[PATCH][NEIGH] Fix race between neigh_parms_release and neightbl_fill_parms

[PATCH][NEIGH] Fix race between neigh_parms_release and neightbl_fill_parms

[Pavel Emelyanov]

The neightbl_fill_parms() is called under the write-locked
tbl->lock and accesses the parms->dev. The negh_parm_release()
calls the dev_put(parms->dev) without this lock. This 
creates a tiny race window on which the parms contains
potentially stale dev pointer.

To fix this race it's enough to move the dev_put() upper
under the tbl->lock, but note, that the parms are held by
neighbors and thus can live after the neigh_parms_release()
is called, so we still can have a parm with bad dev pointer.

I didn't find where the neigh->parms->dev is accessed, but 
still think that putting the dev is to be done in a place,
where the parms are really freed. Am I right with that?

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>

---

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 29b8ee4..cc8a2f1 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -1316,8 +1316,6 @@ void neigh_parms_release(struct neigh_table *tbl, struct neigh_parms *parms)
 			*p = parms->next;
 			parms->dead = 1;
 			write_unlock_bh(&tbl->lock);
-			if (parms->dev)
-				dev_put(parms->dev);
 			call_rcu(&parms->rcu_head, neigh_rcu_free_parms);
 			return;
 		}
@@ -1328,6 +1326,8 @@ void neigh_parms_release(struct neigh_table *tbl, struct neigh_parms *parms)
 
 void neigh_parms_destroy(struct neigh_parms *parms)
 {
+	if (parms->dev)
+		dev_put(parms->dev);
 	kfree(parms);
 }
 

Re: [PATCH][NEIGH] Fix race between neigh_parms_release and neightbl_fill_parms

[David Miller]

From: Pavel Emelyanov <xemul@openvz.org>
Date: Thu, 10 Jan 2008 13:56:53 +0300

> The neightbl_fill_parms() is called under the write-locked
> tbl->lock and accesses the parms->dev. The negh_parm_release()
> calls the dev_put(parms->dev) without this lock. This 
> creates a tiny race window on which the parms contains
> potentially stale dev pointer.
> 
> To fix this race it's enough to move the dev_put() upper
> under the tbl->lock, but note, that the parms are held by
> neighbors and thus can live after the neigh_parms_release()
> is called, so we still can have a parm with bad dev pointer.
> 
> I didn't find where the neigh->parms->dev is accessed, but 
> still think that putting the dev is to be done in a place,
> where the parms are really freed. Am I right with that?
> 
> Signed-off-by: Pavel Emelyanov <xemul@openvz.org>

It is accessed in lookup_neigh_parms(), neightbl_fill_parms(), and
neightbl_fill_info() (hmmm, that BUG_ON(tbl->parms.dev) is cute).

You fix looks correct, patch applied, thanks!

Re: [PATCH][NEIGH] Fix race between neigh_parms_release and neightbl_fill_parms

[Andrey Rahmatullin]

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This patch causes messages like 'unregister_netdevice: waiting for ppp0 to
become free. Usage count = 1' when I try 'ifdown ppp0'. This happens in all
kernel versions containing this commit and doesn't happen in 2.6.24-rc8
with this commit reverted.

-- 
WBR, wRAR (ALT Linux Team)

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