[PATCH 4.9 015/218] SUNRPC: avoid race between mod_timer() and del_timer_sync()

Greg Kroah-Hartman
2022-04-18T14:08:08Z

From: NeilBrown <neilb@suse.de>

commit 3848e96edf4788f772d83990022fa7023a233d83 upstream.

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>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/sunrpc/xprt.c | 7 +++++++
1 file changed, 7 insertions(+)

--- a/net/sunrpc/xprt.c
+++ b/net/sunrpc/xprt.c
@@ -1446,7 +1446,14 @@ static void xprt_destroy(struct rpc_xprt
/* Exclude transport connect/disconnect handlers */
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);

rpc_xprt_debugfs_unregister(xprt);
rpc_destroy_wait_queue(&xprt->binding);