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

[Jiri Olsa <jolsa@redhat.com>]

hi,
any feedback?

jirka

On Fri, Jun 25, 2010 at 09:33:33AM -0400, Jiri Olsa wrote:
> 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 think there's no need to get the cred refference as long as
> the 'cred' handling stays inside the rcu_read_lock block.
> 
> And the condition of __task_cred 'make sure task doesn't go away',
> is done by proc_single_show as this is the proc file.
> 
> wbr,
> jirka
> 
> 
> Signed-off-by: Jiri Olsa <jolsa@redhat.com>
> Acked-by: David Howells <dhowells@redhat.com>
> ---
> diff --git a/fs/proc/array.c b/fs/proc/array.c
> index 9b58d38..ac3b3a4 100644
> --- a/fs/proc/array.c
> +++ b/fs/proc/array.c
> @@ -176,7 +176,7 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
>  		if (tracer)
>  			tpid = task_pid_nr_ns(tracer, ns);
>  	}
> -	cred = get_cred((struct cred *) __task_cred(p));
> +	cred = __task_cred(p);
>  	seq_printf(m,
>  		"State:\t%s\n"
>  		"Tgid:\t%d\n"
> @@ -199,15 +199,14 @@ 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);
>  
>  	for (g = 0; g < min(group_info->ngroups, NGROUPS_SMALL); g++)
>  		seq_printf(m, "%d ", GROUP_AT(group_info, g));
> -	put_cred(cred);
>  
> +	rcu_read_unlock();
>  	seq_printf(m, "\n");
>  }
>