netdev - Re: [PATCH] smsc95xx: Use skb_cow to deal with cloned skbs

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Date:   Tue, 18 Apr 2017 17:56:34 +0100
From:   James Hughes <james.hughes@...pberrypi.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        Steve Glendinning <steve.glendinning@...well.net>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>
Subject: Re: [PATCH] smsc95xx: Use skb_cow to deal with cloned skbs

On 18 April 2017 at 17:52, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2017-04-18 at 17:16 +0100, James Hughes wrote:
>> On 18 April 2017 at 16:55, David Miller <davem@...emloft.net> wrote:
>> > From: Eric Dumazet <eric.dumazet@...il.com>
>> > Date: Tue, 18 Apr 2017 08:51:51 -0700
>> >
>> >> On Tue, 2017-04-18 at 15:48 +0100, James Hughes wrote:
>> >>> The driver was failing to check that the SKB wasn't cloned
>> >>> before adding checksum data or adding header data.
>> >>> Replace existing handling to extend the buffer with
>> >>> skb_cow. Don't use skb_cow_head as the sw checksum
>> >>> code modifies the data portion.
>> >>>
>> >>> Signed-off-by: James Hughes <james.hughes@...pberrypi.org>
>> >>> ---
>> >>>  drivers/net/usb/smsc95xx.c | 10 +++-------
>> >>>  1 file changed, 3 insertions(+), 7 deletions(-)
>> >>>
>> >>> diff --git a/drivers/net/usb/smsc95xx.c b/drivers/net/usb/smsc95xx.c
>> >>> index df60c98..04f6397 100644
>> >>> --- a/drivers/net/usb/smsc95xx.c
>> >>> +++ b/drivers/net/usb/smsc95xx.c
>> >>> @@ -2067,13 +2067,9 @@ static struct sk_buff *smsc95xx_tx_fixup(struct usbnet *dev,
>> >>>      /* We do not advertise SG, so skbs should be already linearized */
>> >>>      BUG_ON(skb_shinfo(skb)->nr_frags);
>> >>>
>> >>> -    if (skb_headroom(skb) < overhead) {
>> >>> -            struct sk_buff *skb2 = skb_copy_expand(skb,
>> >>> -                    overhead, 0, flags);
>> >>> -            dev_kfree_skb_any(skb);
>> >>> -            skb = skb2;
>> >>> -            if (!skb)
>> >>> -                    return NULL;
>> >>> +    /* Make writable and expand space by overhead if required */
>> >>> +    if (skb_cow(skb, overhead)) {
>> >>> +            return NULL;
>> >>>      }
>> >>
>> >> Note that this patch will probably force a copy of all locally generated
>> >> TCP packets.
>> >>
>> >> For them skb_cloned(skb) is true.
>> >>
>> >> I do believe skb_cow_head() would be better, since TCP stack uses the
>> >> __skb_header_release() helper to tell lower stacks they can write the
>> >> header part, even on a clone.
>> >
>> > Agreed.
>>
>> I'm happy to work it as you see fit - you know this code far better than I do.
>>
>> Our reading of the code is that the software checksum path is
>> modifying the data rather than just adding a header. Based on the
>> description of skb_cow_head it therefore isn't appropriate. If that
>> isn't a concern in reality then skb_cow_head is fine and I'll make a
>> V2 patchset.
>> Or do we need to skb_cow if doing the software checksum, but
>> skb_cow_head normally? That can be done instead but requires a
>> slightly larger change.
>>
>> The failure case we were seeing was with a bridged network using
>> SMSC9514 and a Broadcom wifi chip on Raspberry Pi 3. The bridge was
>> making an SKB clone of broadcasts for the two interfaces, and then
>> both drivers were adding headers without checking skb_cloned(skb)
>> first, hence trampling on each other. For small packets the SMSC95xx
>> driver will be computing the software checksum and writing it in to
>> the data, so the wifi driver will also be seeing it. For many drivers
>> that probably won't matter, but is that always true?
>>
>> (Patches for the Broadcom wifi driver will be coming once we've worked
>> out the best way of fixing this - there is no error path easily
>> available if the skb_cow_head call fails).
>>
>
> You misread what the driver does.
>
> The TCP data (payload) is _not_ modified.
>
> Only additional headers are pushed in front of the existing (Ethernet,
> IP, TCP) headers.
>
> For this, skb_cow_head() is the perfect solution.
>

OK, will modify the patch, commit and resubmit. Thanks

James