Re: [PATCH v4 9/9] scsi/scsi_bus: fix races in REPORT LUNS

[Maxim Levitsky <mlevitsk@redhat.com>]

On Mon, 2020-08-31 at 18:01 +0300, Maxim Levitsky wrote:
> Currently scsi_target_emulate_report_luns iterates
> over child devices list twice, and there is guarantee, that
> it will not be changed meanwhile.
> 
> This reason for two loops is that it needs to know how much memory
> to allocate.
> 
> Avoid this by iterating once, and allocating the memory for the output
> dynamically with reserving enought memory so that in practice it won't
> be reallocated often.
Just too many spelling/grammar mistakes in the commit message. Sorry about that.

It should be something like that:

Currently scsi_target_emulate_report_luns iterates over the child device list
twice, and there is no guarantee that this list is the same in both iterations.

The reason for iterating twise is that the first iteration calculates 
how much memory to allocate.

However if we use a dynamic array we can avoid iterating twice, and therefore
we avoid this race.

Best regards,
	Maxim Levitsky


> 
> Bugzilla for reference: https://bugzilla.redhat.com/show_bug.cgi?id=1866707
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
>  hw/scsi/scsi-bus.c | 62 ++++++++++++++++++++++------------------------
>  1 file changed, 29 insertions(+), 33 deletions(-)
> 
> diff --git a/hw/scsi/scsi-bus.c b/hw/scsi/scsi-bus.c
> index feab20b76d..150dee2e6a 100644
> --- a/hw/scsi/scsi-bus.c
> +++ b/hw/scsi/scsi-bus.c
> @@ -438,19 +438,25 @@ struct SCSITargetReq {
>  static void store_lun(uint8_t *outbuf, int lun)
>  {
>      if (lun < 256) {
> +        /* Simple logical unit addressing method*/
> +        outbuf[0] = 0;
>          outbuf[1] = lun;
> -        return;
> +    } else {
> +        /* Flat space addressing method */
> +        outbuf[0] = 0x40 | (lun >> 8);
> +        outbuf[1] = (lun & 255);
>      }
> -    outbuf[1] = (lun & 255);
> -    outbuf[0] = (lun >> 8) | 0x40;
>  }
>  
>  static bool scsi_target_emulate_report_luns(SCSITargetReq *r)
>  {
>      BusChild *kid;
> -    int i, len, n;
>      int channel, id;
> -    bool found_lun0;
> +    uint8_t tmp[8] = {0};
> +    int len = 0;
> +
> +    /* reserve space for 63 LUNs*/
> +    GByteArray *buf = g_byte_array_sized_new(512);
>  
>      if (r->req.cmd.xfer < 16) {
>          return false;
> @@ -460,46 +466,36 @@ static bool scsi_target_emulate_report_luns(SCSITargetReq *r)
>      }
>      channel = r->req.dev->channel;
>      id = r->req.dev->id;
> -    found_lun0 = false;
> -    n = 0;
>  
> -    rcu_read_lock();
>  
> -    QTAILQ_FOREACH_RCU(kid, &r->req.bus->qbus.children, sibling) {
> -        DeviceState *qdev = kid->child;
> -        SCSIDevice *dev = SCSI_DEVICE(qdev);
> +    /* add size (will be updated later to correct value */
> +    g_byte_array_append(buf, tmp, 8);
> +    len += 8;
>  
> -        if (dev->channel == channel && dev->id == id) {
> -            if (dev->lun == 0) {
> -                found_lun0 = true;
> -            }
> -            n += 8;
> -        }
> -    }
> -    if (!found_lun0) {
> -        n += 8;
> -    }
> -
> -    scsi_target_alloc_buf(&r->req, n + 8);
> +    /* add LUN0 */
> +    g_byte_array_append(buf, tmp, 8);
> +    len += 8;
>  
> -    len = MIN(n + 8, r->req.cmd.xfer & ~7);
> -    memset(r->buf, 0, len);
> -    stl_be_p(&r->buf[0], n);
> -    i = found_lun0 ? 8 : 16;
> +    rcu_read_lock();
>      QTAILQ_FOREACH_RCU(kid, &r->req.bus->qbus.children, sibling) {
>          DeviceState *qdev = kid->child;
>          SCSIDevice *dev = SCSI_DEVICE(qdev);
>  
> -        if (dev->channel == channel && dev->id == id) {
> -            store_lun(&r->buf[i], dev->lun);
> -            i += 8;
> +        if (dev->channel == channel && dev->id == id && dev->lun != 0) {
> +            store_lun(tmp, dev->lun);
> +            g_byte_array_append(buf, tmp, 8);
> +            len += 8;
>          }
>      }
> -
>      rcu_read_unlock();
>  
> -    assert(i == n + 8);
> -    r->len = len;
> +    r->buf_len = len;
> +    r->buf = g_byte_array_free(buf, FALSE);
> +    r->len = MIN(len, r->req.cmd.xfer & ~7);
> +
> +    /* store the LUN list length */
> +    stl_be_p(&r->buf[0], len - 8);
> +
>      return true;
>  }
>