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Date:   Mon, 30 Dec 2019 20:19:55 -0800 (PST)
Message-Id: <20191230.201955.97738480102485597.davem@davemloft.net>
To:     vdronov@redhat.com
Cc:     linux-fsdevel@vger.kernel.org, viro@zeniv.linux.org.uk,
        richardcochran@gmail.com, aviro@redhat.com, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] ptp: fix the race between the release of ptp_clock
 and cdev
From:   David Miller <davem@davemloft.net>
In-Reply-To: <20191227022627.24476-1-vdronov@redhat.com>
References: <20191208195340.GX4203@ZenIV.linux.org.uk>
        <20191227022627.24476-1-vdronov@redhat.com>
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From: Vladis Dronov <vdronov@redhat.com>
Date: Fri, 27 Dec 2019 03:26:27 +0100

> In a case when a ptp chardev (like /dev/ptp0) is open but an underlying
> device is removed, closing this file leads to a race. This reproduces
> easily in a kvm virtual machine:
 . ..
> This happens in:
> 
> static void __fput(struct file *file)
> {   ...
>     if (file->f_op->release)
>         file->f_op->release(inode, file); <<< cdev is kfree'd here
>     if (unlikely(S_ISCHR(inode->i_mode) && inode->i_cdev != NULL &&
>              !(mode & FMODE_PATH))) {
>         cdev_put(inode->i_cdev); <<< cdev fields are accessed here
> 
> Namely:
> 
> __fput()
>   posix_clock_release()
>     kref_put(&clk->kref, delete_clock) <<< the last reference
>       delete_clock()
>         delete_ptp_clock()
>           kfree(ptp) <<< cdev is embedded in ptp
>   cdev_put
>     module_put(p->owner) <<< *p is kfree'd, bang!
> 
> Here cdev is embedded in posix_clock which is embedded in ptp_clock.
> The race happens because ptp_clock's lifetime is controlled by two
> refcounts: kref and cdev.kobj in posix_clock. This is wrong.
> 
> Make ptp_clock's sysfs device a parent of cdev with cdev_device_add()
> created especially for such cases. This way the parent device with its
> ptp_clock is not released until all references to the cdev are released.
> This adds a requirement that an initialized but not exposed struct
> device should be provided to posix_clock_register() by a caller instead
> of a simple dev_t.
> 
> This approach was adopted from the commit 72139dfa2464 ("watchdog: Fix
> the race between the release of watchdog_core_data and cdev"). See
> details of the implementation in the commit 233ed09d7fda ("chardev: add
> helper function to register char devs with a struct device").
> 
> Link: https://lore.kernel.org/linux-fsdevel/20191125125342.6189-1-vdronov@redhat.com/T/#u
> Analyzed-by: Stephen Johnston <sjohnsto@redhat.com>
> Analyzed-by: Vern Lovejoy <vlovejoy@redhat.com>
> Signed-off-by: Vladis Dronov <vdronov@redhat.com>

Applied, thanks.