linux-kernel - Re: [PATCH 2/4] cxl/mem: Fix cdev_device

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Date:   Mon, 29 Mar 2021 14:03:37 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     linux-cxl@...r.kernel.org, "Weiny, Ira" <ira.weiny@...el.com>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        "Schofield, Alison" <alison.schofield@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] cxl/mem: Fix cdev_device_add() error handling

On Thu, Mar 25, 2021 at 10:12 AM Jason Gunthorpe <jgg@...dia.com> wrote:
>
> On Wed, Mar 24, 2021 at 02:01:56PM -0700, Dan Williams wrote:
> > If cdev_device_add() fails then the allocation performed by
> > dev_set_name() is leaked. Use put_device(), not open coded release, for
> > device_add() failures.
> >
> > The comment is obsolete because direct err_id failures need not worry
> > about the device being live.
> >
> > The release method expects the percpu_ref is already dead, so
> > percpu_ref_kill() is needed before put_device(). However, given that the
> > cdev was partially live wait_for_completion() also belongs in the
> > release method.
> >
> > Fixes: b39cb1052a5c ("cxl/mem: Register CXL memX devices")
> > Reported-by: Jason Gunthorpe <jgg@...dia.com>
> > Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> >  drivers/cxl/mem.c |   16 ++++++----------
> >  1 file changed, 6 insertions(+), 10 deletions(-)
> >
> > diff --git a/drivers/cxl/mem.c b/drivers/cxl/mem.c
> > index 30bf4f0f3c17..e53d573ae4ab 100644
> > +++ b/drivers/cxl/mem.c
> > @@ -1049,6 +1049,7 @@ static void cxl_memdev_release(struct device *dev)
> >  {
> >       struct cxl_memdev *cxlmd = to_cxl_memdev(dev);
> >
> > +     wait_for_completion(&cxlmd->ops_dead);
>
> This only works because the fops stuff is not right, a kref shouldn't
> have a completion like this.
>
> Also, don't use devm for unregister. That just makes it extra-hard to
> write the driver remove function correctly.

To date there is no driver remove function, however if that changes
then I expect all the devm needs to go.

>
> > @@ -1157,7 +1158,6 @@ static void cxlmdev_unregister(void *_cxlmd)
> >
> >       percpu_ref_kill(&cxlmd->ops_active);
> >       cdev_device_del(&cxlmd->cdev, dev);
> > -     wait_for_completion(&cxlmd->ops_dead);
> >       cxlmd->cxlm = NULL;
> >       put_device(dev);
> >  }
> > @@ -1210,20 +1210,16 @@ static int cxl_mem_add_memdev(struct cxl_mem *cxlm)
> >       cdev_init(cdev, &cxl_memdev_fops);
> >
> >       rc = cdev_device_add(cdev, dev);
> > -     if (rc)
> > -             goto err_add;
> > +     if (rc) {
> > +             percpu_ref_kill(&cxlmd->ops_active);
> > +             put_device(dev);
>
> This must be one high performance ioctl to warrant the percpu ref.. If
> it is not high performance use a rwsem, otherwise I'd suggest srcu as
> a faster/simpler alternative.

The plan is to refactor and share the same reference counted fops
mechanism as debugfs and make that common infrastructure. However, in
the meantime I think global srcu is suitable.

>
> This is a use-after-free:
>
> static long cxl_memdev_ioctl(struct file *file, unsigned int cmd,
>                              unsigned long arg)
> {
>         struct cxl_memdev *cxlmd;
>         struct inode *inode;
>         int rc = -ENOTTY;
>
>         inode = file_inode(file);
>         cxlmd = container_of(inode->i_cdev, typeof(*cxlmd), cdev);
>        ^^^^^ can be freed memory
>
> ioctl needs to store the cxlmd in file->private_data and
> open()/release() need to do get/put device on it so the memory stays
> around. This is why open gets the inode as an argument and ioctl/etc
> does not.

Ugh, exactly why I was motivated to attempt to preclude this with new
core infrastructure that attempted to fix this centrally [1]. Remove
the  possibility of "others" getting this wrong. However after my
initial idea bounced off Greg then I ended up shipping this bug in the
local rewrite. I think the debugfs api gets this right in terms of
centralizing the reference count management, and I want to see
something similar for common driver ioctl patterns.

[1]: http://lore.kernel.org/r/CAPcyv4hGxLZGEkfnqdLfF-a1CzfEjLux-TBxXztbknFhEe9mYA@mail.gmail.com

>
> The ordering cxlmdev_unregister should mirror the ordering in create
> so cdev_device_del should be first

Sure.