netdev - Re: [RFC PATCH net] net: dsa: tag_dsa: fix suspicious rcu_dereference_check() with br_vlan_get_pvid

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Date:   Thu, 30 Sep 2021 00:44:18 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Nikolay Aleksandrov <nikolay@...dia.com>
Cc:     Vladimir Oltean <vladimir.oltean@....com>, netdev@...r.kernel.org,
        "Paul E . McKenney" <paulmck@...nel.org>,
        rcu <rcu@...r.kernel.org>, Roopa Prabhu <roopa@...dia.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Tobias Waldekranz <tobias@...dekranz.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>
Subject: Re: [RFC PATCH net] net: dsa: tag_dsa: fix suspicious
 rcu_dereference_check() with br_vlan_get_pvid_rcu

On Thu, Sep 30, 2021 at 12:21:55AM +0300, Nikolay Aleksandrov wrote:
> To answer the question about the patch - it is correct and ok, the dereference needed
> is done inside the function and after that the value is copied. I'd only check how the
> DSA bridge pointer is handled, it seems it is dereferenced directly and I don't know
> the DSA locking rules (i.e. if anything is held by that time to ensure the pointer is
> valid). As for the patch:
> Reviewed-by: Nikolay Aleksandrov <nikolay@...dia.com>

I think you're right in the fact that nobody ensures dp->bridge_dev
cannot be reset to NULL by dsa_port_bridge_leave(). I'll try to simulate
an issue by spin-looping for a while in the xmit function while I add
and remove the port from a bridge and see what happens. Worst case,
we'll have to add READ_ONCE and WRITE_ONCE annotations for all users,
and work with a local, on-stack reference. Because the bridge net_device
itself will not go away until the xmit itself finishes, but it's just
that it may not be our upper anymore.