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

This Linux kernel change "batman-adv: Reduce claim 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 da1efb8 (patch) which is from upstream commit 4ba104f. 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 4ba104f.

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

commit 4ba104f468bbfc27362c393815d03aa18fb7a20f 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_bla_del_claim 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: 23721387c409 ("batman-adv: add basic bridge loop avoidance code")
Signed-off-by: Sven Eckelmann <[email protected]>
Signed-off-by: Simon Wunderlich <[email protected]>
[bwh: Backported to 3.16: keep using batadv_claim_free_ref()]
Signed-off-by: Ben Hutchings <[email protected].uk>

There are 16 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 net/batman-adv/bridge_loop_avoidance.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/net/batman-adv/bridge_loop_avoidance.c b/net/batman-adv/bridge_loop_avoidance.c
index 335401d..2a1611e 100644
--- a/net/batman-adv/bridge_loop_avoidance.c
+++ b/net/batman-adv/bridge_loop_avoidance.c
@@ -677,6 +677,8 @@ static void batadv_bla_del_claim(struct batadv_priv *bat_priv,
                 const uint8_t *mac, const unsigned short vid)
 {
    struct batadv_bla_claim search_claim, *claim;
+   struct batadv_bla_claim *claim_removed_entry;
+   struct hlist_node *claim_removed_node;

    ether_addr_copy(search_claim.addr, mac);
    search_claim.vid = vid;
@@ -687,10 +689,18 @@ static void batadv_bla_del_claim(struct batadv_priv *bat_priv,
    batadv_dbg(BATADV_DBG_BLA, bat_priv, "bla_del_claim(): %pM, vid %d\n",
           mac, BATADV_PRINT_VID(vid));

-   batadv_hash_remove(bat_priv->bla.claim_hash, batadv_compare_claim,
-              batadv_choose_claim, claim);
-   batadv_claim_free_ref(claim); /* reference from the hash is gone */
+   claim_removed_node = batadv_hash_remove(bat_priv->bla.claim_hash,
+                       batadv_compare_claim,
+                       batadv_choose_claim, claim);
+   if (!claim_removed_node)
+       goto free_claim;

+   /* reference from the hash is gone */
+   claim_removed_entry = hlist_entry(claim_removed_node,
+                     struct batadv_bla_claim, hash_entry);
+   batadv_claim_free_ref(claim_removed_entry);
+
+free_claim:
    /* don't need the reference from hash_find() anymore */
    batadv_claim_free_ref(claim);
 }

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