From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH net] ppp: ensure file->private_data can't be overridden
Date: Fri, 11 Mar 2016 14:42:16 -0500 (EST)
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From: Guillaume Nault <g.nault@alphalink.fr>
Date: Tue, 8 Mar 2016 20:14:30 +0100

> Lock ppp_mutex and check that file->private_data is NULL before
> executing any action in ppp_unattached_ioctl().
> The test done by ppp_ioctl() can't be relied upon, because
> file->private_data may have been updated meanwhile. In which case
> ppp_unattached_ioctl() will override file->private_data and mess up
> reference counters or loose pointer to previously allocated PPP unit.
> 
> In case the test fails, -ENOTTY is returned, just like if ppp_ioctl()
> had rejected the ioctl in the first place.
> 
> Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>

If this thing can disappear on us, then we need to make the entirety
of ppp_ioctl() run with the mutex held to fix this properly.

Otherwise ->private_data could go NULL on us meanwhile as well.

We should hold the mutex, to stabilize the value of ->private_data.