Re: [PATCH net v3 1/1] net/sched: sch_teql: Introduce slaves_lock to avoid race condition and UAF

[Paolo Abeni <pabeni@redhat.com>]

On 6/28/26 1:12 PM, Jamal Hadi Salim wrote:
> The teql master->slaves singly linked list is not protected against
> multiple writes. It can be mod'ed concurently from teql_master_xmit(),
> teql_dequeue(), teql_init() and teql_destroy() without holding any list
> lock or RCU protection.
> 
> zdi-disclosures@trendmicro.com has demonstrated that the qdisc is freed
> after an RCU grace period, but teql_master_xmit() running on another
> CPU can still hold a stale pointer into the list, resulting in a
> slab-use-after-free:
> 
> BUG: KASAN: slab-use-after-free in teql_master_xmit+0xf0f/0x16b0
> Read of size 8 at addr ffff888013fb0440 by task poc/332
> Freed 512-byte region [ffff888013fb0400, ffff888013fb0600) (kmalloc-512)
> 
> The fix?
> Add a per-master slaves_lock spinlock that serializes all mutations of
> master->slaves and the NEXT_SLAVE() links in teql_destroy() and
> teql_qdisc_init(). teql_master_xmit() also takes the same slaves_lock
> around those updates.
> Annotate master->slaves and the per-slave ->next pointer with __rcu and
> use the appropriate RCU accessors everywhere they are touched:
> rcu_assign_pointer() on the writer side (under slaves_lock),
> rcu_dereference_protected() for the writer-side loads (also under
> slaves_lock), rcu_dereference_bh() for the loads in teql_master_xmit() and
> rtnl_dereference() for the loads in teql_master_open()/teql_master_mtu(),
> which run under RTNL.
> Pair this with rcu_read_lock_bh()/rcu_read_unlock_bh() around the list
> traversal in teql_master_xmit(), so that readers either observe a fully
> linked list or are deferred until the in-flight mutation completes. The two
> early-return paths in teql_master_xmit() are updated to release the RCU-bh
> read-side critical section before returning, since leaving it held would
> disable BH on that CPU for good.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Reported-by: zdi-disclosures@trendmicro.com
> Tested-by: Victor Nogueira <victor@mojatatu.com>
> Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>

Looks good, thanks!

Please note that sashiko/gemini found a pre-existing issues which may
require a follow-up/separate fix:

https://sashiko.dev/#/patchset/20260628111229.669751-1-jhs%40mojatatu.com

(the 2nd one in the above link, IDK how to generate a direct link to a
specific comment)

/P