When coming from ndisc_netdev_event() in net/ipv6/ndisc.c,
neigh_ifdown() is called with &nd_tbl, locking this while
clearing the proxy neighbor entries when eg. deleting an
interface. Calling the table's pndisc_destructor() with the
lock still held, however, can cause a deadlock: When a
multicast listener is available an IGMP packet of type
ICMPV6_MGM_REDUCTION may be sent out. When reaching
ip6_finish_output2(), if no neighbor entry for the target
address is found, __neigh_create() is called with &nd_tbl,
which it'll want to lock.

Move the elements into their own list, then unlock the table
and perform the destruction.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=199289
Fixes: 6fd6ce2056de ("ipv6: Do not depend on rt->n in ip6_finish_output2().")
Signed-off-by: Wolfgang Bumiller <w.bumil...@proxmox.com>
---
I do feel bad about moving the unlock call. Perhaps returning the
freelist and dealing with it in neigh_ifdown() would be better?

 net/core/neighbour.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 7b7a14abba28..601df647588c 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -292,7 +292,6 @@ int neigh_ifdown(struct neigh_table *tbl, struct net_device 
*dev)
        write_lock_bh(&tbl->lock);
        neigh_flush_dev(tbl, dev);
        pneigh_ifdown(tbl, dev);
-       write_unlock_bh(&tbl->lock);
 
        del_timer_sync(&tbl->proxy_timer);
        pneigh_queue_purge(&tbl->proxy_queue);
@@ -683,7 +682,7 @@ int pneigh_delete(struct neigh_table *tbl, struct net *net, 
const void *pkey,
 
 static int pneigh_ifdown(struct neigh_table *tbl, struct net_device *dev)
 {
-       struct pneigh_entry *n, **np;
+       struct pneigh_entry *n, **np, *freelist = NULL;
        u32 h;
 
        for (h = 0; h <= PNEIGH_HASHMASK; h++) {
@@ -691,16 +690,23 @@ static int pneigh_ifdown(struct neigh_table *tbl, struct 
net_device *dev)
                while ((n = *np) != NULL) {
                        if (!dev || n->dev == dev) {
                                *np = n->next;
-                               if (tbl->pdestructor)
-                                       tbl->pdestructor(n);
-                               if (n->dev)
-                                       dev_put(n->dev);
-                               kfree(n);
+                               n->next = freelist;
+                               freelist = n;
                                continue;
                        }
                        np = &n->next;
                }
        }
+       write_unlock_bh(&tbl->lock);
+       while ((n = freelist)) {
+               freelist = n->next;
+               n->next = NULL;
+               if (tbl->pdestructor)
+                       tbl->pdestructor(n);
+               if (n->dev)
+                       dev_put(n->dev);
+               kfree(n);
+       }
        return -ENOENT;
 }
 
-- 
2.11.0


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