linux-kernel - Re: [PATCH] cred - synchronize rcu before releasing cred

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Date:	Wed, 16 Jun 2010 14:57:08 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	dhowells@...hat.com, linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] cred - synchronize rcu before releasing cred

On Wed, Jun 16, 2010 at 02:45:14PM +0200, Eric Dumazet wrote:
> Le mercredi 16 juin 2010 à 14:24 +0200, Jiri Olsa a écrit :
> > hi,
> > 
> > BZ 591015 - kernel BUG at kernel/cred.c:168
> > https://bugzilla.redhat.com/show_bug.cgi?id=591015
> > 
> > Above bugzilla reported bug during the releasing of
> > old cred structure.
> > 
> > There is reproducer attached to the bugzilla.
> > 
> > The issue is caused by releasing old cred struct while other
> > kernel path might be still using it. This leads to cred->usage
> > inconsistency inside the __put_cred and triggering the bug.
> > 
> > Following kernel paths are affected:
> > 
> > The CPU1 path is setting the new groups creds.
> > The CPU2 path is cat /proc/PID/status
> > 
> > 
> > CPU 1                              CPU 2
> > 
> > sys_setgroups                      proc_pid_status      
> >   set_current_groups                 task_state
> >     commit_creds                       rcu_read_lock
> >       put_cred                         ...
> >         __put_cred                     get_cred
> >           BUG_ON(usage != 0)           ...
> >                                        rcu_read_unlock
> >                                        
> > 
> > 
> > If __put_cred got executed during the CPU2 holding the reference
> > the BUG_ON inside __put_cred is trigered.
> > 
> > I put synchronize_rcu before put_cred call, so we are sure
> > all the rcu_read_lock blocks are finished, and if needed, using
> > new creds.
> > 
> > Also I moved rcu_read_unlock below the put_cred for consistency.
> > 
> > 
> > wbr,
> > jirka
> > 
> > 
> > Signed-off-by: Jiri Olsa <jolsa@...hat.com>
> > ---
> >  fs/proc/array.c |    3 ++-
> >  kernel/cred.c   |    6 ++++++
> >  2 files changed, 8 insertions(+), 1 deletions(-)
> > 
> > diff --git a/fs/proc/array.c b/fs/proc/array.c
> > index 9b58d38..c0e60d1 100644
> > --- a/fs/proc/array.c
> > +++ b/fs/proc/array.c
> > @@ -199,7 +199,6 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
> >  		"FDSize:\t%d\n"
> >  		"Groups:\t",
> >  		fdt ? fdt->max_fds : 0);
> > -	rcu_read_unlock();
> >  
> >  	group_info = cred->group_info;
> >  	task_unlock(p);
> > @@ -208,6 +207,8 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
> >  		seq_printf(m, "%d ", GROUP_AT(group_info, g));
> >  	put_cred(cred);
> >  
> > +	rcu_read_unlock();
> > +
> >  	seq_printf(m, "\n");
> >  }
> >  
> > diff --git a/kernel/cred.c b/kernel/cred.c
> > index a2d5504..4872f12 100644
> > --- a/kernel/cred.c
> > +++ b/kernel/cred.c
> > @@ -510,6 +510,12 @@ int commit_creds(struct cred *new)
> >  	    new->fsgid != old->fsgid)
> >  		proc_id_connector(task, PROC_EVENT_GID);
> >  
> > +	/*
> > +	 * New cred is set already, now let the old one
> > +	 * chance to disappear on other CPUs.
> > +	 */
> > +	synchronize_rcu();
> > +
> >  	/* release the old obj and subj refs both */
> >  	put_cred(old);
> >  	put_cred(old);
> > --
> 
> I respectfully suggest that every patch adding a synchronize_rcu() call
> be reviewed by Paul himself. This is really the last option that should
> be considered, since it adds a big delay.

no problem, I wasn't aware of this

> 
> Can you explain why other solutions cannot be used ?

I'm not sure which ones do you mean.. I'm not RCU expert, and
this one occured to me as probable fix, but if there're other
ways, no problem ;)

thanks,
jirka
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