Patch "net/sctp: always initialise sctp_ht_iter::start_fail" has been added to the 4.7-stable tree

This is a note to let you know that I've just added the patch titled

net/sctp: always initialise sctp_ht_iter::start_fail

to the 4.7-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
net-sctp-always-initialise-sctp_ht_iter-start_fail.patch
and it can be found in the queue-4.7 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Vegard Nossum <[email protected]>
Date: Fri, 12 Aug 2016 09:50:51 +0200
Subject: net/sctp: always initialise sctp_ht_iter::start_fail

From: Vegard Nossum <[email protected]>

[ Upstream commit 54236ab09e9696a27baaae693c288920a26e8588 ]

sctp_transport_seq_start() does not currently clear iter->start_fail on success, but relies on it being zero when it is allocated (by seq_open_net()).

This can be a problem in the following sequence:

open() // allocates iter (and implicitly sets iter->start_fail = 0) read()
- iter->start() // fails and sets iter->start_fail = 1
- iter->stop() // doesn't call sctp_transport_walk_stop() (correct)
read() again
- iter->start() // succeeds, but doesn't change iter->start_fail
- iter->stop() // doesn't call sctp_transport_walk_stop() (wrong)

We should initialize sctp_ht_iter::start_fail to zero if ->start() succeeds, otherwise it's possible that we leave an old value of 1 there, which will cause ->stop() to not call sctp_transport_walk_stop(), which causes all sorts of problems like not calling rcu_read_unlock() (and preempt_enable()), eventually leading to more warnings like this:

    BUG: sleeping function called from invalid context at mm/slab.h:388
    in_atomic(): 0, irqs_disabled(): 0, pid: 16551, name: trinity-c2
    Preemption disabled at:[<ffffffff819bceb6>] rhashtable_walk_start+0x46/0x150

     [<ffffffff81149abb>] preempt_count_add+0x1fb/0x280
     [<ffffffff83295892>] _raw_spin_lock+0x12/0x40
     [<ffffffff819bceb6>] rhashtable_walk_start+0x46/0x150
     [<ffffffff82ec665f>] sctp_transport_walk_start+0x2f/0x60
     [<ffffffff82edda1d>] sctp_transport_seq_start+0x4d/0x150
     [<ffffffff81439e50>] traverse+0x170/0x850
     [<ffffffff8143aeec>] seq_read+0x7cc/0x1180
     [<ffffffff814f996c>] proc_reg_read+0xbc/0x180
     [<ffffffff813d0384>] do_loop_readv_writev+0x134/0x210
     [<ffffffff813d2a95>] do_readv_writev+0x565/0x660
     [<ffffffff813d6857>] vfs_readv+0x67/0xa0
     [<ffffffff813d6c16>] do_preadv+0x126/0x170
     [<ffffffff813d710c>] SyS_preadv+0xc/0x10
     [<ffffffff8100334c>] do_syscall_64+0x19c/0x410
     [<ffffffff83296225>] return_from_SYSCALL_64+0x0/0x6a
     [<ffffffffffffffff>] 0xffffffffffffffff

Notice that this is a subtly different stacktrace from the one in commit 5fc382d875 ("net/sctp: terminate rhashtable walk correctly").

---
 net/sctp/proc.c |    1 +
 1 file changed, 1 insertion(+)

@@ -293,6 +293,7 @@ static void *sctp_transport_seq_start(st
 		return ERR_PTR(err);
 	}
 
+	iter->start_fail = 0;
 	return sctp_transport_get_idx(seq_file_net(seq), &iter->hti, *pos);
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.7/net-sctp-always-initialise-sctp_ht_iter-start_fail.patch queue-4.7/net-irda-handle-iriap_register_lsap-allocation-failure.patch queue-4.7/tipc-fix-null-pointer-dereference-in-shutdown.patch