[bionic/linux-azure][PATCH 2/2] srcu: Lock srcu_data structure in srcu_gp_start()

[Marcelo Henrique Cerri]

From: Dennis Krein <Dennis.Krein at netapp.com>

BugLink: http://bugs.launchpad.net/bugs/1802021

The srcu_gp_start() function is called with the srcu_struct structure's
->lock held, but not with the srcu_data structure's ->lock.  This is
problematic because this function accesses and updates the srcu_data
structure's ->srcu_cblist, which is protected by that lock.  Failing to
hold this lock can result in corruption of the SRCU callback lists,
which in turn can result in arbitrarily bad results.

This commit therefore makes srcu_gp_start() acquire the srcu_data
structure's ->lock across the calls to rcu_segcblist_advance() and
rcu_segcblist_accelerate(), thus preventing this corruption.

Reported-by: Bart Van Assche <bvanassche at acm.org>
Reported-by: Christoph Hellwig <hch at infradead.org>
Reported-by: Sebastian Kuzminsky <seb.kuzminsky at gmail.com>
Signed-off-by: Dennis Krein <Dennis.Krein at netapp.com>
Signed-off-by: Paul E. McKenney <paulmck at linux.ibm.com>
Tested-by: Dennis Krein <Dennis.Krein at netapp.com>
Cc: <stable at vger.kernel.org> # 4.16.x
(cherry picked from commit eb4c2382272ae7ae5d81fdfa5b7a6c86146eaaa4)
Signed-off-by: Marcelo Henrique Cerri <marcelo.cerri at canonical.com>
---
 kernel/rcu/srcutree.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index d5cea81378cc..b3e5e9873582 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -441,10 +441,12 @@ static void srcu_gp_start(struct srcu_struct *sp)
 
 	lockdep_assert_held(&sp->lock);
 	WARN_ON_ONCE(ULONG_CMP_GE(sp->srcu_gp_seq, sp->srcu_gp_seq_needed));
+	spin_lock_rcu_node(sdp);  /* Interrupts already disabled. */
 	rcu_segcblist_advance(&sdp->srcu_cblist,
 			      rcu_seq_current(&sp->srcu_gp_seq));
 	(void)rcu_segcblist_accelerate(&sdp->srcu_cblist,
 				       rcu_seq_snap(&sp->srcu_gp_seq));
+	spin_unlock_rcu_node(sdp);  /* Interrupts remain disabled. */
 	smp_mb(); /* Order prior store to ->srcu_gp_seq_needed vs. GP start. */
 	rcu_seq_start(&sp->srcu_gp_seq);
 	state = rcu_seq_state(READ_ONCE(sp->srcu_gp_seq));
-- 
2.17.1