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From:   NeilBrown <neilb@suse.de>
Subject: [PATCH] SUNRPC: avoid race between mod_timer() and del_timer_sync()
To:     Trond Myklebust <trond.myklebust@hammerspace.com>,
        Anna Schumaker <anna@kernel.org>
Cc:     Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Date:   Tue, 08 Mar 2022 13:42:17 +1100
Message-id: <164670733789.31932.14711754930977072270@noble.neil.brown.name>
Precedence: bulk


xprt_destory() claims XPRT_LOCKED and then calls del_timer_sync().
Both xprt_unlock_connect() and xprt_release() call
 ->release_xprt()
which drops XPRT_LOCKED and *then* xprt_schedule_autodisconnect()
which calls mod_timer().

This may result in mod_timer() being called *after* del_timer_sync().
When this happens, the timer may fire long after the xprt has been freed,
and run_timer_softirq() will probably crash.

The pairing of ->release_xprt() and xprt_schedule_autodisconnect() is
always called under ->transport_lock.  So if we take ->transport_lock to
call del_timer_sync(), we can be sure that mod_timer() will run first
(if it runs at all).

Cc: stable@vger.kernel.org
Signed-off-by: NeilBrown <neilb@suse.de>
---
 net/sunrpc/xprt.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/sunrpc/xprt.c b/net/sunrpc/xprt.c
index a02de2bddb28..5388263f8fc8 100644
--- a/net/sunrpc/xprt.c
+++ b/net/sunrpc/xprt.c
@@ -2112,7 +2112,14 @@ static void xprt_destroy(struct rpc_xprt *xprt)
 	 */
 	wait_on_bit_lock(&xprt->state, XPRT_LOCKED, TASK_UNINTERRUPTIBLE);
 
+	/*
+	 * xprt_schedule_autodisconnect() can run after XPRT_LOCKED
+	 * is cleared.  We use ->transport_lock to ensure the mod_timer()
+	 * can only run *before* del_time_sync(), never after.
+	 */
+	spin_lock(&xprt->transport_lock);
 	del_timer_sync(&xprt->timer);
+	spin_unlock(&xprt->transport_lock);
 
 	/*
 	 * Destroy sockets etc from the system workqueue so they can
-- 
2.35.1