Re: [RFC PATCH] tracing: Fix syscall tracepoint use-after-free

[Alexei Starovoitov]

On Wed, Oct 23, 2024 at 7:56 AM Jordan Rife <jrife@xxxxxxxxxx> wrote:
>
> Mathieu's patch alone does not seem to be enough to prevent the
> use-after-free issue reported by syzbot.
>
> Link: https://lore.kernel.org/bpf/67121037.050a0220.10f4f4.000f.GAE@xxxxxxxxxx/T/#u
>
> I reran the repro script with his patch applied to my tree and was
> still able to get the same KASAN crash to occur.
>
> In this case, when bpf_link_free is invoked it kicks off three instances
> of call_rcu*.
>
> bpf_link_free()
>   ops->release()
>      bpf_raw_tp_link_release()
>        bpf_probe_unregister()
>          tracepoint_probe_unregister()
>            tracepoint_remove_func()
>              release_probes()
>                call_rcu()               [1]
>   bpf_prog_put()
>     __bpf_prog_put()
>       bpf_prog_put_deferred()
>         __bpf_prog_put_noref()
>            call_rcu()                   [2]
>   call_rcu()                            [3]
>
> With Mathieu's patch, [1] is chained with call_rcu_tasks_trace()
> making the grace period suffiently long to safely free the probe itself.
> The callback for [2] and [3] may be invoked before the
> call_rcu_tasks_trace() grace period has elapsed leading to the link or
> program itself being freed while still in use. I was able to prevent
> any crashes with the patch below which also chains
> call_rcu_tasks_trace() and call_rcu() at [2] and [3].
>
> ---
>  kernel/bpf/syscall.c | 24 ++++++++++--------------
>  1 file changed, 10 insertions(+), 14 deletions(-)
>
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 59de664e580d..5290eccb465e 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -2200,6 +2200,14 @@ static void __bpf_prog_put_rcu(struct rcu_head *rcu)
>         bpf_prog_free(aux->prog);
>  }
>
> +static void __bpf_prog_put_tasks_trace_rcu(struct rcu_head *rcu)
> +{
> +       if (rcu_trace_implies_rcu_gp())
> +               __bpf_prog_put_rcu(rcu);
> +       else
> +               call_rcu(rcu, __bpf_prog_put_rcu);
> +}
> +
>  static void __bpf_prog_put_noref(struct bpf_prog *prog, bool deferred)
>  {
>         bpf_prog_kallsyms_del_all(prog);
> @@ -2212,10 +2220,7 @@ static void __bpf_prog_put_noref(struct bpf_prog *prog, bool deferred)
>                 btf_put(prog->aux->attach_btf);
>
>         if (deferred) {
> -               if (prog->sleepable)
> -                       call_rcu_tasks_trace(&prog->aux->rcu, __bpf_prog_put_rcu);
> -               else
> -                       call_rcu(&prog->aux->rcu, __bpf_prog_put_rcu);
> +               call_rcu_tasks_trace(&prog->aux->rcu, __bpf_prog_put_tasks_trace_rcu);
>         } else {
>                 __bpf_prog_put_rcu(&prog->aux->rcu);
>         }
> @@ -2996,24 +3001,15 @@ static void bpf_link_defer_dealloc_mult_rcu_gp(struct rcu_head *rcu)
>  static void bpf_link_free(struct bpf_link *link)
>  {
>         const struct bpf_link_ops *ops = link->ops;
> -       bool sleepable = false;
>
>         bpf_link_free_id(link->id);
>         if (link->prog) {
> -               sleepable = link->prog->sleepable;
>                 /* detach BPF program, clean up used resources */
>                 ops->release(link);
>                 bpf_prog_put(link->prog);
>         }
>         if (ops->dealloc_deferred) {
> -               /* schedule BPF link deallocation; if underlying BPF program
> -                * is sleepable, we need to first wait for RCU tasks trace
> -                * sync, then go through "classic" RCU grace period
> -                */
> -               if (sleepable)
> -                       call_rcu_tasks_trace(&link->rcu, bpf_link_defer_dealloc_mult_rcu_gp);
> -               else
> -                       call_rcu(&link->rcu, bpf_link_defer_dealloc_rcu_gp);
> +               call_rcu_tasks_trace(&link->rcu, bpf_link_defer_dealloc_mult_rcu_gp);

This patch is completely wrong.
Looks like Mathieu patch broke bpf program contract somewhere.
The tracepoint type bpf programs must be called with rcu_read_lock held.
Looks like it's not happening anymore.