netdev - Re: [PATCH] tun: avoid double free in tun_free

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Message-ID: <20211208155838.24556030@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 8 Dec 2021 15:58:38 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     George Kennedy <george.kennedy@...cle.com>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        Greg KH <gregkh@...uxfoundation.org>, davem@...emloft.net,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tun: avoid double free in tun_free_netdev

On Wed, 8 Dec 2021 11:44:02 -0500 George Kennedy wrote:
> > It looks like a lot of the problem is duplicate unwind.
> > Why does err_free_flow, err_free_stat etc unwinds need to exist if
> > the free_netdev is going to do same thing.  
> 
> Maybe instead do not call security_tun_dev_free_security(tun->security) 
> in err_free_flow if it's going to be done anyway in tun_free_netdev().

That won't be good either. register_netdevice() has multiple failure
modes, it may or may not call the destructor depending on where it
fails. Either the stuff that destructor undoes needs to be moved to
ndo_init (which is what destructor always pairs with), or you can check
dev->reg_state. If dev->reg_state is NETREG_UNREGISTERING that means
the destructor will be caller later.

The ndo_init way is preferable, just cut and past the appropriate lines
preceding registration into a ndo_init callback.