rxrpc: Fix locking issue

There's a locking issue with the per-netns list of calls in rxrpc.  The
pieces of code that add and remove a call from the list use write_lock()
and the calls procfile uses read_lock() to access it.  However, the timer
callback function may trigger a removal by trying to queue a call for
processing and finding that it's already queued - at which point it has a
spare refcount that it has to do something with.  Unfortunately, if it puts
the call and this reduces the refcount to 0, the call will be removed from
the list.  Unfortunately, since the _bh variants of the locking functions
aren't used, this can deadlock.

================================
WARNING: inconsistent lock state
5.18.0-rc3-build4+ #10 Not tainted
--------------------------------
inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.
ksoftirqd/2/25 [HC0[0]:SC1[1]:HE1:SE0] takes:
ffff888107ac4038 (&rxnet->call_lock){+.?.}-{2:2}, at: rxrpc_put_call+0x103/0x14b
{SOFTIRQ-ON-W} state was registered at:
...
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&rxnet->call_lock);
  <Interrupt>
    lock(&rxnet->call_lock);

 *** DEADLOCK ***

1 lock held by ksoftirqd/2/25:
 #0: ffff8881008ffdb0 ((&call->timer)){+.-.}-{0:0}, at: call_timer_fn+0x5/0x23d

Changes
=======
ver #2)
 - Changed to using list_next_rcu() rather than rcu_dereference() directly.

Fixes: 17926a79320a ("[AF_RXRPC]: Provide secure RxRPC sockets for use by userspace and kernel both")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 32 insertions, 0 deletions

diff --git a/fs/seq_file.c b/fs/seq_file.c
index 7ab8a58c29b6..9456a2032224 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -931,6 +931,38 @@ struct list_head *seq_list_next(void *v, struct list_head *head, loff_t *ppos)
 }
 EXPORT_SYMBOL(seq_list_next);
 
+struct list_head *seq_list_start_rcu(struct list_head *head, loff_t pos)
+{
+	struct list_head *lh;
+
+	list_for_each_rcu(lh, head)
+		if (pos-- == 0)
+			return lh;
+
+	return NULL;
+}
+EXPORT_SYMBOL(seq_list_start_rcu);
+
+struct list_head *seq_list_start_head_rcu(struct list_head *head, loff_t pos)
+{
+	if (!pos)
+		return head;
+
+	return seq_list_start_rcu(head, pos - 1);
+}
+EXPORT_SYMBOL(seq_list_start_head_rcu);
+
+struct list_head *seq_list_next_rcu(void *v, struct list_head *head,
+				    loff_t *ppos)
+{
+	struct list_head *lh;
+
+	lh = list_next_rcu((struct list_head *)v);
+	++*ppos;
+	return lh == head ? NULL : lh;
+}
+EXPORT_SYMBOL(seq_list_next_rcu);
+
 /**
  * seq_hlist_start - start an iteration of a hlist
  * @head: the head of the hlist