RE: [PATCH 4.14 022/284] SUNRPC: avoid race between mod_timer() and del_timer_sync()

David Laight
2022-09-11T12:43:21Z

From: NeilBrown
> Sent: 08 September 2022 01:58
>
> On Thu, 08 Sep 2022, Eugeniu Rosca wrote:
> > Hello all,
> >
> > On Mo, Apr 18, 2022 at 02:10:03 +0200, Greg Kroah-Hartman wrote:
> > > 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
> > > @@ -1520,7 +1520,14 @@ static void xprt_destroy(struct rpc_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);
>
> I think it is sufficient to change the to spin_{,un}lock_bh()
> in older kernels. The spinlock call need to match other uses of the
> same lock.

Every time I see this patch it looks wrong.
You need something to stop the code that is calling mod_timer()
running after the spin_unlock().
Now it might be that there is some other state that is
already set - in which case you only need to wait for the
spin_lock to be released - since it can't be obtained again
(to start the timer).

So I'd expect to see:
spin_lock();
if (nothing_set_earlier)
xprt->destroying = 1;
spin_unlock()
del_timer_sync();

Looking at the code (for a change) is looks even worse.

del_timer_sync() isn't anywhere near enough.
All the timer callback function does is schedule some work.
So you also need to wait for the work to complete.

Changing it all to use delayed_work might reduce the problems.

Oh, any using proper mutex/locks instead of wait_on_bit_lock().

David

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