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

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Date:	Wed, 16 Jun 2010 14:45:14 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jiri Olsa <jolsa@...hat.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

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.

Can you explain why other solutions cannot be used ?

Thanks


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