Re: [PATCH net] gianfar: prevent integer wrapping in the rx handler

[David Miller <davem@davemloft.net>]

From: Andy Spencer <aspencer@spacex.com>
Date: Thu, 25 Jan 2018 19:37:50 -0800

> When the frame check sequence (FCS) is split across the last two frames
> of a fragmented packet, part of the FCS gets counted twice, once when
> subtracting the FCS, and again when subtracting the previously received
> data.
> 
> For example, if 1602 bytes are received, and the first fragment contains
> the first 1600 bytes (including the first two bytes of the FCS), and the
> second fragment contains the last two bytes of the FCS:
> 
>   'skb->len == 1600' from the first fragment
> 
>   size  = lstatus & BD_LENGTH_MASK; # 1602
>   size -= ETH_FCS_LEN;              # 1598
>   size -= skb->len;                 # -2
> 
> Since the size is unsigned, it wraps around and causes a BUG later in
> the packet handling, as shown below:
> 
>   kernel BUG at ./include/linux/skbuff.h:2068!
>   Oops: Exception in kernel mode, sig: 5 [#1]
>   ...
>   NIP [c021ec60] skb_pull+0x24/0x44
>   LR [c01e2fbc] gfar_clean_rx_ring+0x498/0x690
>   Call Trace:
>   [df7edeb0] [c01e2c1c] gfar_clean_rx_ring+0xf8/0x690 (unreliable)
>   [df7edf20] [c01e33a8] gfar_poll_rx_sq+0x3c/0x9c
>   [df7edf40] [c023352c] net_rx_action+0x21c/0x274
>   [df7edf90] [c0329000] __do_softirq+0xd8/0x240
>   [df7edff0] [c000c108] call_do_irq+0x24/0x3c
>   [c0597e90] [c00041dc] do_IRQ+0x64/0xc4
>   [c0597eb0] [c000d920] ret_from_except+0x0/0x18
>   --- interrupt: 501 at arch_cpu_idle+0x24/0x5c
> 
> Change the size to a signed integer and then trim off any part of the
> FCS that was received prior to the last fragment.
> 
> Fixes: 6c389fc931bc ("gianfar: fix size of scatter-gathered frames")
> Signed-off-by: Andy Spencer <aspencer@spacex.com>

Applied.