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Date:   Tue, 05 Jan 2021 16:52:47 -0800 (PST)
From:   David Miller <>
Subject: Re: [PATCH net,stable v3] net: cdc_ncm: correct overhead in

From: Jouni Seppänen <>
Date: Tue,  5 Jan 2021 06:52:49 +0200

> From: Jouni K. Seppänen <>
> Aligning to tx_ndp_modulus is not sufficient because the next align
> call can be cdc_ncm_align_tail, which can add up to ctx->tx_modulus +
> ctx->tx_remainder - 1 bytes. This used to lead to occasional crashes
> on a Huawei 909s-120 LTE module as follows:
> - the condition marked /* if there is a remaining skb [...] */ is true
>   so the swaps happen
> - skb_out is set from ctx->tx_curr_skb
> - skb_out->len is exactly 0x3f52
> - ctx->tx_curr_size is 0x4000 and delayed_ndp_size is 0xac
>   (note that the sum of skb_out->len and delayed_ndp_size is 0x3ffe)
> - the for loop over n is executed once
> - the cdc_ncm_align_tail call marked /* align beginning of next frame */
>   increases skb_out->len to 0x3f56 (the sum is now 0x4002)
> - the condition marked /* check if we had enough room left [...] */ is
>   false so we break out of the loop
> - the condition marked /* If requested, put NDP at end of frame. */ is
>   true so the NDP is written into skb_out
> - now skb_out->len is 0x4002, so padding_count is minus two interpreted
>   as an unsigned number, which is used as the length argument to memset,
>   leading to a crash with various symptoms but usually including
>> Call Trace:
>>  <IRQ>
>>  cdc_ncm_fill_tx_frame+0x83a/0x970 [cdc_ncm]
>>  cdc_mbim_tx_fixup+0x1d9/0x240 [cdc_mbim]
>>  usbnet_start_xmit+0x5d/0x720 [usbnet]
> The cdc_ncm_align_tail call first aligns on a ctx->tx_modulus
> boundary (adding at most ctx->tx_modulus-1 bytes), then adds
> ctx->tx_remainder bytes. Alternatively, the next alignment call can
> occur in cdc_ncm_ndp16 or cdc_ncm_ndp32, in which case at most
> ctx->tx_ndp_modulus-1 bytes are added.
> A similar problem has occurred before, and the code is nontrivial to
> reason about, so add a guard before the crashing call. By that time it
> is too late to prevent any memory corruption (we'll have written past
> the end of the buffer already) but we can at least try to get a warning
> written into an on-disk log by avoiding the hard crash caused by padding
> past the buffer with a huge number of zeros.
> Signed-off-by: Jouni K. Seppänen <>
> Fixes: 4a0e3e989d66 ("cdc_ncm: Add support for moving NDP to end of NCM frame")
> Link:
> Reported-by: kernel test robot <>
> Reviewed-by: Bjørn Mork <>

Applied and queued up for -stable, thanks.

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