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

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Date:   Mon, 06 Nov 2023 11:55:49 +0100
From:   Bjørn Mork <bjorn@...k.no>
To:     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

Oliver Neukum <oneukum@...e.com> writes:

> yes it looks like NCM does funky things, but what does that mean?
>
> ndp_to_end_store()
>
>         /* flush pending data before changing flag */
>         netif_tx_lock_bh(dev->net);
>         usbnet_start_xmit(NULL, dev->net);
>         spin_lock_bh(&ctx->mtx);
>         if (enable)
>
> expects some odd semantics from it. The proposed patch simply
> increases the drop counter, which is by itself questionable, as
> we drop nothing.
>
> But it definitely does no IO, so we flush nothing.
> That is, we clearly have bug(s) but the patch only papers over
> them.
> And frankly, the basic question needs to be answered:
> Are you allowed to call ndo_start_xmit() with a NULL skb?
>
> My understanding until now was that you must not.

Yuck.  I see that I'm to blame for that code, so I've tried to figure
out what the idea behind it could possibly have been.

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
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.

Bjørn