In the Linux kernel, the following vulnerability has been resolved: bpf: Fix invalid prog->stats access when update_effective_progs fails

In the Linux kernel, the following vulnerability has been resolved:

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.