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

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

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

Author: Pu Lehui

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;
-		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);
+		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);
 	}