clsact: Fix use-after-free in init/destroy rollback asymmetry

Fix a use-after-free in the clsact qdisc upon init/destroy rollback asymmetry.
The latter is achieved by first fully initializing a clsact instance, and
then in a second step having a replacement failure for the new clsact qdisc
instance. clsact_init() initializes ingress first and then takes care of the
egress part. This can fail midway, for example, via tcf_block_get_ext(). Upon
failure, the kernel will trigger the clsact_destroy() callback.

Commit 1cb6f0bae504 ("bpf: Fix too early release of tcx_entry") details the
way how the transition is happening. If tcf_block_get_ext on the q->ingress_block
ends up failing, we took the tcx_miniq_inc reference count on the ingress
side, but not yet on the egress side. clsact_destroy() tests whether the
{ingress,egress}_entry was non-NULL. However, even in midway failure on the
replacement, both are in fact non-NULL with a valid egress_entry from the
previous clsact instance.

What we really need to test for is whether the qdisc instance-specific ingress
or egress side previously got initialized. This adds a small helper for checking
the miniq initialization called mini_qdisc_pair_inited, and utilizes that upon
clsact_destroy() in order to fix the use-after-free scenario. Convert the
ingress_destroy() side as well so both are consistent to each other.

Fixes: 1cb6f0bae504 ("bpf: Fix too early release of tcx_entry")
Reported-by: Keenan Dong <keenanat2000@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20260313065531.98639-1-daniel@iogearbox.net
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index cafb266a0b80..c3d657359a3d 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -1457,6 +1457,11 @@ void mini_qdisc_pair_init(struct mini_Qdisc_pair *miniqp, struct Qdisc *qdisc,
 void mini_qdisc_pair_block_init(struct mini_Qdisc_pair *miniqp,
 				struct tcf_block *block);
 
+static inline bool mini_qdisc_pair_inited(struct mini_Qdisc_pair *miniqp)
+{
+	return !!miniqp->p_miniq;
+}
+
 void mq_change_real_num_tx(struct Qdisc *sch, unsigned int new_real_tx);
 
 int sch_frag_xmit_hook(struct sk_buff *skb, int (*xmit)(struct sk_buff *skb));