netdev - Re: [PATCH 1/2] bpf: cgroup: Fix optlen WARN_ON

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Date:   Fri, 22 Jan 2021 09:04:07 -0800
From:   Stanislav Fomichev <sdf@...gle.com>
To:     Loris Reiff <loris.reiff@...lor.ch>
Cc:     bpf <bpf@...r.kernel.org>, Netdev <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>, kpsingh@...nel.org
Subject: Re: [PATCH 1/2] bpf: cgroup: Fix optlen WARN_ON_ONCE toctou

On Fri, Jan 22, 2021 at 8:43 AM Loris Reiff <loris.reiff@...lor.ch> wrote:
>
> A toctou issue in `__cgroup_bpf_run_filter_getsockopt` can trigger a
> WARN_ON_ONCE in a check of `copy_from_user`.
> `*optlen` is checked to be non-negative in the individual getsockopt
> functions beforehand. Changing `*optlen` in a race to a negative value
> will result in a `copy_from_user(ctx.optval, optval, ctx.optlen)` with
> `ctx.optlen` being a negative integer.
>
> Fixes: 0d01da6afc54 ("bpf: implement getsockopt and setsockopt hooks")
> Signed-off-by: Loris Reiff <loris.reiff@...lor.ch>
> ---
>  kernel/bpf/cgroup.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> index 96555a8a2..6ec8f02f4 100644
> --- a/kernel/bpf/cgroup.c
> +++ b/kernel/bpf/cgroup.c
> @@ -1442,6 +1442,11 @@ int __cgroup_bpf_run_filter_getsockopt(struct sock *sk, int level,
>                         goto out;
>                 }
>
> +               if (ctx.optlen < 0) {
> +                       ret = -EFAULT;
> +                       goto out;
> +               }
> +
>                 if (copy_from_user(ctx.optval, optval,
>                                    min(ctx.optlen, max_optlen)) != 0) {
>                         ret = -EFAULT;
> --
> 2.29.2
Good point, user's optlen can be concurrently changed after the kernel
updated it.

Reviewed-by: Stanislav Fomichev <sdf@...gle.com>