bpf: Fix invalid prog->stats access when update_effective_progs fails

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 members of
dummy_bpf_prog.prog in the softirq triggers invalid mem access.

To fix it, skip updating stats when stats is NULL.

Fixes: 492ecee892 ("bpf: enable program stats")
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Link: https://lore.kernel.org/r/20251115102343.2200727-1-pulehui@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/filter.h

@@ -712,11 +712,13 @@ static __always_inline u32 __bpf_prog_run(const struct bpf_prog *prog,
 		ret = dfunc(ctx, prog->insnsi, prog->bpf_func);
 
 		duration = sched_clock() - start;
+		if (likely(prog->stats)) {
 			stats = this_cpu_ptr(prog->stats);
 			flags = u64_stats_update_begin_irqsave(&stats->syncp);
 			u64_stats_inc(&stats->cnt);
 			u64_stats_add(&stats->nsecs, duration);
 			u64_stats_update_end_irqrestore(&stats->syncp, flags);
+		}
 	} else {
 		ret = dfunc(ctx, prog->insnsi, prog->bpf_func);
 	}

kernel/bpf/syscall.c

@@ -2463,6 +2463,9 @@ void notrace bpf_prog_inc_misses_counter(struct bpf_prog *prog)
 	struct bpf_prog_stats *stats;
 	unsigned int flags;
 
+	if (unlikely(!prog->stats))
+		return;
+
 	stats = this_cpu_ptr(prog->stats);
 	flags = u64_stats_update_begin_irqsave(&stats->syncp);
 	u64_stats_inc(&stats->misses);