bpf: Fix invalid mem access when update_effective_progs fails in __cgroup_bpf_detach

Syzkaller triggers an invalid memory access issue following fault
injection in update_effective_progs. The issue can be described as
follows:

__cgroup_bpf_detach
  update_effective_progs
    compute_effective_progs
      bpf_prog_array_alloc <-- fault inject
  purge_effective_progs
    /* change to dummy_bpf_prog */
    array->items[index] = &dummy_bpf_prog.prog

---softirq start---
__do_softirq
  ...
    __cgroup_bpf_run_filter_skb
      __bpf_prog_run_save_cb
        bpf_prog_run
          stats = this_cpu_ptr(prog->stats)
          /* invalid memory access */
          flags = u64_stats_update_begin_irqsave(&stats->syncp)
---softirq end---

  static_branch_dec(&cgroup_bpf_enabled_key[atype])

The reason is that fault injection caused update_effective_progs to fail
and then changed the original prog into dummy_bpf_prog.prog in
purge_effective_progs. Then a softirq came, and accessing the stats of
dummy_bpf_prog.prog in the softirq triggers invalid mem access.

To fix it, we can use static per-cpu variable to initialize the stats
of dummy_bpf_prog.prog.

Fixes: 4c46091 ("bpf: Fix KASAN use-after-free Read in compute_effective_progs")
Signed-off-by: Pu Lehui <[email protected]>

Author: Pu Lehui

kernel/bpf/core.c

@@ -2536,11 +2536,14 @@ static unsigned int __bpf_prog_ret1(const void *ctx,
 	return 1;
 }
 
+static DEFINE_PER_CPU(struct bpf_prog_stats, __dummy_stats);
+
 static struct bpf_prog_dummy {
 	struct bpf_prog prog;
 } dummy_bpf_prog = {
 	.prog = {
 		.bpf_func = __bpf_prog_ret1,
+		.stats = &__dummy_stats,
 	},
 };