Re: [PATCH] Fix: tracepoint: use after rcu free

[Mathieu Desnoyers]

----- Original Message -----
> From: "Steven Rostedt" <rostedt@xxxxxxxxxxx>
> To: "Mathieu Desnoyers" <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx, "Sasha Levin" <sasha.levin@xxxxxxxxxx>, "Oleg Nesterov" <oleg@xxxxxxxxxx>, "Dave
> Jones" <davej@xxxxxxxxxx>
> Sent: Thursday, May 8, 2014 9:05:20 AM
> Subject: Re: [PATCH] Fix: tracepoint: use after rcu free
> 
> On Thu,  8 May 2014 07:47:49 -0400
> Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> 
> > commit de7b2973903c6cc50b31ee5682a69b2219b9919d
> > Author: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > Date:   Tue Apr 8 17:26:21 2014 -0400
> 
> Hi Mathieu,
> 
> Thanks for the quick fix.
> 
> 
> > 
> >     tracepoint: Use struct pointer instead of name hash for reg/unreg
> >     tracepoints
> > 
> > introduces a use after free by calling release_probes on the old struct
> 
> Note when talking about commits already in mainline, use the first 12
> characters of the commit sha1 followed by the subject. Like so:
> 
>  Commit de7b2973903c "tracepoint: Use struct pointer instead of name
>  hash for reg/unreg tracepoints" introduces a use after free ...

OK, will do.

> 
> 
> > tracepoint array before the newly allocated array is published with
> > rcu_assign_pointer. There is a race window where tracepoints (RCU
> > readers) can perform a "use-after-grace-period-after-free", which shows
> > up as a GPF in stress-tests.
> > 
> > Link: https://lkml.org/lkml/2014/5/6/738
> 
> Also, Link tags need to point to lkml.kernel.org.
> 
> Link: http://lkml.kernel.org/r/53698021.5020108@xxxxxxxxxx
> 
> Which is "lkml.kernel.org/r/" followed by the Message-ID.

OK

> 
> We can also include a (even though it's redundant)
> 
>  Fixes: de7b2973903c "tracepoint: Use struct pointer instead of name hash for
>  reg/unreg tracepoints"

Thanks for the pointers, I'll keep those in mind for my next patches.

> 
> I'll update the change log to reflect my comments and then pull this
> into my urgent branch and start testing it.

Thanks!

Mathieu

> 
> Thanks,
> 
> -- Steve
> 
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > CC: Sasha Levin <sasha.levin@xxxxxxxxxx>
> > CC: Oleg Nesterov <oleg@xxxxxxxxxx>
> > CC: Dave Jones <davej@xxxxxxxxxx>
> > ---
> >  kernel/tracepoint.c |    4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
> > index ac5b23c..6620e58 100644
> > --- a/kernel/tracepoint.c
> > +++ b/kernel/tracepoint.c
> > @@ -188,7 +188,6 @@ static int tracepoint_add_func(struct tracepoint *tp,
> >  		WARN_ON_ONCE(1);
> >  		return PTR_ERR(old);
> >  	}
> > -	release_probes(old);
> >  
> >  	/*
> >  	 * rcu_assign_pointer has a smp_wmb() which makes sure that the new
> > @@ -200,6 +199,7 @@ static int tracepoint_add_func(struct tracepoint *tp,
> >  	rcu_assign_pointer(tp->funcs, tp_funcs);
> >  	if (!static_key_enabled(&tp->key))
> >  		static_key_slow_inc(&tp->key);
> > +	release_probes(old);
> >  	return 0;
> >  }
> >  
> > @@ -221,7 +221,6 @@ static int tracepoint_remove_func(struct tracepoint
> > *tp,
> >  		WARN_ON_ONCE(1);
> >  		return PTR_ERR(old);
> >  	}
> > -	release_probes(old);
> >  
> >  	if (!tp_funcs) {
> >  		/* Removed last function */
> > @@ -232,6 +231,7 @@ static int tracepoint_remove_func(struct tracepoint
> > *tp,
> >  			static_key_slow_dec(&tp->key);
> >  	}
> >  	rcu_assign_pointer(tp->funcs, tp_funcs);
> > +	release_probes(old);
> >  	return 0;
> >  }
> >  
> 
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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