linux-kernel - Re: [PATCH] switchtec: cleanup cdev init

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Message-ID: <20170221183753.GB13138@obsidianresearch.com>
Date:   Tue, 21 Feb 2017 11:37:53 -0700
From:   Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Keith Busch <keith.busch@...el.com>,
        Myron Stowe <myron.stowe@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Jonathan Corbet <corbet@....net>,
        "David S. Miller" <davem@...emloft.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Emil Velikov <emil.l.velikov@...il.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Ryusuke Konishi <konishi.ryusuke@....ntt.co.jp>,
        Stefan Berger <stefanb@...ux.vnet.ibm.com>,
        Wei Zhang <wzhang@...com>, linux-doc@...r.kernel.org,
        linux-pci@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvme@...ts.infradead.org,
        Stephen Bates <stephen.bates@...rosemi.com>,
        Kurt Schwemmer <kurt.schwemmer@...rosemi.com>
Subject: Re: [PATCH] switchtec: cleanup cdev init

On Sun, Feb 19, 2017 at 09:22:35PM -0700, Logan Gunthorpe wrote:

> Really, in any situation where there's a cdev and a device in the same
> structure, the life cycles of the two become linked but their reference
> counts are not and that is the problem here.

Yes, the cdev must hold a kref on the containing struct otherwise
userspace can trigger a use after free. This cannot be fixed with an
approach inside the open/release function either as the cdev core code
itself relies on the memory to exist.

I've suggested something like this before:

https://lkml.org/lkml/2015/7/8/1066

So I hope this will make it in, it is a step in the right direction.

If it does, would you make another patch to go further? I think
cdev_init should take enough arguments to hold the enclosing kref, API
wise there should be no API to init a cdev without the caller
specifying the enclosing struct's kref. That is the only way we will
stamp this bug-class out.

Eg look at kernel/time/posix-clock.c, it is wrong in the same way as
well - the kref_put in posix_clock_release is not enough to make it
work, clk->cdev is referenced after posix_clock_release returns by the
cdev core so this has a use-after-free.

Jason