Re: [BUG] skb corruption and kernel panic at forwarding with fragmentation

On Wed, Jan 6, 2016 at 10:59 PM, Cong Wang [off-list ref] wrote:

On Wed, Jan 6, 2016 at 11:15 AM, Konstantin Khlebnikov [off-list ref] wrote:

quoted

Looks like this happens because ip_options_fragment() relies on
correct ip options length in ip control block in skb. But in
ip_finish_output_gso() control block in segments is reused by
skb_gso_segment(). following ip_fragment() sees some garbage.

In my case there was no ip options but length becomes non-zero and
ip_options_fragment() picked some bytes from payload and decides to
fill huge range with IPOPT_NOOP (1). One of that ones flipped nr_frags
in skb_shared_info at the end of data =)

Hmm, it looks like SKB_GSO_CB should be cleared after skb_gso_segment()
since all the gso information should be saved in shared_info after it finishes.

Does a memset(0) on SKB_GSO_CB after skb_gso_segment() work as well?

This will break present logic around ip_options_fragment() - it clears
options from
second and following fragments. With zeroed cb it will do nothing.

ip_options_fragment() can get required information directly from ip header but
it also resets fields in IPCB -- probably it should stay valid here
and somebody else will use it later.