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

[Eugeniu Rosca]

Hello Neil,
Hello Trond,

On Do, Sep 08, 2022 at 01:09:29 +0000, Trond Myklebust wrote:
> On Thu, 2022-09-08 at 10:58 +1000, NeilBrown wrote:
> > 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@xxxxxxx>
> > > > 
> > > > 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@xxxxxxxxxxxxxxx
> > > > Signed-off-by: NeilBrown <neilb@xxxxxxx>
> > > > Signed-off-by: Trond Myklebust <https://urldefense.proofpoint.com/v2/url?u=http-3A__trond.myklebust-40hammerspace.com&d=DwIGaQ&c=euGZstcaTDllvimEN8b7jXrwqOf-v5A_CdpgnVfiiMM&r=SAhjP5GOmrADp1v_EE5jWoSuMlYCIt9gKduw-DCBPLs&m=HrAc1Ouz3sNPrdBv5QBgh7SToNI8M0iGJyDPgOTT5AE&s=cNzW7c7t83SH27ck7hxvneR9awrt17JualiCD6TZtdI&e=>
> > > > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > > ---
> > > >  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.
> 
> Agreed. On older kernels, the xprt->transport_lock served the same
> purpose, but it had to take a bh-safe spinlock in order to avoid
> certain races with the socket callbacks. Since then, a number of
> changes to both the socket layer and the SUNRPC code have made it
> possible to eliminate bh-safe requirement.
> 
> > 
> > Can you confirm doing that removes the problem?

Your proposal [*] seems to resolve the issue for me.

Any chance to get a stable patch, to which I will gladly provide
the Reviewed-by/Tested-by signatures?

> > 
> > NeilBrown
> > 

[*] diff --git a/net/sunrpc/xprt.c b/net/sunrpc/xprt.c
index e7d55d63d4f1..7f9b94acf597 100644
--- a/net/sunrpc/xprt.c
+++ b/net/sunrpc/xprt.c
@@ -1525,9 +1525,9 @@ static void xprt_destroy(struct rpc_xprt *xprt)
 	 * 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);
+	spin_lock_bh(&xprt->transport_lock);
 	del_timer_sync(&xprt->timer);
-	spin_unlock(&xprt->transport_lock);
+	spin_unlock_bh(&xprt->transport_lock);
 
 	/*
 	 * Destroy sockets etc from the system workqueue so they can

Best Regards,
Eugeniu Rosca