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

[Jiri Olsa <jolsa@redhat.com>]

On Fri, Jun 18, 2010 at 12:50:40AM +0100, David Howells wrote:
> Jiri Olsa <jolsa@redhat.com> wrote:
> 
> > maybe I have better solution...
> > 
> > I think there's no need to get the cred refference as long as
> > the 'cred' handling stays inside the rcu_read_lock block.
> 
> I think this is right.  There should be no need for a synchronize_rcu() call
> to be added in commit_creds() with this as commit_creds() calls put_cred()
> which will defer the destruction until the RCU grace period is up anyway.
> 
> Whilst I'd prefer to call get_cred() in task_state(), as you point out (and I
> hadn't considered), this may see an cred struct that has been detached from
> its pointer on another CPU and had its usage count reduced to 0.
> 
> In such a case, we can't simply increment the count and then decrement it
> again later as it's already on the RCU destruction queue and can't necessarily
> be removed so that it can be added back in.
> 
> What could be done, though I'm not sure it's worth it, is to use
> atomic_inc_not_zero() and loop around if the cred struct has gone out of
> service when we try and access it and reread the pointer.
> 
> The advantage of this would be that we could manage to hold the RCU read lock
> for as little time as possible.
> 
> Acked-by: David Howells <dhowells@redhat.com>

thanks,

in case the there's need for some consistent comment :)
patch stays

wbr,
jirka



---
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>
---
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");
 }