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Date:   Sun, 8 Dec 2019 19:49:07 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Vladis Dronov <vdronov@redhat.com>
Cc:     Richard Cochran <richardcochran@gmail.com>,
        linux-fsdevel@vger.kernel.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] fs: fix use-after-free in __fput() when a chardev is
 removed but a file is still open
Message-ID: <20191208194907.GW4203@ZenIV.linux.org.uk>
References: <20191125125342.6189-1-vdronov@redhat.com>
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On Mon, Nov 25, 2019 at 01:53:42PM +0100, Vladis Dronov wrote:
> In a case when a chardev file (like /dev/ptp0) is open but an underlying
> device is removed, closing this file leads to a use-after-free. This
> reproduces easily in a KVM virtual machine:
> 
> # cat openptp0.c
> int main() { ... fp = fopen("/dev/ptp0", "r"); ... sleep(10); }

> 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
> 
> because of:
> 
> __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
> 
> The fix is to call cdev_put() before file->f_op->release(). This fix the
> class of bugs when a chardev device is removed when its file is open, for
> example:

And what's to prevent rmmod coming and freeing ->release code right as you
are executing it?