netdev - Re: [PATCH net 1/3] ice: fix NULL pointer dereference in ice_update_vsi_tx_ring

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Date:   Wed, 16 Mar 2022 13:24:18 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     "Nguyen, Anthony L" <anthony.l.nguyen@...el.com>
Cc:     "kuba@...nel.org" <kuba@...nel.org>, lkp <lkp@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "Lobakin, Alexandr" <alexandr.lobakin@...el.com>,
        "Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>,
        "dan.carpenter@...cle.com" <dan.carpenter@...cle.com>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "G, GurucharanX" <gurucharanx.g@...el.com>
Subject: Re: [PATCH net 1/3] ice: fix NULL pointer dereference in ice_update_vsi_tx_ring_stats()

On Wed, Mar 16, 2022 at 12:01 PM Nguyen, Anthony L
<anthony.l.nguyen@...el.com> wrote:
>
> On Tue, 2022-03-15 at 20:29 -0700, Jakub Kicinski wrote:
> > On Tue, 15 Mar 2022 14:12:23 -0700 Tony Nguyen wrote:
> > > From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>

<snip>

> > > pointer is valid, but later on ring is accessed to propagate
> > > gathered Tx
> > > stats onto VSI stats.
> > >
> > > Change the existing logic to move to next ring when ring is NULL.
> > >
> > > Fixes: e72bba21355d ("ice: split ice_ring onto Tx/Rx separate
> > > structs")
> > > Reported-by: kernel test robot <lkp@...el.com>
> > > Reported-by: Dan Carpenter <dan.carpenter@...cle.com>
> > > Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
> > > Acked-by: Alexander Lobakin <alexandr.lobakin@...el.com>
> > > Tested-by: Gurucharan G <gurucharanx.g@...el.com> (A Contingent
> > > worker at Intel)
> > > Signed-off-by: Tony Nguyen <anthony.l.nguyen@...el.com>
> > > ---
> > >  drivers/net/ethernet/intel/ice/ice_main.c | 5 +++--
> > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/net/ethernet/intel/ice/ice_main.c
> > > b/drivers/net/ethernet/intel/ice/ice_main.c
> > > index 493942e910be..d4a7c39fd078 100644
> > > --- a/drivers/net/ethernet/intel/ice/ice_main.c
> > > +++ b/drivers/net/ethernet/intel/ice/ice_main.c
> > > @@ -5962,8 +5962,9 @@ ice_update_vsi_tx_ring_stats(struct ice_vsi
> > > *vsi,
> > >                 u64 pkts = 0, bytes = 0;
> > >
> > >                 ring = READ_ONCE(rings[i]);
> >
> > Not really related to this patch but why is there a read_once() here?
> > Aren't stats read under rtnl_lock? What is this protecting against?
>
> It looks like it was based on a patch from i40e [1]. From the commit, I
> gather this is the reason:
>
> "Previously the stats were 64 bit but highly racy due to the fact that
> 64 bit transactions are not atomic on 32 bit systems."
>
> Thanks,
>
> Tony
>
> [1]
> https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=980e9b1186424fa3eb766d59fc91003d0ed1ed6a
>
>
> (Resending as some non-text formatting snuck in to my reply. Sorry for
> the spam)

Actually the rcu locking and READ_ONCE has to do with the fact that
the driver code for the igb/ixgbe/i40e driver had a bunch of code that
could kick in from the sysfs and/or PCIe paths that would start
tearing things down without necessarily holding the rtnl_lock as it
should have.

It might make sense to reevaluate the usage of the READ_ONCE and rcu
locking to determine if it is actually needed since much of that is a
hold over from when we were doing this in the watchdog and not while
holding the rtnl_lock in the stats call.