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SubjectRe: [PATCH net-next] net: dsa: remove tree refcount
Date
Hi Andrew,

Andrew Lunn <andrew@lunn.ch> writes:

> On Thu, Nov 09, 2017 at 12:36:52PM -0500, Vivien Didelot wrote:
>> Setting the refcount to 0 when allocating a tree to match the number of
>> switch devices it holds may cause an 'increment on 0; use-after-free'.
>>
>> Tracking the number of devices in a tree with a kref is not really
>> appropriate anyway so removes it completely in favor of a basic counter.
>
> How are you protecting this basic counter? switches can come and go at
> random, modules are loaded and unloaded, probing can happen in
> parallel, probes can fail with EPROBE_DEFFER causing a switch to
> unregister itself while others are registering themselves, etc.

As for the kref, the counter is protected by dsa2_mutex which locks
switch registration, nothing changed.

> The point of using a kref is that it is a well known kernel method of
> safely handling this situation. When the last member of the tree goes
> away, we safely and atomically remove the tree. It worked well for a
> few years, until you refactored it. Maybe the correct solution is to
> revert your change?

The kref doesn't add any value here and make the code more complex. If
you prefer to keep it, a simple alternative can be provided to init the
refcount to 1 at initialization and decrement it after registration:

diff --git a/net/dsa/dsa2.c b/net/dsa/dsa2.c
index fd54a8e17986..1fb8beb66493 100644
--- a/net/dsa/dsa2.c
+++ b/net/dsa/dsa2.c
@@ -51,9 +51,7 @@ static struct dsa_switch_tree *dsa_tree_alloc(int index)
INIT_LIST_HEAD(&dst->list);
list_add_tail(&dsa_tree_list, &dst->list);

- /* Initialize the reference counter to the number of switches, not 1 */
kref_init(&dst->refcount);
- refcount_set(&dst->refcount.refcount, 0);

return dst;
}
@@ -69,7 +67,9 @@ static struct dsa_switch_tree *dsa_tree_touch(int index)
struct dsa_switch_tree *dst;

dst = dsa_tree_find(index);
- if (!dst)
+ if (dst)
+ dsa_tree_get(dst);
+ else
dst = dsa_tree_alloc(index);
return dst;
@@ -765,6 +765,8 @@ int dsa_register_switch(struct dsa_switch *ds)

mutex_lock(&dsa2_mutex);
err = dsa_switch_probe(ds);
+ if (ds->dst)
+ dsa_tree_put(ds->dst);
mutex_unlock(&dsa2_mutex);
return err;


Getting rid of the refcount seems simpler, but we can use this
alternative instead. Let me know what you prefer.


Thanks,

Vivien

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