Re: [Qemu-devel] [PATCH 3/3] scsi: megasas: check 'read_queue_head' index value

[Alexander Graf <agraf@suse.de>]

On 05/25/2016 12:31 PM, P J P wrote:
> From: Prasad J Pandit <pjp@fedoraproject.org>
>
> While doing MegaRAID SAS controller command frame lookup, routine
> 'megasas_lookup_frame' uses 'read_queue_head' value as an index
> into 'frames[MEGASAS_MAX_FRAMES=2048]' array. Limit its value
> within array bounds to avoid any OOB access.
>
> Reported-by: Li Qiang <liqiang6-s@360.cn>
> Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
> ---
>   hw/scsi/megasas.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/hw/scsi/megasas.c b/hw/scsi/megasas.c
> index 7c08932..76d57f9 100644
> --- a/hw/scsi/megasas.c
> +++ b/hw/scsi/megasas.c
> @@ -649,8 +649,10 @@ static int megasas_init_firmware(MegasasState *s, MegasasCmd *cmd)
>       pa_lo = le32_to_cpu(initq->pi_addr_lo);
>       pa_hi = le32_to_cpu(initq->pi_addr_hi);
>       s->producer_pa = ((uint64_t) pa_hi << 32) | pa_lo;
> -    s->reply_queue_head = ldl_le_pci_dma(pcid, s->producer_pa);
> -    s->reply_queue_tail = ldl_le_pci_dma(pcid, s->consumer_pa);
> +    s->reply_queue_head = \
> +        ldl_le_pci_dma(pcid, s->producer_pa) % MEGASAS_MAX_FRAMES;
> +    s->reply_queue_tail = \
> +        ldl_le_pci_dma(pcid, s->consumer_pa) % MEGASAS_MAX_FRAMES;

You're using up 2 lines for each of those assignments now anyway, so why 
not just split it into

   s->reply_queue_head = ldl_le_pci_dma(pcid, s->producer_pa);
   s->reply_queue_head %= MEGASAS_MAX_FRAMES;

and that way avoid a real multi-line statement (which is hard to read).

Whether the bitmask really is what hardware would do, I don't know. I'll 
let Hannes comment there. Maybe the hardware simply does

   s->reply_queue_head = MIN(ldl_le_pci_dma(pcid, s->producer_pa), 
MEGASAS_MAX_FRAMES);

But I agree that the mask is more likely.


Alex