Re: [PATCH bpf-next] bpf: Fix race between btf_put and btf_idr walk.

[Eduard Zingerman <eddyz87@gmail.com>]

On Thu, 2023-04-20 at 18:49 -0700, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.org>
> 
> Florian and Eduard reported hard dead lock:
> [   58.433327]  _raw_spin_lock_irqsave+0x40/0x50
> [   58.433334]  btf_put+0x43/0x90
> [   58.433338]  bpf_find_btf_id+0x157/0x240
> [   58.433353]  btf_parse_fields+0x921/0x11c0
> 
> This happens since btf->refcount can be 1 at the time of btf_put() and
> btf_put() will call btf_free_id() which will try to grab btf_idr_lock
> and will dead lock.
> Avoid the issue by doing btf_put() without locking.
> 
> Reported-by: Florian Westphal <fw@strlen.de>
> Reported-by: Eduard Zingerman <eddyz87@gmail.com>
> Fixes: 3d78417b60fb ("bpf: Add bpf_btf_find_by_name_kind() helper.")
> Fixes: 1e89106da253 ("bpf: Add bpf_core_add_cands() and wire it into bpf_core_apply_relo_insn().")
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>

I applied the patch from Dave, that fixes address computation
in bpf_refcount_acquire_impl() and tested this patch using the
following reproducing script (to obtain a race between test module
unload and bpf_find_btf_id():

  for j in $(seq 1 100);
    do echo ">>>> $j <<<<";
    for i in $(seq 1 4); do (./test_progs --allow=refcounted_kptr &); done;
    sleep 1;
  done

W/o this patch I see dead locks, with this patch I don't see dead locks.

Tested-by: Eduard Zingerman <eddyz87@gmail.com>

> ---
>  kernel/bpf/btf.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index a0887ee44e89..7db4ec125fbd 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -577,8 +577,8 @@ static s32 bpf_find_btf_id(const char *name, u32 kind, struct btf **btf_p)
>  			*btf_p = btf;
>  			return ret;
>  		}
> -		spin_lock_bh(&btf_idr_lock);
>  		btf_put(btf);
> +		spin_lock_bh(&btf_idr_lock);
>  	}
>  	spin_unlock_bh(&btf_idr_lock);
>  	return ret;
> @@ -8354,12 +8354,10 @@ bpf_core_find_cands(struct bpf_core_ctx *ctx, u32 local_type_id)
>  		btf_get(mod_btf);
>  		spin_unlock_bh(&btf_idr_lock);
>  		cands = bpf_core_add_cands(cands, mod_btf, btf_nr_types(main_btf));
> -		if (IS_ERR(cands)) {
> -			btf_put(mod_btf);
> +		btf_put(mod_btf);
> +		if (IS_ERR(cands))
>  			return ERR_CAST(cands);
> -		}
>  		spin_lock_bh(&btf_idr_lock);
> -		btf_put(mod_btf);
>  	}
>  	spin_unlock_bh(&btf_idr_lock);
>  	/* cands is a pointer to kmalloced memory here if cands->cnt > 0