Re: [PATCH] device-dax: don't set kobj parent during cdev init

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Fri, Feb 10, 2017 at 02:25:35PM -0800, Dan Williams wrote:
> On Fri, Feb 10, 2017 at 12:17 PM, Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> > On Fri, Feb 10, 2017 at 11:41:20AM -0800, Dan Williams wrote:
> >> On Fri, Feb 10, 2017 at 11:19 AM, Logan Gunthorpe <logang@deltatee.com> wrote:
> >> > I copied this code and per feedback from Greg Kroah-Hartman [1] the
> >> > cdev's kobject's parent should not be set to the related device.
> >> > This should have minor consequences but isn't doing what anyone
> >> > expects it to.
> >> >
> >> > This patch then fixes device-dax so it doesn't make the same mistake.
> >> >
> >> > [1] https://lkml.org/lkml/2017/2/10/370
> >> >
> >> > Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
> >>
> >> Thanks for following up with this fix, but this causes a
> >> use-after-free regression:
> >>
> >>  general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC
> >>  [..]
> >>  Call Trace:
> >>   vsnprintf+0x2d7/0x500
> >>   snprintf+0x49/0x60
> >>   dev_vprintk_emit+0x68/0x230
> >>   ? debug_lockdep_rcu_enabled+0x1d/0x20
> >>   ? trace_hardirqs_off+0xd/0x10
> >>   ? cmpxchg_double_slab.isra.70+0x15a/0x1c0
> >>   ? __slab_free+0x134/0x290
> >>   dev_printk_emit+0x4e/0x70
> >>   __dynamic_dev_dbg+0xc8/0x110
> >>   ? __lock_acquire+0x33d/0x1290
> >>   dax_dev_huge_fault+0xee/0x570 [dax]
> >>   __handle_mm_fault+0x5aa/0x10a0
> >>   handle_mm_fault+0x154/0x350
> >>   ? handle_mm_fault+0x3c/0x350
> >>   __do_page_fault+0x26b/0x4c0
> >>   trace_do_page_fault+0x58/0x270
> >>   do_async_page_fault+0x1a/0xa0
> >>   async_page_fault+0x28/0x30
> >>
> >> I added this reference explicitly so the parent struct device has the
> >> correct lifetime after this feedback from Al.
> >>
> >>    https://lists.01.org/pipermail/linux-nvdimm/2016-August/006563.html
> >>
> >> ...so I'm wondering what the actual problem is with setting cdev->parent?
> >
> > It shouldn't do anything at all.  The kobject in a cdev isn't a "normal"
> > kobject, it doesn't show up in sysfs, or anywhere else.  It's used for
> > an internal representation to the cdev code (a kmap) to look up the
> > object to call when userspace opens the device node in a quick manner.
> >
> > Now changing from initialize/add to just register, does do different
> > things, perhaps that is the issue here.  Just try removing the
> > cdev->kobject parent stuff and see if that causes a problem or not.
> >
> 
>  That doesn't help.  I rely on the "kobject_get(p->kobj.parent);" in
> cdev_add() to pin my device and cdev_default_release() to free it.

"pin it" where?  Why do you need this?  That feels really "odd" to me...
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