[PATCH bpf-next v5 3/7] bpf: Pin associated struct_ops when registering async callback

Amery Hung
2025-11-04T17:26:59Z

Take a refcount of the associated struct_ops map to prevent the map from
being freed when an async callback scheduled from a struct_ops program
runs.

Since struct_ops programs do not take refcounts on the struct_ops map,
it is possible for a struct_ops map to be freed when an async callback
scheduled from it runs. To prevent this, take a refcount on prog->aux->
st_ops_assoc and save it in a newly created struct bpf_async_res for
every async mechanism. The reference needs to be preserved in
bpf_async_res since prog->aux->st_ops_assoc can be poisoned anytime
and reference leak could happen.

bpf_async_res will contain a async callback's BPF program and resources
related to the BPF program. The resources will be acquired when
registering a callback and released when cancelled or when the map
associated with the callback is freed.

Also rename drop_prog_refcnt to bpf_async_cb_reset to better reflect
what it now does.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
---
kernel/bpf/helpers.c | 105 +++++++++++++++++++++++++++++--------------
1 file changed, 72 insertions(+), 33 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 930e132f440f..5c081cd604d5 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1092,9 +1092,14 @@ static void *map_key_from_value(struct bpf_map *map, void *value, u32 *arr_idx)
return (void *)value - round_up(map->key_size, 8);
}

+struct bpf_async_res {
+ struct bpf_prog *prog;
+ struct bpf_map *st_ops_assoc;
+};
+
struct bpf_async_cb {
struct bpf_map *map;
- struct bpf_prog *prog;
+ struct bpf_async_res res;
void __rcu *callback_fn;
void *value;
union {
@@ -1299,8 +1304,8 @@ static int __bpf_async_init(struct bpf_async_kern *async, struct bpf_map *map, u
break;
}
cb->map = map;
- cb->prog = NULL;
cb->flags = flags;
+ memset(&cb->res, 0, sizeof(cb->res));
rcu_assign_pointer(cb->callback_fn, NULL);

WRITE_ONCE(async->cb, cb);
@@ -1351,11 +1356,47 @@ static const struct bpf_func_proto bpf_timer_init_proto = {
.arg3_type = ARG_ANYTHING,
};

+static void bpf_async_res_put(struct bpf_async_res *res)
+{
+ bpf_prog_put(res->prog);
+
+ if (res->st_ops_assoc)
+ bpf_map_put(res->st_ops_assoc);
+}
+
+static int bpf_async_res_get(struct bpf_async_res *res, struct bpf_prog *prog)
+{
+ struct bpf_map *st_ops_assoc = NULL;
+ int err;
+
+ prog = bpf_prog_inc_not_zero(prog);
+ if (IS_ERR(prog))
+ return PTR_ERR(prog);
+
+ st_ops_assoc = READ_ONCE(prog->aux->st_ops_assoc);
+ if (prog->type == BPF_PROG_TYPE_STRUCT_OPS &&
+ st_ops_assoc && st_ops_assoc != BPF_PTR_POISON) {
+ st_ops_assoc = bpf_map_inc_not_zero(st_ops_assoc);
+ if (IS_ERR(st_ops_assoc)) {
+ err = PTR_ERR(st_ops_assoc);
+ goto put_prog;
+ }
+ }
+
+ res->prog = prog;
+ res->st_ops_assoc = st_ops_assoc;
+ return 0;
+put_prog:
+ bpf_prog_put(prog);
+ return err;
+}
+
static int __bpf_async_set_callback(struct bpf_async_kern *async, void *callback_fn,
struct bpf_prog_aux *aux, unsigned int flags,
enum bpf_async_type type)
{
struct bpf_prog *prev, *prog = aux->prog;
+ struct bpf_async_res res;
struct bpf_async_cb *cb;
int ret = 0;

@@ -1376,20 +1417,18 @@ static int __bpf_async_set_callback(struct bpf_async_kern *async, void *callback
ret = -EPERM;
goto out;
}
- prev = cb->prog;
+ prev = cb->res.prog;
if (prev != prog) {
- /* Bump prog refcnt once. Every bpf_timer_set_callback()
+ /* Get prog and related resources once. Every bpf_timer_set_callback()
* can pick different callback_fn-s within the same prog.
*/
- prog = bpf_prog_inc_not_zero(prog);
- if (IS_ERR(prog)) {
- ret = PTR_ERR(prog);
+ ret = bpf_async_res_get(&res, prog);
+ if (ret)
goto out;
- }
if (prev)
- /* Drop prev prog refcnt when swapping with new prog */
- bpf_prog_put(prev);
- cb->prog = prog;
+ /* Put prev prog and related resources when swapping with new prog */
+ bpf_async_res_put(&cb->res);
+ cb->res = res;
}
rcu_assign_pointer(cb->callback_fn, callback_fn);
out:
@@ -1423,7 +1462,7 @@ BPF_CALL_3(bpf_timer_start, struct bpf_async_kern *, timer, u64, nsecs, u64, fla
return -EINVAL;
__bpf_spin_lock_irqsave(&timer->lock);
t = timer->timer;
- if (!t || !t->cb.prog) {
+ if (!t || !t->cb.res.prog) {
ret = -EINVAL;
goto out;
}
@@ -1451,14 +1490,14 @@ static const struct bpf_func_proto bpf_timer_start_proto = {
.arg3_type = ARG_ANYTHING,
};

-static void drop_prog_refcnt(struct bpf_async_cb *async)
+static void bpf_async_cb_reset(struct bpf_async_cb *cb)
{
- struct bpf_prog *prog = async->prog;
+ struct bpf_prog *prog = cb->res.prog;

if (prog) {
- bpf_prog_put(prog);
- async->prog = NULL;
- rcu_assign_pointer(async->callback_fn, NULL);
+ bpf_async_res_put(&cb->res);
+ memset(&cb->res, 0, sizeof(cb->res));
+ rcu_assign_pointer(cb->callback_fn, NULL);
}
}

@@ -1512,7 +1551,7 @@ BPF_CALL_1(bpf_timer_cancel, struct bpf_async_kern *, timer)
goto out;
}
drop:
- drop_prog_refcnt(&t->cb);
+ bpf_async_cb_reset(&t->cb);
out:
__bpf_spin_unlock_irqrestore(&timer->lock);
/* Cancel the timer and wait for associated callback to finish
@@ -1545,7 +1584,7 @@ static struct bpf_async_cb *__bpf_async_cancel_and_free(struct bpf_async_kern *a
cb = async->cb;
if (!cb)
goto out;
- drop_prog_refcnt(cb);
+ bpf_async_cb_reset(cb);
/* The subsequent bpf_timer_start/cancel() helpers won't be able to use
* this timer, since it won't be initialized.
*/
@@ -3112,7 +3151,7 @@ __bpf_kfunc int bpf_wq_start(struct bpf_wq *wq, unsigned int flags)
if (flags)
return -EINVAL;
w = READ_ONCE(async->work);
- if (!w || !READ_ONCE(w->cb.prog))
+ if (!w || !READ_ONCE(w->cb.res.prog))
return -EINVAL;

schedule_work(&w->work);
@@ -4034,8 +4073,8 @@ struct bpf_task_work_ctx {
refcount_t refcnt;
struct callback_head work;
struct irq_work irq_work;
- /* bpf_prog that schedules task work */
- struct bpf_prog *prog;
+ /* bpf_prog that schedules task work and related resources */
+ struct bpf_async_res res;
/* task for which callback is scheduled */
struct task_struct *task;
/* the map and map value associated with this context */
@@ -4053,9 +4092,9 @@ struct bpf_task_work_kern {

static void bpf_task_work_ctx_reset(struct bpf_task_work_ctx *ctx)
{
- if (ctx->prog) {
- bpf_prog_put(ctx->prog);
- ctx->prog = NULL;
+ if (ctx->res.prog) {
+ bpf_async_res_put(&ctx->res);
+ memset(&ctx->res, 0, sizeof(ctx->res));
}
if (ctx->task) {
bpf_task_release(ctx->task);
@@ -4233,19 +4272,19 @@ static int bpf_task_work_schedule(struct task_struct *task, struct bpf_task_work
struct bpf_map *map, bpf_task_work_callback_t callback_fn,
struct bpf_prog_aux *aux, enum task_work_notify_mode mode)
{
- struct bpf_prog *prog;
struct bpf_task_work_ctx *ctx;
+ struct bpf_async_res res;
int err;

BTF_TYPE_EMIT(struct bpf_task_work);

- prog = bpf_prog_inc_not_zero(aux->prog);
- if (IS_ERR(prog))
- return -EBADF;
+ err = bpf_async_res_get(&res, aux->prog);
+ if (err)
+ return err;
task = bpf_task_acquire(task);
if (!task) {
err = -EBADF;
- goto release_prog;
+ goto release_res;
}

ctx = bpf_task_work_acquire_ctx(tw, map);
@@ -4256,7 +4295,7 @@ static int bpf_task_work_schedule(struct task_struct *task, struct bpf_task_work

ctx->task = task;
ctx->callback_fn = callback_fn;
- ctx->prog = prog;
+ ctx->res = res;
ctx->mode = mode;
ctx->map = map;
ctx->map_val = (void *)tw - map->record->task_work_off;
@@ -4268,8 +4307,8 @@ static int bpf_task_work_schedule(struct task_struct *task, struct bpf_task_work

release_all:
bpf_task_release(task);
-release_prog:
- bpf_prog_put(prog);
+release_res:
+ bpf_async_res_put(&res);
return err;
}

--
2.47.3