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

seq_read() can call ->start() twice on the same iterator more than once
(e.g. once through traverse() and once in seq_read() itself).

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 <redacted>
Cc: stable@vger.kernel.org
Signed-off-by: Vegard Nossum <redacted>
---
 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

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?

On 07/23/2016 03:39 PM, Marcelo Ricardo Leitner wrote:

On Sat, Jul 23, 2016 at 11:52:23AM +0200, Vegard Nossum wrote:

quoted

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?

I think you're right.

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

The sctp_ht_iter is allocated in
sctp_assocs_seq_open()/sctp_remaddr_seq_open(), so I assume it's
allocated on open().

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

I was using trinity, so I'm not quite sure a priori, but the problem was
100% reproducible before I applied the patch and seeing that it gets
allocated on open() and is never cleared anywhere else, your suggestion
sounds like the most plausible explanation :-)

How about rewording the first paragraph as:

"""
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)
"""

Let me know how that sounds.

Thanks for looking so closely at it!


Vegard

On Sat, Jul 23, 2016 at 04:00:39PM +0200, Vegard Nossum wrote:

On 07/23/2016 03:39 PM, Marcelo Ricardo Leitner wrote:

quoted

On Sat, Jul 23, 2016 at 11:52:23AM +0200, Vegard Nossum wrote:

quoted

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?

I think you're right.

quoted

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

The sctp_ht_iter is allocated in
sctp_assocs_seq_open()/sctp_remaddr_seq_open(), so I assume it's
allocated on open().

quoted

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

I was using trinity, so I'm not quite sure a priori, but the problem was
100% reproducible before I applied the patch and seeing that it gets
allocated on open() and is never cleared anywhere else, your suggestion
sounds like the most plausible explanation :-)

How about rewording the first paragraph as:

"""
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)
"""

Let me know how that sounds.

LGTM, thanks!

  Marcelo

Thanks for looking so closely at it!


Vegard
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