Re: [PATCH] net/sctp: always initialise sctp_ht_iter::start_fail

[Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>]

On Sat, Jul 23, 2016 at 11:52:23AM +0200, Vegard Nossum wrote:
> seq_read() can call ->start() twice on the same iterator more than once
> (e.g. once through traverse() and once in seq_read() itself).

But when traverse() returns the error, it goes to Done label, skipping
the call to ->start() from seq_read(), or am I missing something? 

Though yes, if sctp_ht_iter memory is actually re-used without
initializting between seq_read()s, it triggers the issue you described.

How did you trigger this, reading after an error on the file descriptor?

> 
> 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 previous bug
> I reported.)
> 
> Cc: Xin Long <lucien.xin@gmail.com>
> Cc: Herbert Xu <herbert@gondor.apana.org.au>
> Cc: Eric W. Biederman <ebiederm@xmission.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
> ---
>  net/sctp/proc.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/net/sctp/proc.c b/net/sctp/proc.c
> index 4cb5aed..ef8ba77 100644
> --- a/net/sctp/proc.c
> +++ b/net/sctp/proc.c
> @@ -293,6 +293,7 @@ static void *sctp_transport_seq_start(struct seq_file *seq, loff_t *pos)
>  		return ERR_PTR(err);
>  	}
>  
> +	iter->start_fail = 0;
>  	return sctp_transport_get_idx(seq_file_net(seq), &iter->hti, *pos);
>  }
>  
> -- 
> 1.9.1
> 
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