[PATCH v3] lib/rhashtable: clear stale iter->p on table restart

[PATCH v3] lib/rhashtable: clear stale iter->p on table restart

[Cen Zhang (Microsoft)]

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 (lib/rhashtable.c:1016)
   __netlink_diag_dump+0x160/0x760 (net/netlink/diag.c:122)
   netlink_diag_dump+0xc2/0x240
   netlink_dump+0x5bc/0x1270
   netlink_recvmsg+0x7a3/0x980
   sock_recvmsg+0x1bc/0x200
   __sys_recvfrom+0x1d4/0x2c0

Fixes: 5d240a8936f6 ("rhashtable: improve rhashtable_walk stability when
stop/start used.")
Reported-by: AutonomousCodeSecurity@microsoft.com
Closes: https://lore.kernel.org/linux-crypto/CAB8m9Wh559e+=n8z51gB8DrbEyCc2mc0MgGjrRR6_VXBmU=2AQ@mail.gmail.com
Signed-off-by: Cen Zhang (Microsoft) <blbllhy@gmail.com>
---
v3: Solved patch format issue
v2: Fix commit subject in Fixes tag
Link: https://lore.kernel.org/linux-crypto/CAB8m9Wh559e+=n8z51gB8DrbEyCc2mc0MgGjrRR6_VXBmU=2AQ@mail.gmail.com

 lib/rhashtable.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index 40cfb38ac..d459bef24 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -878,6 +878,7 @@ int rhashtable_walk_start_check(struct rhashtable_iter *iter)
 		iter->walker.tbl = rht_dereference_rcu(ht->tbl, ht);
 		iter->slot = 0;
 		iter->skip = 0;
+		iter->p = NULL;
 		return -EAGAIN;
 	}
 
-- 
2.53.0

Re: [PATCH v3] lib/rhashtable: clear stale iter->p on table restart

[Herbert Xu]

On Tue, Jul 07, 2026 at 12:41:15PM -0400, Cen Zhang (Microsoft) wrote:
> 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.

Maybe I'm misreading the original patch (in the Fixes header).  But
it seems the whole point of having it is to look for iter->p in the
new table.  Even if the hash table remains the same iter->p could have
been freed since we hold no reference to that object.

If that is the case, then resetting iter->p on a resize doesn't
fix this at all since the root cause is that iter->p is being
held with no reference.

I think we should just revert the original patch since the whole
concept doesn't seem to work (although it's salvageable for the
non-rhlist case).

Thanks,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt