFSZ=16]' could exceed its size,
> as a check was missing to validate it. Add check to avoid OOB
> r/w access.
>
> Reported-by: Huawei PSIRT <address@hidden>
> Signed-off-by: Prasad J Pandit <address@hidden>
> ---
>  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 591c817..80e4287 100644
> --- a/hw/scsi/esp.c
> +++ b/hw/scsi/esp.c
> @@ -407,8 +407,10 @@ uint64_t esp_reg_read(ESPState *s, uint32_t saddr)
>                  qemu_log_mask(LOG_UNIMP,
>                                "esp: PIO data read not implemented\n");
>                  s->rregs[ESP_FIFO] = 0;
> -            } else {
> +            } else if (s->ti_rptr < TI_BUFSZ) {
>                  s->rregs[ESP_FIFO] = s->ti_buf[s->ti_rptr++];
> +            } else {
> +                trace_esp_error_fifo_overrun();

Isn't this an underrun, not an overrun?

In any case, something weird seems to be going on here:
it looks like the amount of data in the fifo should be
constrained by ti_size (which we're already checking), so
when can we get into a situation where ti_rptr can
get beyond the buffer size? It seems to me that we should
fix whatever that underlying bug is, and then have an
assert() on ti_rptr here.

>              }
>              esp_raise_irq(s);
>          }
> @@ -456,7 +458,7 @@ void esp_reg_write(ESPState *s, uint32_t saddr, uint64_t 
> val)
>              } else {
>                  trace_esp_error_fifo_overrun();
>              }
> -        } else if (s->ti_size == TI_BUFSZ - 1) {
> +        } else if (s->ti_wptr == TI_BUFSZ - 1) {
>              trace_esp_error_fifo_overrun();
>          } else {
>              s->ti_size++;

Similarly, this looks odd -- the ti_size check should be
sufficient if the rest of the code is correctly managing
the ti_size/ti_wptr/ti_rptr values.

It would probably also be possible to rewrite this
FIFO handling into a less error prone style that
isn't reliant on keeping three different indexes
in sync to avoid buffer overruns.

> --
> 2.5.5

thanks
-- PMM