netdev - Re: [PATCH] usbnet: dereference after null check in usbnet_start_xmit() and __usbnet_read

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Date:	Thu, 20 Aug 2015 09:29:05 +0200
From:	Bjørn Mork <bjorn@...k.no>
To:	Vivek Kumar Bhagat <vivek.bhagat@...sung.com>
Cc:	"netdev\@vger.kernel.org" <netdev@...r.kernel.org>,
	Nitin Jhanwar <nitin.j@...sung.com>,
	HEMANSHU SRIVASTAVA <hemanshu.s@...sung.com>
Subject: Re: [PATCH] usbnet: dereference after null check in usbnet_start_xmit() and __usbnet_read_cmd()

Vivek Kumar Bhagat <vivek.bhagat@...sung.com> writes:

> Dear Bjorn,
>
>>>This is wrong.  There are usbnet minidrivers depending on info->tx_fixup
>>> being called with a NULL skb.
> Also, if dev_hard_start_xmit() ensures that skb can not be NULL in usbnet_start_xmit()
> then we should remove below check.
>     if (skb)      <--- This check is confusing which says skb can be NULL.
>                 skb_tx_timestamp(skb); 

No, that test is there because of the ugly hack in cdc_ncm.  It doesn't
go through dev_hard_start_xmit(), but calls usbnet_start_xmit() directly
with a NULL skb as a signal to itself.  Yes, I told you it was ugly ;)

I do agree that it would be nice to make this go away.  But until that
happens usbnet_start_xmit() has to deal with NULL skbs, forwarding them
to the tx_fixup hook.

Bjørn
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