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

1 files changed, 49 insertions, 8 deletions

diff --git a/include/linux/bpf-cgroup.h b/include/linux/bpf-cgroup.h
index c42e02b4d84b..6a29fe11485d 100644
--- a/include/linux/bpf-cgroup.h
+++ b/include/linux/bpf-cgroup.h
@@ -20,14 +20,25 @@ struct bpf_sock_ops_kern;
 struct bpf_cgroup_storage;
 struct ctl_table;
 struct ctl_table_header;
+struct task_struct;
 
 #ifdef CONFIG_CGROUP_BPF
 
 extern struct static_key_false cgroup_bpf_enabled_key[MAX_BPF_ATTACH_TYPE];
 #define cgroup_bpf_enabled(type) static_branch_unlikely(&cgroup_bpf_enabled_key[type])
 
-DECLARE_PER_CPU(struct bpf_cgroup_storage*,
-		bpf_cgroup_storage[MAX_BPF_CGROUP_STORAGE_TYPE]);
+#define BPF_CGROUP_STORAGE_NEST_MAX	8
+
+struct bpf_cgroup_storage_info {
+	struct task_struct *task;
+	struct bpf_cgroup_storage *storage[MAX_BPF_CGROUP_STORAGE_TYPE];
+};
+
+/* For each cpu, permit maximum BPF_CGROUP_STORAGE_NEST_MAX number of tasks
+ * to use bpf cgroup storage simultaneously.
+ */
+DECLARE_PER_CPU(struct bpf_cgroup_storage_info,
+		bpf_cgroup_storage_info[BPF_CGROUP_STORAGE_NEST_MAX]);
 
 #define for_each_cgroup_storage_type(stype) \
 	for (stype = 0; stype < MAX_BPF_CGROUP_STORAGE_TYPE; stype++)
@@ -161,13 +172,42 @@ static inline enum bpf_cgroup_storage_type cgroup_storage_type(
 	return BPF_CGROUP_STORAGE_SHARED;
 }
 
-static inline void bpf_cgroup_storage_set(struct bpf_cgroup_storage
-					  *storage[MAX_BPF_CGROUP_STORAGE_TYPE])
+static inline int bpf_cgroup_storage_set(struct bpf_cgroup_storage
+					 *storage[MAX_BPF_CGROUP_STORAGE_TYPE])
 {
 	enum bpf_cgroup_storage_type stype;
+	int i, err = 0;
+
+	preempt_disable();
+	for (i = 0; i < BPF_CGROUP_STORAGE_NEST_MAX; i++) {
+		if (unlikely(this_cpu_read(bpf_cgroup_storage_info[i].task) != NULL))
+			continue;
+
+		this_cpu_write(bpf_cgroup_storage_info[i].task, current);
+		for_each_cgroup_storage_type(stype)
+			this_cpu_write(bpf_cgroup_storage_info[i].storage[stype],
+				       storage[stype]);
+		goto out;
+	}
+	err = -EBUSY;
+	WARN_ON_ONCE(1);
+
+out:
+	preempt_enable();
+	return err;
+}
+
+static inline void bpf_cgroup_storage_unset(void)
+{
+	int i;
+
+	for (i = 0; i < BPF_CGROUP_STORAGE_NEST_MAX; i++) {
+		if (unlikely(this_cpu_read(bpf_cgroup_storage_info[i].task) != current))
+			continue;
 
-	for_each_cgroup_storage_type(stype)
-		this_cpu_write(bpf_cgroup_storage[stype], storage[stype]);
+		this_cpu_write(bpf_cgroup_storage_info[i].task, NULL);
+		return;
+	}
 }
 
 struct bpf_cgroup_storage *
@@ -448,8 +488,9 @@ static inline int cgroup_bpf_prog_query(const union bpf_attr *attr,
 	return -EINVAL;
 }
 
-static inline void bpf_cgroup_storage_set(
-	struct bpf_cgroup_storage *storage[MAX_BPF_CGROUP_STORAGE_TYPE]) {}
+static inline int bpf_cgroup_storage_set(
+	struct bpf_cgroup_storage *storage[MAX_BPF_CGROUP_STORAGE_TYPE]) { return 0; }
+static inline void bpf_cgroup_storage_unset(void) {}
 static inline int bpf_cgroup_storage_assign(struct bpf_prog_aux *aux,
 					    struct bpf_map *map) { return 0; }
 static inline struct bpf_cgroup_storage *bpf_cgroup_storage_alloc(