Re: [RFC PATCH] usb: gadget: ncm: Fix handling of zero block length packets

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Feb 26, 2024 at 04:58:16PM +0530, Krishna Kurapati wrote:
> While connecting to a Linux host with CDC_NCM_NTB_DEF_SIZE_TX
> set to 65536, it has been observed that we receive short packets,
> which come at interval of 5-10 seconds sometimes and have block
> length zero but still contain 1-2 valid datagrams present.
> 
> According to the NCM spec:
> 
> "If wBlockLength = 0x0000, the block is terminated by a
> short packet. In this case, the USB transfer must still
> be shorter than dwNtbInMaxSize or dwNtbOutMaxSize. If
> exactly dwNtbInMaxSize or dwNtbOutMaxSize bytes are sent,
> and the size is a multiple of wMaxPacketSize for the
> given pipe, then no ZLP shall be sent.
> 
> wBlockLength= 0x0000 must be used with extreme care, because
> of the possibility that the host and device may get out of
> sync, and because of test issues.
> 
> wBlockLength = 0x0000 allows the sender to reduce latency by
> starting to send a very large NTB, and then shortening it when
> the sender discovers that there’s not sufficient data to justify
> sending a large NTB"
> 
> However, there is a potential issue with the current implementation,
> as it checks for the occurrence of multiple NTBs in a single
> giveback by verifying if the leftover bytes to be processed is zero
> or not. If the block length reads zero, we would process the same
> NTB infintely because the leftover bytes is never zero and it leads
> to a crash. Fix this by bailing out if block length reads zero.
> 
> Fixes: 427694cfaafa ("usb: gadget: ncm: Handle decoding of multiple NTB's in unwrap call")
> Signed-off-by: Krishna Kurapati <quic_kriskura@quicinc.com>
> ---
> 
> PS: Although this issue was seen after CDC_NCM_NTB_DEF_SIZE_TX
> was modified to 64K on host side, I still believe this
> can come up at any time as per the spec. Also I assumed
> that the giveback where block length is zero, has only
> one NTB and not multiple ones.

Hi,

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