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From:   Magnus Karlsson <magnus.karlsson@gmail.com>
Date:   Fri, 2 Sep 2022 08:15:34 +0200
Message-ID: <CAJ8uoz0RXAFPfu1v_1UuV-iUZ846ZbHNNB=oCQ=sV=pbLSzvxw@mail.gmail.com>
Subject: Re: [PATCH v1] selftests/xsk: Avoid use-after-free on ctx
To:     Ian Rogers <irogers@google.com>
Cc:     =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>,
        Magnus Karlsson <magnus.karlsson@intel.com>,
        Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
        Jonathan Lemon <jonathan.lemon@gmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        Mykola Lysenko <mykolal@fb.com>, Shuah Khan <shuah@kernel.org>,
        netdev@vger.kernel.org, bpf@vger.kernel.org,
        linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org
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On Thu, Sep 1, 2022 at 10:56 PM Ian Rogers <irogers@google.com> wrote:
>
> The put lowers the reference count to 0 and frees ctx, reading it
> afterwards is invalid. Move the put after the uses and determine the
> last use by the reference count being 1.

Thanks for spotting and fixing this Ian.

Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>

> Fixes: 39e940d4abfa ("selftests/xsk: Destroy BPF resources only when ctx refcount drops to 0")
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
>  tools/testing/selftests/bpf/xsk.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/xsk.c b/tools/testing/selftests/bpf/xsk.c
> index f2721a4ae7c5..0b3ff49c740d 100644
> --- a/tools/testing/selftests/bpf/xsk.c
> +++ b/tools/testing/selftests/bpf/xsk.c
> @@ -1237,15 +1237,15 @@ void xsk_socket__delete(struct xsk_socket *xsk)
>         ctx = xsk->ctx;
>         umem = ctx->umem;
>
> -       xsk_put_ctx(ctx, true);
> -
> -       if (!ctx->refcount) {
> +       if (ctx->refcount == 1) {
>                 xsk_delete_bpf_maps(xsk);
>                 close(ctx->prog_fd);
>                 if (ctx->has_bpf_link)
>                         close(ctx->link_fd);
>         }
>
> +       xsk_put_ctx(ctx, true);
> +
>         err = xsk_get_mmap_offsets(xsk->fd, &off);
>         if (!err) {
>                 if (xsk->rx) {
> --
> 2.37.2.789.g6183377224-goog
>