Subject: Invalid sk_policy[] access (was Re: Recent spontaneous reboots on multiple machines)

From: Sowmini Varadhan <[email-masked]>
Date: Tue, 23 Feb 2016 07:12:44 -0500

I figured out what's the root-cause of my panics.

In my case, for the stack shown in 
 http://marc.info/?l=linux-sparc&m=145610295109214&w=2
(which also has all the details about the issue),

tcp_make_synack has been called with attach_req set to true
so it sets up the skb->sk via:

        if (attach_req) {
                skb_set_owner_w(skb, req_to_sk(req));
        } else { /* .. */

Now, req is a struct inet_request_sock, and we are casting this
as a struct sock, to later get the ->sk_policy[1] in the xfrm
code. Consider the sizes of these structures between 32 and 64 bits:

  sizeof             32-bit        64-bit
-------------------------------------------
request_sock          256           312
inet_request_sock     272           328
sock                  688          1216

And offsetof sk_policy[1] is 256 on the 32-bit v440, whereas
it is 520 on my 64-bit T5. 

Thus on the v440, the sk_policy[1] is pointing at somewhere
in the middle of stuff set up by tcp_openreq_init() (the ireq
flags initialization).

Even on the 64-bit arch, trying to do req_to_sk(req) and accessing
fields beyond the sock_common, e.g., between offset 312 and 328 
may not give you the fields you are looking for? 

so how is this supposed to work? (Evidently it worked for Meelis
before, but I dont know if that was before or after  commit
9e17f8a475).

--Sowmini