Re: [PATCH v3 net-next 1/2] net: skbuff: Remove errornous length validation in skb_vlan_pop()

On Tue, Sep 20, 2016 at 2:48 AM, Shmulik Ladkani
[off-list ref] wrote:

In 93515d53b1
  "net: move vlan pop/push functions into common code"
skb_vlan_pop was moved from its private location in openvswitch to
skbuff common code.

In case skb has non hw-accel vlan tag, the original 'pop_vlan()' assured
that skb->len is sufficient (if skb->len < VLAN_ETH_HLEN then pop was
considered a no-op).

This validation was moved as is into the new common 'skb_vlan_pop'.

Alas, in its original location (openvswitch), there was a guarantee that
'data' points to the mac_header, therefore the 'skb->len < VLAN_ETH_HLEN'
condition made sense.
However there's no such guarantee in the generic 'skb_vlan_pop'.

For short packets received in rx path going through 'skb_vlan_pop',
this causes 'skb_vlan_pop' to fail pop-ing a valid vlan hdr (in the non
hw-accel case) or to fail moving next tag into hw-accel tag.

Remove the 'skb->len < VLAN_ETH_HLEN' condition entirely:
It is superfluous since inner '__skb_vlan_pop' already verifies there
are VLAN_ETH_HLEN writable bytes at the mac_header.

Note this presents a slight change to skb_vlan_pop() users:
In case total length is smaller than VLAN_ETH_HLEN, skb_vlan_pop() now
returns an error, as opposed to previous "no-op" behavior.
Existing callers (e.g. tc act vlan, ovs) usually drop the packet if
'skb_vlan_pop' fails.

Fixes: 93515d53b1 ("net: move vlan pop/push functions into common code")
Signed-off-by: Shmulik Ladkani <redacted>
Cc: Pravin Shelar <redacted>
---
 v3: Elaborate log message to explain the change presented to
     'skb_vlan_pop' users for packets smaller than VLAN_ETH_HLEN

 v2: Remove 'skb->len < VLAN_ETH_HLEN' condition entirely (instead of
     testing skb->mac_len as was in v1), suggested by Pravin Shelar

 net/core/skbuff.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

Reviewed-by: Pravin B Shelar <redacted>