linux-kernel - Re: [PATCH] lguest: explicitly setup /dev/lguest private

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Date:	Tue, 7 Apr 2015 10:55:44 +0200
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Daniel Baluta <daniel.baluta@...el.com>
Cc:	Tom Van Braeckel <tomvanbraeckel@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	lguest <lguest@...ts.ozlabs.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kbuild test robot <fengguang.wu@...el.com>, lkp@...org
Subject: Re: [PATCH] lguest: explicitly setup /dev/lguest private_data

On Tue, Apr 07, 2015 at 11:34:25AM +0300, Daniel Baluta wrote:
> On Tue, Apr 7, 2015 at 11:18 AM, Tom Van Braeckel
> <tomvanbraeckel@...il.com> wrote:
> > The private_data member of the /dev/lguest device file is used to hold
> > the current struct lguest and needs to be set to NULL to signify that
> > no initialization has taken place.
> >
> > We explicitly set it to NULL to be independent of whatever value the
> > misc subsystem initializes it to.
> >
> > Signed-off-by: Tom Van Braeckel <tomvanbraeckel@...il.com>
> > ---
> > Backstory:
> > ==========
> > The misc subsystem used to initialize a file's private_data to point to
> > the misc device when a driver had registered a custom open file
> > operation and initialized it to NULL when a custom open file operation
> > had *not* been provided.
> >
> > This subtle quirk was confusing, to the point where kernel code
> > registered *empty* file open operations to have private_data point to
> > the misc device structure.
> >
> > And it lead to bugs, where the addition or removal of a custom open
> > file operation surprisingly changed the initial contents of a file's
> > private_data structure.
> >
> > The misc subsystem is currently underdoing changes to *always* set
> > private_data to point to the misc device instead of only doing this
> > when a custom open file operation has been registered.
> >
> > Intel's 0day kernel testing robot discovered that the lguest driver
> > depended on it implicitly being initialized to NULL, as Fengguang Wu
> > reported. Thanks a lot for all the hard work!
> >
> >  drivers/lguest/lguest_user.c | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/lguest/lguest_user.c b/drivers/lguest/lguest_user.c
> > index c4c6113..054bf70 100644
> > --- a/drivers/lguest/lguest_user.c
> > +++ b/drivers/lguest/lguest_user.c
> > @@ -98,6 +98,17 @@ static int trap(struct lg_cpu *cpu, const unsigned long __user *input)
> >         return 0;
> >  }
> >
> > +/*
> > + * Set up the /dev/lguest file structure
> > + * The file's private_data will hold the "struct lguest" after
> > + * initialization and is used to check whether it is already initialized.
> > + */
> > +static int open(struct inode *inode, struct file *file)
> > +{
> > +       file->private_data = NULL;
> > +       return 0;
> > +}
> > +
> >  /*L:040
> >   * Once our Guest is initialized, the Launcher makes it run by reading
> >   * from /dev/lguest.
> > @@ -405,10 +416,11 @@ static int close(struct inode *inode, struct file *file)
> >   *
> >   * We begin our understanding with the Host kernel interface which the Launcher
> >   * uses: reading and writing a character device called /dev/lguest.  All the
> > - * work happens in the read(), write() and close() routines:
> > + * work happens in the open(), read(), write() and close() routines:
> >   */
> >  static const struct file_operations lguest_fops = {
> >         .owner   = THIS_MODULE,
> > +       .open    = open,
> >         .release = close,
> >         .write   = write,
> >         .read    = read,
> 
> Hmm, isn't this already fixed?
> 
> https://lkml.org/lkml/2015/3/23/319

Ah, this might be a cross-tree issue then, the 0-day bot tested my tree
without this change in it, and hit the problem.  So all is good when we
merge with Linus for 4.1-rc1.

But to be "safe" I could queue this up to my tree as well, any objection
to that?

thanks,

greg k-h
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