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 <pulehui@huawei.com>
Link: https://lore.kernel.org/r/20251115102343.2200727-1-pulehui@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/include/linux/filter.h b/include/linux/filter.h
index 973233b82dc1fd422f26ac221eeb46c66c47767a..569de3b14279af88fe9914fdf6de879c3d4486f2 100644 (file)
--- a/include/linux/filter.h
+++ b/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);
        }

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index a2a441185f8109398ec0f592e32e51041b53e915..792623a7c90b529183f49c93900ba26af3152045 100644 (file)
--- a/kernel/bpf/syscall.c
+++ b/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);