Re: [Qemu-devel] [PATCH v2] scsi: check page count while initialising descriptor rings

[Dmitry Fleytman]

> On 31 Aug 2016, at 09:49 AM, P J P <address@hidden> wrote:
> 
> From: Prasad J Pandit <address@hidden>
> 
> Vmware Paravirtual SCSI emulation uses command descriptors to
> process SCSI commands. These descriptors come with their ring
> buffers. A guest could set the page count for these rings to
> an arbitrary value, leading to infinite loop or OOB access.
> Add check to avoid it.
> 
> Reported-by: Tom Victor <address@hidden>
> Signed-off-by: Prasad J Pandit <address@hidden>
> ---
> hw/scsi/vmw_pvscsi.c | 17 +++++++++--------
> 1 file changed, 9 insertions(+), 8 deletions(-)
> 
> Update:
>    Moved Request and Confirm rings page count check to the parent
> function -> pvscsi_on_cmd_setup_rings().
> 
> diff --git a/hw/scsi/vmw_pvscsi.c b/hw/scsi/vmw_pvscsi.c
> index 5116f4a..79aa88c 100644
> --- a/hw/scsi/vmw_pvscsi.c
> +++ b/hw/scsi/vmw_pvscsi.c
> @@ -160,10 +160,6 @@ pvscsi_ring_init_data(PVSCSIRingInfo *m, 
> PVSCSICmdDescSetupRings *ri)
>     uint32_t req_ring_size, cmp_ring_size;
>     m->rs_pa = ri->ringsStatePPN << VMW_PAGE_SHIFT;
> 
> -    if ((ri->reqRingNumPages > PVSCSI_SETUP_RINGS_MAX_NUM_PAGES)
> -        || (ri->cmpRingNumPages > PVSCSI_SETUP_RINGS_MAX_NUM_PAGES)) {
> -        return -1;
> -    }

Hello Prasad,

Why did you decide to move this logic out of pvscsi_ring_init_data()?
Why not just amend existing “if" as you did in v1 of this patch?

~Dmitry

>     req_ring_size = ri->reqRingNumPages * PVSCSI_MAX_NUM_REQ_ENTRIES_PER_PAGE;
>     cmp_ring_size = ri->cmpRingNumPages * PVSCSI_MAX_NUM_CMP_ENTRIES_PER_PAGE;
>     txr_len_log2 = pvscsi_log2(req_ring_size - 1);
> @@ -746,7 +742,7 @@ pvscsi_dbg_dump_tx_rings_config(PVSCSICmdDescSetupRings 
> *rc)
> 
>     trace_pvscsi_tx_rings_num_pages("Confirm Ring", rc->cmpRingNumPages);
>     for (i = 0; i < rc->cmpRingNumPages; i++) {
> -        trace_pvscsi_tx_rings_ppn("Confirm Ring", rc->reqRingPPNs[i]);
> +        trace_pvscsi_tx_rings_ppn("Confirm Ring", rc->cmpRingPPNs[i]);
>     }
> }
> 
> @@ -777,12 +773,17 @@ pvscsi_on_cmd_setup_rings(PVSCSIState *s)
>     PVSCSICmdDescSetupRings *rc =
>         (PVSCSICmdDescSetupRings *) s->curr_cmd_data;
> 
> +    if (!rc->reqRingNumPages
> +        || rc->reqRingNumPages > PVSCSI_SETUP_RINGS_MAX_NUM_PAGES
> +        || !rc->cmpRingNumPages
> +        || rc->cmpRingNumPages > PVSCSI_SETUP_RINGS_MAX_NUM_PAGES) {
> +        return PVSCSI_COMMAND_PROCESSING_FAILED;
> +    }
> +
>     trace_pvscsi_on_cmd_arrived("PVSCSI_CMD_SETUP_RINGS");
> 
>     pvscsi_dbg_dump_tx_rings_config(rc);
> -    if (pvscsi_ring_init_data(&s->rings, rc) < 0) {
> -        return PVSCSI_COMMAND_PROCESSING_FAILED;
> -    }
> +    pvscsi_ring_init_data(&s->rings, rc);
> 
>     s->rings_info_valid = TRUE;
>     return PVSCSI_COMMAND_PROCESSING_SUCCEEDED;
> -- 
> 2.5.5
>