Re: [PATCH] ibmvnic: fix OOB array access in ibmvnic_xmit on queue count reduction

[Tyllis Xu]

I'll try out the suggested changes and use more
of the existing handling to create a new patch.
I'll also remove the unlikely(). Thank you for
your feedback!


On Mon, Mar 23, 2026 at 9:45 AM Simon Horman <horms@xxxxxxxxxx> wrote:
>
> On Fri, Mar 20, 2026 at 10:54:39PM -0500, Tyllis Xu wrote:
> > When the number of TX queues is reduced (e.g., via ethtool -L), the
> > Qdisc layer retains previously enqueued skbs with queue mappings from
> > before the reduction. After the reset completes and tx_queues_active is
> > set to true, netif_tx_start_all_queues() drains these stale skbs through
> > ibmvnic_xmit(). The queue index from skb_get_queue_mapping() may exceed
> > the newly allocated array bounds, causing out-of-bounds reads on
> > tx_scrq[] and tx_pool[]/tso_pool[], and out-of-bounds writes on
> > tx_stats_buffers[] in the function's exit path.
> >
> > The existing tx_queues_active guard does not help here: it is set to
> > true by __ibmvnic_open() before netif_tx_start_all_queues() restarts
> > queue draining, so stale skbs pass the check with an invalid queue index.
> >
> > Add a bounds check against num_active_tx_scrqs immediately after the
> > tx_queues_active guard. Use a dedicated out_unlock label to skip the
> > per-queue stats updates (which also index tx_stats_buffers[queue_num])
> > when the queue index is invalid.
> >
> > Fixes: 4219196d1f66 ("ibmvnic: fix race between xmit and reset")
> > Reported-by: Yuhao Jiang <danisjiang@xxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Tyllis Xu <LivelyCarpet87@xxxxxxxxx>
> > ---
> >  drivers/net/ethernet/ibm/ibmvnic.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/drivers/net/ethernet/ibm/ibmvnic.c b/drivers/net/ethernet/ibm/ibmvnic.c
> > index 5a510eed335e..c939391474cb 100644
> > --- a/drivers/net/ethernet/ibm/ibmvnic.c
> > +++ b/drivers/net/ethernet/ibm/ibmvnic.c
> > @@ -2453,6 +2453,11 @@ static netdev_tx_t ibmvnic_xmit(struct sk_buff *skb, struct net_device *netdev)
> >               goto out;
> >       }
> >
> > +     if (unlikely(queue_num >= adapter->num_active_tx_scrqs)) {
> > +             dev_kfree_skb_any(skb);
> > +             goto out_unlock;
> > +     }
> > +
>
> This doesn't seem quite right. Shouldn't it be as per other
> blocks in this function that drop packets. In which case
> it could re-use the existing handling in the conditional immediately above
> this hunk.
>
> Also, I don't think unlikely() seems in keeping with the existing
> implementation of this function.
>
> I'm suggesting something like (completely untested):
>
> diff --git a/drivers/net/ethernet/ibm/ibmvnic.c b/drivers/net/ethernet/ibm/ibmvnic.c
> index 5a510eed335e..67e1e62631e3 100644
> --- a/drivers/net/ethernet/ibm/ibmvnic.c
> +++ b/drivers/net/ethernet/ibm/ibmvnic.c
> @@ -2457,7 +2457,8 @@ static netdev_tx_t ibmvnic_xmit(struct sk_buff *skb, struct net_device *netdev)
>         txq = netdev_get_tx_queue(netdev, queue_num);
>         ind_bufp = &tx_scrq->ind_buf;
>
> -       if (ibmvnic_xmit_workarounds(skb, netdev)) {
> +       if (ibmvnic_xmit_workarounds(skb, netdev) ||
> +           queue_num >= adapter->num_active_tx_scrqs) {
>                 tx_dropped++;
>                 tx_send_failed++;
>                 ret = NETDEV_TX_OK;
>
> Where the next line is:
>
>                 goto out;
>
> ...
>
> > @@ -2672,6 +2677,9 @@ static netdev_tx_t ibmvnic_xmit(struct sk_buff *skb, struct net_device *netdev)
> >       adapter->tx_stats_buffers[queue_num].bytes += tx_bytes;
> >       adapter->tx_stats_buffers[queue_num].dropped_packets += tx_dropped;
> >
> > +     return ret;
> > +out_unlock:
> > +     rcu_read_unlock();
> >       return ret;
> >  }
>
> My previous comment not, withstanding:
>
> The RCU read side critical section is already enormous.
> So perhaps making it slightly better doesn't make a difference.
>
> If so, can we go for this slightly flow here (completely untested).
>
> diff --git a/drivers/net/ethernet/ibm/ibmvnic.c b/drivers/net/ethernet/ibm/ibmvnic.c
> index 5a510eed335e..1e1cd8c11cf9 100644
> --- a/drivers/net/ethernet/ibm/ibmvnic.c
> +++ b/drivers/net/ethernet/ibm/ibmvnic.c
> @@ -2664,14 +2664,14 @@ static netdev_tx_t ibmvnic_xmit(struct sk_buff *skb, struct net_device *netdev)
>                 netif_carrier_off(netdev);
>         }
>  out:
> -       rcu_read_unlock();
>         adapter->tx_send_failed += tx_send_failed;
>         adapter->tx_map_failed += tx_map_failed;
>         adapter->tx_stats_buffers[queue_num].batched_packets += tx_bpackets;
>         adapter->tx_stats_buffers[queue_num].direct_packets += tx_dpackets;
>         adapter->tx_stats_buffers[queue_num].bytes += tx_bytes;
>         adapter->tx_stats_buffers[queue_num].dropped_packets += tx_dropped;
> -
> +out_unlock:
> +       rcu_read_unlock();
>         return ret;
>  }
>
>
> --
> pw-bot: changes-requested