batman-adv: Reduce tt_global hash refcnt only for removed entry [Linux 3.16.72]

This Linux kernel change "batman-adv: Reduce tt_global hash refcnt only for removed entry" is included in the Linux 3.16.72 release. This change is authored by Sven Eckelmann <sven [at] narfation.org> on Sat Feb 23 14:27:10 2019 +0100. The commit for this change in Linux stable tree is 6aa8ab0 (patch) which is from upstream commit f131a56. The same Linux upstream change may have been applied to various maintained Linux releases and you can find all Linux releases containing changes from upstream f131a56.

batman-adv: Reduce tt_global hash refcnt only for removed entry

commit f131a56880d10932931e74773fb8702894a94a75 upstream.

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: 7683fdc1e886 ("batman-adv: protect the local and the global trans-tables with rcu")
Reported-by: Martin Weinelt <[email protected]>
Signed-off-by: Sven Eckelmann <[email protected]>
Acked-by: Antonio Quartulli <[email protected]>
Signed-off-by: Simon Wunderlich <[email protected]>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <[email protected]>

There are 18 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 net/batman-adv/translation-table.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/net/batman-adv/translation-table.c b/net/batman-adv/translation-table.c
index 64efccf..422d54f 100644
--- a/net/batman-adv/translation-table.c
+++ b/net/batman-adv/translation-table.c
@@ -483,14 +483,26 @@ 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_free_ref(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_free_ref(tt_removed_entry);
 }

 /**

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