netdev - Re: [PATCH bpf v2 2/4] bpf: map_poke_descriptor is being called with an unstable poke

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Message-ID: <20210622215439.wfi56kb77ewq2lx6@ast-mbp.dhcp.thefacebook.com>
Date:   Tue, 22 Jun 2021 14:54:39 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     maciej.fijalkowski@...el.com, ast@...nel.org, daniel@...earbox.net,
        andriin@...com, netdev@...r.kernel.org
Subject: Re: [PATCH bpf v2 2/4] bpf: map_poke_descriptor is being called with
 an unstable poke_tab[]

On Wed, Jun 16, 2021 at 03:55:19PM -0700, John Fastabend wrote:
> When populating poke_tab[] of a subprog we call map_poke_track() after
> doing bpf_jit_add_poke_descriptor(). But, bpf_jit_add_poke_descriptor()
> may, likely will, realloc the poke_tab[] structure and free the old
> one. So that prog->aux->poke_tab is not stable. However, the aux pointer
> is referenced from bpf_array_aux and poke_tab[] is used to 'track'
> prog<->map link. This way when progs are released the entry in the
> map is dropped and vice versa when the map is released we don't drop
> it too soon if a prog is in the process of calling it.
> 
> I wasn't able to trigger any errors here, for example having map_poke_run
> run with a poke_tab[] pointer that was free'd from
> bpf_jit_add_poke_descriptor(), but it looks possible and at very least
> is very fragile.
> 
> This patch moves poke_track call out of loop that is calling add_poke
> so that we only ever add stable aux->poke_tab pointers to the map's
> bpf_array_aux struct. Further, we need this in the next patch to fix
> a real bug where progs are not 'untracked'.
> 
> Signed-off-by: John Fastabend <john.fastabend@...il.com>
> ---
>  kernel/bpf/verifier.c |    6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 6e2ebcb0d66f..066fac9b5460 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -12126,8 +12126,12 @@ static int jit_subprogs(struct bpf_verifier_env *env)
>  			}
>  
>  			func[i]->insnsi[insn_idx - subprog_start].imm = ret + 1;
> +		}
>  
> -			map_ptr = func[i]->aux->poke_tab[ret].tail_call.map;
> +		for (j = 0; j < func[i]->aux->size_poke_tab; j++) {
> +			int ret;
> +
> +			map_ptr = func[i]->aux->poke_tab[j].tail_call.map;

I don't see why it's necessary.
poke_tab pointer will be re-read after bpf_jit_add_poke_descriptor().
The compiler is not allowed to cache it.

I've applied the patch 1.