linux-kernel - Re: [PATCH] net: usbnet: Fix potential NULL pointer dereference

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Message-ID: <0ed0f964-f8c8-4776-a2ae-ca25071bd0cd@suse.com>
Date:   Mon, 6 Nov 2023 13:53:12 +0100
From:   Oliver Neukum <oneukum@...e.com>
To:     Bjørn Mork <bjorn@...k.no>,
        Oliver Neukum <oneukum@...e.com>
Cc:     Ren Mingshuai <renmingshuai@...wei.com>, kuba@...nel.org,
        caowangbao@...wei.com, davem@...emloft.net, khlebnikov@...nvz.org,
        liaichun@...wei.com, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, yanan@...wei.com
Subject: Re: [PATCH] net: usbnet: Fix potential NULL pointer dereference

On 06.11.23 11:55, Bjørn Mork wrote:

> I believe that code is based on the (safe?) assumption that the struct
> usbnet driver_info->tx_fixup points to cdc_ncm_tx_fixup().  And

That seems to be a correct assumption, but one that is far from obvious.
Could you add a big, fat comment?

> cdc_ncm_tx_fixup does lots of weird stuff, including special handling of
> NULL skb. It might return a valid skb for further processing by
> usbnet_start_xmit().  If it doesn't, then we jump straight to
> "not_drop", like we do when cdc_ncm_tx_fixup decides to eat the passed
> skb.
> 
> But "funky" is i precise description of all this...  If someone feels
> like it, then all that open coded skb queing inside cdc_ncm should be
> completely rewritten.

I understand what you mean, but I need a generic answer. Can you call
ndo_start_xmit() with skb == NULL?

	Regards
		Oliver