linux-kernel - Re: [PATCH 3/7] seccomp_filter: Enable ftrace-based system call filtering

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Date:	Thu, 28 Apr 2011 12:39:57 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
	kees.cook@...onical.com, eparis@...hat.com, agl@...omium.org,
	mingo@...e.hu, jmorris@...ei.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Michal Marek <mmarek@...e.cz>,
	Oleg Nesterov <oleg@...hat.com>,
	Roland McGrath <roland@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Jiri Slaby <jslaby@...e.cz>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH 3/7] seccomp_filter: Enable ftrace-based system call
 filtering

Quoting Steven Rostedt (rostedt@...dmis.org):
> On Thu, 2011-04-28 at 11:55 -0500, Serge E. Hallyn wrote:
> 
> > ...
> > 
> > >  void __secure_computing(int this_syscall)
> > >  {
> > > -	int mode = current->seccomp.mode;
> > > +	int mode = -1;
> > >  	int * syscall;
> > > -
> > > +	/* Do we need an RCU read lock to access current's state? */
> > 
> > Nope.
> 
> Correct.
> 
> > > - out:
> > > +	rcu_assign_pointer(current->seccomp.state, state);
> > > +	synchronize_rcu();
> > > +	put_seccomp_state(orig_state);  /* for the get */
> > > +
> > > +out:
> > > +	put_seccomp_state(orig_state);  /* for the task */
> > > +	return ret;
> > > +
> > > +free_state:
> > > +	put_seccomp_state(orig_state);  /* for the get */
> > > +	put_seccomp_state(state);  /* drop the dup */
> > >  	return ret;
> > >  }
> > 
> > This looks exactly right.  The only case where put_seccomp_state()
> > might actually lead to freeing the state is where the current's
> > state gets reassigned.  So you need to synchronize_rcu() before
> > that (as you do).  The other cases will only decrement the usage
> > counter, can race with a reader doing (inc; get) but not with a
> > final free, which can only be done here.
> 
> Technically incorrect ;)
> 
> "final free, which can only be done here."
> 
> This is not the only place that a free will happen. But the code is
> correct none-the-less.
> 
> Reader on another CPU ups the orig_state refcount under rcu_readlock,
> but after it ups the refcount it releases the rcu_readlock and continues
> to read this state.
> 
> Current on this CPU calls this function does the synchronize_rcu() and
> calls put on the state. But since the reader still has a ref count on
> it, it does not get freed here.
> 
> When the reader is finally done with the state it calls the put() which
> does the final free on it.
> 
> The code still looks correct, I'm just nitpicking your analysis.

:)  I appreciate the precision.

> > (Rambling above is just me pursuading myself)
> 
> Me rambling too.
> 
> > 
> > > diff --git a/kernel/seccomp_filter.c b/kernel/seccomp_filter.c
> > 
> > Unfortunately your use of filters doesn't seem exactly right.
> > 
> > > +/* seccomp_copy_all_filters - copies all filters from src to dst.
> > > + *
> > > + * @dst: the list_head for seccomp_filters to populate.
> > > + * @src: the list_head for seccomp_filters to copy from.
> > > + * Returns non-zero on failure.
> > > + */
> > > +int seccomp_copy_all_filters(struct list_head *dst,
> > > +			     const struct list_head *src)
> > > +{
> > > +	struct seccomp_filter *filter;
> > > +	int ret = 0;
> > > +	BUG_ON(!dst || !src);
> > > +	if (list_empty(src))
> > > +		goto done;
> > > +	rcu_read_lock();
> > > +	list_for_each_entry(filter, src, list) {
> > > +		struct seccomp_filter *new_filter = copy_seccomp_filter(filter);
> > 
> > copy_seccomp_filter() causes kzalloc to be called.  You can't do that under
> > rcu_read_lock().
> 
> Unless you change the kzalloc to do GFP_ATOMIC. Not sure I'd recommend
> doing that.
> 
> > 
> > I actually thought you were going to be more extreme about the seccomp
> > state than you are:  I thought you were going to tie a filter list to
> > seccomp state.  So adding or removing a filter would have required
> > duping the seccomp state, duping all the filters, making the change in
> > the copy, and then swapping the new state into place.  Slow in the
> > hopefully rare update case, but safe.
> > 
> > You don't have to do that, but then I'm pretty sure you'll need to add
> > reference counts to each filter and use rcu cycles to a reader from
> > having the filter disappear mid-read.
> 
> Or you can preallocate the new filters, call rcu_read_lock(), check if
> the number of old filters is the same or less, if more, call
> rcu_read_unlock, and try allocating more, and then call rcu_read_lock()
> again and repeat. Then just copy the filters to the preallocate ones.
> rcu_read_unlock() and then free any unused allocated filters.
> 
> Maybe a bit messy, but not that bad.

Sounds good.

thanks,
-serge
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