netdev - Re: [PATCH net] ibmvnic: Clean RX pools only during a hard reset

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Message-Id: <93621ba4-4718-468d-5801-7c2f1b67c326@linux.vnet.ibm.com>
Date:   Mon, 19 Feb 2018 13:24:52 -0600
From:   Thomas Falcon <tlfalcon@...ux.vnet.ibm.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, nfont@...ux.vnet.ibm.com,
        jallen@...ux.vnet.ibm.com
Subject: Re: [PATCH net] ibmvnic: Clean RX pools only during a hard reset

On 02/19/2018 10:37 AM, David Miller wrote:
> From: Thomas Falcon <tlfalcon@...ux.vnet.ibm.com>
> Date: Sun, 18 Feb 2018 10:08:40 -0600
>
>> Sorry, this fixes a bug in commit d0869c0071e4. The cause of the
>> bug is that "stale" RX buffers containing packet data are returned
>> to the driver after device close and open. While most buffers will be
>> returned with an error and handled by the polling routine, some buffers
>> will be returned as containing valid data. Unfortunately, the socket
>> buffers allocated were already freed when the device was closed, so
>> attempts to process them result in a panic.
>>
>> RX pools still need to be cleaned in some cases, such as during
>> a fatal reset. In all other cases, the socket buffers will either
>> be freed in the polling routine or processed by the kernel.
>>
>> Fixes: d0869c0071e4 ("ibmvnic: Clean RX pool buffers during device close")
>> Signed-off-by: Thomas Falcon <tlfalcon@...ux.vnet.ibm.com>
> It really shouldn't matter who, or how many times, clear_rx_pools() is
> called.
>
> Anyone who calls it and frees the SKBs will mark the SKB slots as NULL,
> so any subsequent call cannot possibly double free the buffers.
>
> At best you need to explain the problem better in the commit message.

Sorry, I should explain it better. It's not there is a double free.  It's that the driver is receiving RX descriptors from the previous session for which socket buffers have been freed. The driver's polling routine tries to copy data to the socket buffer, but it's been freed, so it's trying to copy to a NULL pointer.

Tom