[PATCH bpf v2] bpf, cgroup: Fix invalid storage access after __cgroup_bpf_attach failed

[Pu Lehui]

From: Pu Lehui <pulehui@xxxxxxxxxx>

Sashiko reported a potential invalid storage access issue after
replacing a cgroup bpf prog.

This occurs in the following scenario:
1. prog1 with storage is attached to a cgroup in multi-attach mode.
2. prog1 is replaced with prog2 using BPF_F_REPLACE in multi-attach
   mode, but fails midway (e.g. in bpf_trampoline_link_cgroup_shim or
   update_effective_progs).
3. A new prog3 is attached to the cgroup in multi-attach mode.

The reason is that __cgroup_bpf_attach overwrites pl->storage with the
new storage prior to attachment completion. When attachment fails
midway, the cleanup path calls bpf_cgroup_storages_free(new_storage) to
free the newly allocated storage, but fails to restore pl->storage back
to old_storage.

Consequently, the still-active prog1 holds invalid or dangling storage
pointers, leading to an invalid memory access when prog1 executes and
calls bpf_get_local_storage. Additionally, original pl->flags and
cgrp->bpf.flags[atype] are left unrestored.

Fix this by saving old_pl_flags, old_storage, and old_flags prior to the
update, and properly restoring all of them in the cleanup path on error.

Fixes: 7d9c3427894f ("bpf: Make cgroup storages shared between programs on the same cgroup")
Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
Signed-off-by: Pu Lehui <pulehui@xxxxxxxxxx>
---
v2:
- Remove the link relative code as link only support
  BPF_F_ALLOW_MULTI attach, so will not occur UAF pl.

v1: https://lore.kernel.org/bpf/20260728132336.2857800-1-pulehui@xxxxxxxxxxxxxxx

 kernel/bpf/cgroup.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index e2fa0ebeed83..be24ca453cab 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -813,8 +813,10 @@ static int __cgroup_bpf_attach(struct cgroup *cgrp,
 	struct bpf_prog *old_prog = NULL;
 	struct bpf_cgroup_storage *storage[MAX_BPF_CGROUP_STORAGE_TYPE] = {};
 	struct bpf_cgroup_storage *new_storage[MAX_BPF_CGROUP_STORAGE_TYPE] = {};
+	struct bpf_cgroup_storage *old_storage[MAX_BPF_CGROUP_STORAGE_TYPE] = {};
 	struct bpf_prog *new_prog = prog ? : link->link.prog;
 	enum cgroup_bpf_attach_type atype;
+	u32 old_flags, old_pl_flags;
 	struct bpf_prog_list *pl;
 	struct hlist_head *progs;
 	int err;
@@ -865,6 +867,8 @@ static int __cgroup_bpf_attach(struct cgroup *cgrp,
 
 	if (pl) {
 		old_prog = pl->prog;
+		old_pl_flags = pl->flags;
+		bpf_cgroup_storages_assign(old_storage, pl->storage);
 	} else {
 		pl = kmalloc_obj(*pl);
 		if (!pl) {
@@ -884,6 +888,7 @@ static int __cgroup_bpf_attach(struct cgroup *cgrp,
 	pl->link = link;
 	pl->flags = flags;
 	bpf_cgroup_storages_assign(pl->storage, storage);
+	old_flags = cgrp->bpf.flags[atype];
 	cgrp->bpf.flags[atype] = saved_flags;
 
 	if (type == BPF_LSM_CGROUP) {
@@ -915,12 +920,15 @@ static int __cgroup_bpf_attach(struct cgroup *cgrp,
 	if (old_prog) {
 		pl->prog = old_prog;
 		pl->link = NULL;
+		pl->flags = old_pl_flags;
+		bpf_cgroup_storages_assign(pl->storage, old_storage);
 	}
 	bpf_cgroup_storages_free(new_storage);
 	if (!old_prog) {
 		hlist_del(&pl->node);
 		kfree(pl);
 	}
+	cgrp->bpf.flags[atype] = old_flags;
 	return err;
 }
 
-- 
2.34.1