linux-kernel - Re: [PATCH v5] Fix rmmod/read/write races in /proc entries

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Date:	Fri, 16 Mar 2007 12:16:13 +0300
From:	Alexey Dobriyan <adobriyan@...ru>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	viro@....linux.org.uk, linux-kernel@...r.kernel.org,
	devel@...nvz.org, adobriyan@...il.com
Subject: Re: [PATCH v5] Fix rmmod/read/write races in /proc entries

On Thu, Mar 15, 2007 at 05:53:04PM -0800, Andrew Morton wrote:
> My, what a lot of code you have here.  I note that nobody can be assed even
> reviewing it.  Now why is that?

I hope, Al could find some time again.

> On Sun, 11 Mar 2007 20:04:56 +0300 Alexey Dobriyan <adobriyan@...ru> wrote:
> > Fix following races:
> > ===========================================
> > 1. Write via ->write_proc sleeps in copy_from_user(). Module disappears
> >    meanwhile. Or, more generically, system call done on /proc file, method
> >    supplied by module is called, module dissapeares meanwhile.
> >
> >    pde = create_proc_entry()
> >    if (!pde)
> > 	return -ENOMEM;
> >    pde->write_proc = ...
> > 				open
> > 				write
> > 				copy_from_user
> >    pde = create_proc_entry();
> >    if (!pde) {
> > 	remove_proc_entry();
> > 	return -ENOMEM;
> > 	/* module unloaded */
> >    }
>
> We usually fix that race by pinning the module: make whoever registered the
> proc entries also register their THIS_MODULE, do a try_module_get() on it
> before we start to play with data structures which the module owns.
>
> Can we do that here?

We can, but it will be unreliable:

Typical proc entry creation sequence is

	pde = create_proc_entry(...);
	if (pde)
		pde->owner = THIS_MODULE;

Right after create_proc_entry() ->owner is NULL, so try_module_get()
won't do anything, but proc_delete_inode() could put module which was
never getted.

This should fixable by always setting ->owner before proc entry is
glued to proc entries tree. Something like this:

	#define create_proc_entry(...) __create_proc_entry(..., THIS_MODULE)

However, I think it's not enough: delete_module(2) first waits for
refcount becoming zero, only then calls modules's exit function which
starts removing proc entries. In between, proc entries are accessible
and fully-functional, so try_module_get() can again get module and
module_put(pde->owner) can happen AFTER module dissapears.
What will it put?

And how can you fix that? The only way I know is to REMOVE ->owner
completely, once we agree on this pde_users/pde_unload_lock stuff.

> And is the above race fix related to the below one in any fashion?
> > ==========================================
> > 2. bogo-revoke aka proc_kill_inodes()
> >
> >   remove_proc_entry		vfs_read
> >   proc_kill_inodes		[check ->f_op validness]
> > 				[check ->f_op->read validness]
> > 				[verify_area, security permissions checks]
> > 	->f_op = NULL;
> > 				if (file->f_op->read)
> > 					/* ->f_op dereference, boom */
>
> So you fixed this via sort-of-refcounting on pde->pde_users.
>
> hmm.

Probably, you're right and they are independently fixable. It's all
about following 3 lines after all. My turn to hmm...

> > -		proc_kill_inodes(de);
> > +		if (!S_ISREG(de->mode))
> > +			proc_kill_inodes(de);



> > +	spin_lock(&pde->pde_unload_lock);
> > +	pde->pde_users--;
> > +	if (pde->pde_unload_completion && pde->pde_users == 0)
> > +		complete(pde->pde_unload_completion);
> > +out_unlock:
> > +	spin_unlock(&pde->pde_unload_lock);
>
> The above six lines happen rather a lot - perhaps it could be placed in a
> helper funtion?

OK. Should I send incremental updates or full patch again?

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