Date:   Tue, 18 Apr 2017 09:52:27 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     James Hughes <james.hughes@...pberrypi.org>
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 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.

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