[PATCH] lib/rhashtable: clear stale iter->p on table restart

[Cen Zhang]

rhashtable_walk_start_check() has two restart paths when resuming a
walk.  When iter->walker.tbl is valid, it re-validates iter->p against
the table and sets iter->p = NULL if the object is gone.  When
iter->walker.tbl is NULL (table was freed during resize), it resets
slot and skip but forgets to clear iter->p.

rhashtable_walk_next() then dereferences the stale iter->p, reading
freed memory.  This is a use-after-free.

Any caller that does multi-fragment rhashtable walks across
walk_stop/walk_start boundaries is affected.  Concrete cases include
netlink_diag (__netlink_diag_dump in net/netlink/diag.c) and TIPC
(tipc_nl_sk_walk in net/tipc/socket.c).

Crash stack (netlink_diag):
  BUG: KASAN: slab-use-after-free in rhashtable_walk_next+0x365/0x3c0
  Read of size 8 at addr ffff88801a9d2438 (freed kmalloc-2k, offset 1080)
  Call Trace:
   rhashtable_walk_next+0x365/0x3c0
   __netlink_diag_dump+0x160/0x760
   netlink_diag_dump+0xc2/0x240
   netlink_dump+0x5bc/0x1270
   netlink_recvmsg+0x7a3/0x980
   sock_recvmsg+0x1bc/0x200
   __sys_recvfrom+0x1d4/0x2c0

Fixes: 5d240a8936f6 ("rhashtable: add rhashtable_walk_start_check()")
Reported-by: AutonomousCodeSecurity@xxxxxxxxxxxxx
Signed-off-by: Cen Zhang (Microsoft) <blbllhy@xxxxxxxxx>
---
 lib/rhashtable.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/lib/rhashtable.c b/lib/rhashtable.c
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -877,6 +877,7 @@ int rhashtable_walk_start_check(struct
rhashtable_iter *iter)
        if (!iter->walker.tbl) {
                iter->walker.tbl = rht_dereference_rcu(ht->tbl, ht);
                iter->slot = 0;
                iter->skip = 0;
+               iter->p = NULL;
                return -EAGAIN;
        }