Brendan reports that the use of netem's packet corruption capability
leads to strange crashes.  This seems to be caused by
commit d66280b1 ("net: netem: use a list in addition to rbtree")
which uses skb->next pointer to construct a fast-path queue of
in-order skbs.

Packet corruption code has to invoke skb_gso_segment() in case
of skbs in need of GSO.  skb_gso_segment() returns a list of
skbs.  If next pointers of the skbs on that list do not get cleared
fast path list may point to freed skbs or skbs which are also on
the RB tree.

Let's say skb gets segmented into 3 frames:

A -> B -> C

A gets hooked to the t_head t_tail list by tfifo_enqueue(), but it's
next pointer didn't get cleared so we have:

h t
|/
A -> B -> C

Now if B and C get also get enqueued successfully all is fine, because
tfifo_enqueue() will overwrite the list in order.  IOW:

Enqueue B:

h    t
|    |
A -> B    C

Enqueue C:

h         t
|         |
A -> B -> C

But if B and C get reordered we may end up with:

h t            RB tree
|/                |
A -> B -> C       B
                   \
                    C

Or if they get dropped just:

h t
|/
A -> B -> C

where A and B are already freed.

To reproduce either limit has to be set low to cause freeing of
segs or reorders have to happen (due to delay jitter).

Note that we only have to mark the first segment as not on the
list, "finish_segs" handling of other frags already does that.

Another caveat is that qdisc_drop_all() still has to free all
segments correctly in case of drop of first segment, therefore
we re-link segs before calling it.

v2:
 - re-link before drop, v1 was leaking non-first segs if limit
   was hit at the first seg
 - better commit message which lead to discovering the above :)
Reported-by: Brendan Galloway <brendan.galloway@netronome.com>
Fixes: d66280b1 ("net: netem: use a list in addition to rbtree")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>