Re: [PATCH v2 1/1] xen-netfront: do not use ~0U as error return value for xennet_fill_frags()

[David Miller <davem@davemloft.net>]

From: Dongli Zhang <dongli.zhang@oracle.com>
Date: Tue,  1 Oct 2019 21:56:41 +0800

> xennet_fill_frags() uses ~0U as return value when the sk_buff is not able
> to cache extra fragments. This is incorrect because the return type of
> xennet_fill_frags() is RING_IDX and 0xffffffff is an expected value for
> ring buffer index.
> 
> In the situation when the rsp_cons is approaching 0xffffffff, the return
> value of xennet_fill_frags() may become 0xffffffff which xennet_poll() (the
> caller) would regard as error. As a result, queue->rx.rsp_cons is set
> incorrectly because it is updated only when there is error. If there is no
> error, xennet_poll() would be responsible to update queue->rx.rsp_cons.
> Finally, queue->rx.rsp_cons would point to the rx ring buffer entries whose
> queue->rx_skbs[i] and queue->grant_rx_ref[i] are already cleared to NULL.
> This leads to NULL pointer access in the next iteration to process rx ring
> buffer entries.
> 
> The symptom is similar to the one fixed in
> commit 00b368502d18 ("xen-netfront: do not assume sk_buff_head list is
> empty in error handling").
> 
> This patch changes the return type of xennet_fill_frags() to indicate
> whether it is successful or failed. The queue->rx.rsp_cons will be
> always updated inside this function.
> 
> Fixes: ad4f15dc2c70 ("xen/netfront: don't bug in case of too many frags")
> Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>

Applied and queued up for -stable.