[PATCH bpf] bpf: Fix deadlock when freeing cgroup storage

Abel Wu
2024-12-18T09:22:11Z

The following commit
bc235cdb423a ("bpf: Prevent deadlock from recursive bpf_task_storage_[get|delete]")
first introduced deadlock prevention for fentry/fexit programs attaching
on bpf_task_storage helpers. That commit also employed the logic in map
free path in its v6 version.

Later bpf_cgrp_storage was first introduced in
c4bcfb38a95e ("bpf: Implement cgroup storage available to non-cgroup-attached bpf progs")
which faces the same issue as bpf_task_storage, instead of its busy
counter, NULL was passed to bpf_local_storage_map_free() which opened
a window to cause deadlock:

<TASK>
_raw_spin_lock_irqsave+0x3d/0x50
bpf_local_storage_update+0xd1/0x460
bpf_cgrp_storage_get+0x109/0x130
bpf_prog_72026450ec387477_cgrp_ptr+0x38/0x5e
bpf_trace_run1+0x84/0x100
cgroup_storage_ptr+0x4c/0x60
bpf_selem_unlink_storage_nolock.constprop.0+0x135/0x160
bpf_selem_unlink_storage+0x6f/0x110
bpf_local_storage_map_free+0xa2/0x110
bpf_map_free_deferred+0x5b/0x90
process_one_work+0x17c/0x390
worker_thread+0x251/0x360
kthread+0xd2/0x100
ret_from_fork+0x34/0x50
ret_from_fork_asm+0x1a/0x30
</TASK>

[ Since the verifier treats 'void *' as scalar which
prevents me from getting a pointer to 'struct cgroup *',
I added a raw tracepoint in cgroup_storage_ptr() to
help reproducing this issue. ]

Although it is tricky to reproduce, the risk of deadlock exists and
worthy of a fix, by passing its busy counter to the free procedure so
it can be properly incremented before storage/smap locking.

Fixes: c4bcfb38a95e ("bpf: Implement cgroup storage available to non-cgroup-attached bpf progs")
Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
---
kernel/bpf/bpf_cgrp_storage.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/bpf_cgrp_storage.c b/kernel/bpf/bpf_cgrp_storage.c
index 20f05de92e9c..7996fcea3755 100644
--- a/kernel/bpf/bpf_cgrp_storage.c
+++ b/kernel/bpf/bpf_cgrp_storage.c
@@ -154,7 +154,7 @@ static struct bpf_map *cgroup_storage_map_alloc(union bpf_attr *attr)

static void cgroup_storage_map_free(struct bpf_map *map)
{
- bpf_local_storage_map_free(map, &cgroup_cache, NULL);
+ bpf_local_storage_map_free(map, &cgroup_cache, &bpf_cgrp_storage_busy);
}

/* *gfp_flags* is a hidden argument provided by the verifier */
--
2.37.3