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Date:   Mon, 24 Aug 2020 21:08:40 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Yonghong Song <yhs@...com>, 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 1:09 PM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> 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>

Applied. Thanks

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