[maint] batman-adv: Reduce tt_global hash refcnt only for removed entry
Commit Message
The batadv_hash_remove is a function which searches the hashtable for an
entry using a needle, a hashtable bucket selection function and a compare
function. It will lock the bucket list and delete an entry when the compare
function matches it with the needle. It returns the pointer to the
hlist_node which matches or NULL when no entry matches the needle.
The batadv_tt_global_free is not itself protected in anyway to avoid that
any other function is modifying the hashtable between the search for the
entry and the call to batadv_hash_remove. It can therefore happen that the
entry either doesn't exist anymore or an entry was deleted which is not the
same object as the needle. In such an situation, the reference counter (for
the reference stored in the hashtable) must not be reduced for the needle.
Instead the reference counter of the actually removed entry has to be
reduced.
Otherwise the reference counter will underflow and the object might be
freed before all its references were dropped. The kref helpers reported
this problem as:
refcount_t: underflow; use-after-free.
Fixes: 7bad46397eff ("batman-adv: protect the local and the global trans-tables with rcu")
Reported-by: Martin Weinelt <martin@linuxlounge.net>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
---
Cc: Antonio Quartulli <a@unstable.cc>
Comments
Hi,
On 24/02/2019 00:09, Sven Eckelmann wrote:
> The batadv_hash_remove is a function which searches the hashtable for an
> entry using a needle, a hashtable bucket selection function and a compare
> function. It will lock the bucket list and delete an entry when the compare
> function matches it with the needle. It returns the pointer to the
> hlist_node which matches or NULL when no entry matches the needle.
>
> The batadv_tt_global_free is not itself protected in anyway to avoid that
> any other function is modifying the hashtable between the search for the
> entry and the call to batadv_hash_remove. It can therefore happen that the
> entry either doesn't exist anymore or an entry was deleted which is not the
> same object as the needle. In such an situation, the reference counter (for
> the reference stored in the hashtable) must not be reduced for the needle.
> Instead the reference counter of the actually removed entry has to be
> reduced.
>
> Otherwise the reference counter will underflow and the object might be
> freed before all its references were dropped. The kref helpers reported
> this problem as:
>
> refcount_t: underflow; use-after-free.
>
> Fixes: 7bad46397eff ("batman-adv: protect the local and the global trans-tables with rcu")
> Signed-off-by: Sven Eckelmann <sven@narfation.org>
> ---
Acked-by: Antonio Quartulli <a@unstable.cc>
@@ -616,14 +616,26 @@ static void batadv_tt_global_free(struct batadv_priv *bat_priv,
struct batadv_tt_global_entry *tt_global,
const char *message)
{
+ struct batadv_tt_global_entry *tt_removed_entry;
+ struct hlist_node *tt_removed_node;
+
batadv_dbg(BATADV_DBG_TT, bat_priv,
"Deleting global tt entry %pM (vid: %d): %s\n",
tt_global->common.addr,
batadv_print_vid(tt_global->common.vid), message);
- batadv_hash_remove(bat_priv->tt.global_hash, batadv_compare_tt,
- batadv_choose_tt, &tt_global->common);
- batadv_tt_global_entry_put(tt_global);
+ tt_removed_node = batadv_hash_remove(bat_priv->tt.global_hash,
+ batadv_compare_tt,
+ batadv_choose_tt,
+ &tt_global->common);
+ if (!tt_removed_node)
+ return;
+
+ /* drop reference of remove hash entry */
+ tt_removed_entry = hlist_entry(tt_removed_node,
+ struct batadv_tt_global_entry,
+ common.hash_entry);
+ batadv_tt_global_entry_put(tt_removed_entry);
}
/**
--
2.20.1