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bpf: Fix NULL pointer dereference in bpf_get_local_storage() helper

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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
This commit is contained in:
Yonghong Song 2021-03-22 22:51:46 -07:00 • committed by Alexei Starovoitov
parent a46410d5e4
commit b910eaaaa4
5 changed files with 86 additions and 19 deletions
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15
kernel/bpf/helpers.c
View file

@ -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];