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: 492ecee892c2 ("bpf: enable program stats")
Signed-off-by: Pu Lehui <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

include/linux/filter.h

···
 		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);
 	}

kernel/bpf/syscall.c

···
 	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);