bpf: Fix NULL pointer dereference in bpf_get_local_storage() helper
Jiri Olsa reported a bug ([1]) in kernel where cgroup local
storage pointer may be NULL in bpf_get_local_storage() helper.
There are two issues uncovered by this bug:
(1). kprobe or tracepoint prog incorrectly sets cgroup local storage
before prog run,
(2). due to change from preempt_disable to migrate_disable,
preemption is possible and percpu storage might be overwritten
by other tasks.
This issue (1) is fixed in [2]. This patch tried to address issue (2).
The following shows how things can go wrong:
task 1: bpf_cgroup_storage_set() for percpu local storage
preemption happens
task 2: bpf_cgroup_storage_set() for percpu local storage
preemption happens
task 1: run bpf program
task 1 will effectively use the percpu local storage setting by task 2
which will be either NULL or incorrect ones.
Instead of just one common local storage per cpu, this patch fixed
the issue by permitting 8 local storages per cpu and each local
storage is identified by a task_struct pointer. This way, we
allow at most 8 nested preemption between bpf_cgroup_storage_set()
and bpf_cgroup_storage_unset(). The percpu local storage slot
is released (calling bpf_cgroup_storage_unset()) by the same task
after bpf program finished running.
bpf_test_run() is also fixed to use the new bpf_cgroup_storage_set()
interface.
The patch is tested on top of [2] with reproducer in [1].
Without this patch, kernel will emit error in 2-3 minutes.
With this patch, after one hour, still no error.
[1] https://lore.kernel.org/bpf/CAKH8qBuXCfUz=w8L+Fj74OaUpbosO29niYwTki7e3Ag044_aww@mail.gmail.com/T
[2] https://lore.kernel.org/bpf/20210309185028.3763817-1-yhs@fb.com
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Roman Gushchin <guro@fb.com>
Link: https://lore.kernel.org/bpf/20210323055146.3334476-1-yhs@fb.com
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 0748002..f306611 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -382,8 +382,8 @@ const struct bpf_func_proto bpf_get_current_ancestor_cgroup_id_proto = {
};
#ifdef CONFIG_CGROUP_BPF
-DECLARE_PER_CPU(struct bpf_cgroup_storage*,
- bpf_cgroup_storage[MAX_BPF_CGROUP_STORAGE_TYPE]);
+DECLARE_PER_CPU(struct bpf_cgroup_storage_info,
+ bpf_cgroup_storage_info[BPF_CGROUP_STORAGE_NEST_MAX]);
BPF_CALL_2(bpf_get_local_storage, struct bpf_map *, map, u64, flags)
{
@@ -392,10 +392,17 @@ BPF_CALL_2(bpf_get_local_storage, struct bpf_map *, map, u64, flags)
* verifier checks that its value is correct.
*/
enum bpf_cgroup_storage_type stype = cgroup_storage_type(map);
- struct bpf_cgroup_storage *storage;
+ struct bpf_cgroup_storage *storage = NULL;
void *ptr;
+ int i;
- storage = this_cpu_read(bpf_cgroup_storage[stype]);
+ for (i = 0; i < BPF_CGROUP_STORAGE_NEST_MAX; i++) {
+ if (unlikely(this_cpu_read(bpf_cgroup_storage_info[i].task) != current))
+ continue;
+
+ storage = this_cpu_read(bpf_cgroup_storage_info[i].storage[stype]);
+ break;
+ }
if (stype == BPF_CGROUP_STORAGE_SHARED)
ptr = &READ_ONCE(storage->buf)->data[0];
