netdev - Re: [PATCH bpf] bpf: fix a buffer out-of-bound access when filling raw_tp link

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Date:   Fri, 21 Aug 2020 13:09:06 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>,
        Andrii Nakryiko <andriin@...com>
Subject: Re: [PATCH bpf] bpf: fix a buffer out-of-bound access when filling
 raw_tp link_info

On Fri, Aug 21, 2020 at 12:11 PM Yonghong Song <yhs@...com> wrote:
>
> Commit f2e10bff16a0 ("bpf: Add support for BPF_OBJ_GET_INFO_BY_FD for bpf_link")
> added link query for raw_tp. One of fields in link_info is to
> fill a user buffer with tp_name. The Scurrent checking only
> declares "ulen && !ubuf" as invalid. So "!ulen && ubuf" will be
> valid. Later on, we do "copy_to_user(ubuf, tp_name, ulen - 1)" which
> may overwrite user memory incorrectly.
>
> This patch fixed the problem by disallowing "!ulen && ubuf" case as well.
>
> Fixes: f2e10bff16a0 ("bpf: Add support for BPF_OBJ_GET_INFO_BY_FD for bpf_link")
> Cc: Andrii Nakryiko <andriin@...com>
> Signed-off-by: Yonghong Song <yhs@...com>
> ---
>  kernel/bpf/syscall.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 86299a292214..ac6c784c0576 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -2634,7 +2634,7 @@ static int bpf_raw_tp_link_fill_link_info(const struct bpf_link *link,
>         u32 ulen = info->raw_tracepoint.tp_name_len;
>         size_t tp_len = strlen(tp_name);
>
> -       if (ulen && !ubuf)
> +       if (!ulen ^ !ubuf)
>                 return -EINVAL;

I think my original idea was to allow ulen == 0 && ubuf != NULL as a
still valid way to get real ulen, but it's clearly wrong with ulen-1
below. So instead of special-casing ulen==0 for the case I wanted to
support, it's easier to disallow ulen==0 && ubuf!=NULL.

So thanks for the fix!

Acked-by: Andrii Nakryiko <andriin@...com>

>
>         info->raw_tracepoint.tp_name_len = tp_len + 1;
> --
> 2.24.1
>