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Date:   Mon, 21 Sep 2020 12:21:20 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Tony Ambardar <tony.ambardar@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        linux-arch@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH bpf v1 1/3] bpf: fix sysfs export of empty BTF section

On Sat, Sep 19, 2020 at 10:05 PM Tony Ambardar <tony.ambardar@...il.com> wrote:
>
> If BTF data is missing or removed from the ELF section it is still exported
> via sysfs as a zero-length file:
>
>   root@...nWrt:/# ls -l /sys/kernel/btf/vmlinux
>   -r--r--r--    1 root    root    0 Jul 18 02:59 /sys/kernel/btf/vmlinux
>
> Moreover, reads from this file succeed and leak kernel data:
>
>   root@...nWrt:/# hexdump -C /sys/kernel/btf/vmlinux|head -10
>   000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
>   *
>   000cc0 00 00 00 00 00 00 00 00 00 00 00 00 80 83 b0 80 |................|
>   000cd0 00 10 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
>   000ce0 00 00 00 00 00 00 00 00 00 00 00 00 57 ac 6e 9d |............W.n.|
>   000cf0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
>   *
>   002650 00 00 00 00 00 00 00 10 00 00 00 01 00 00 00 01 |................|
>   002660 80 82 9a c4 80 85 97 80 81 a9 51 68 00 00 00 02 |..........Qh....|
>   002670 80 25 44 dc 80 85 97 80 81 a9 50 24 81 ab c4 60 |.%D.......P$...`|
>
> This situation was first observed with kernel 5.4.x, cross-compiled for a
> MIPS target system. Fix by adding a sanity-check for export of zero-length
> data sections.
>
> Fixes: 341dfcf8d78e ("btf: expose BTF info through sysfs")
>
> Signed-off-by: Tony Ambardar <Tony.Ambardar@...il.com>
> ---

Apparently sysfs infrastructure doesn't validate read position and
size when bin_attribute's size is 0, and just expects read callback to
handle such situation explicitly. Preventing sysfs entry from
registering seems like a good solution. Thanks!

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

>  kernel/bpf/sysfs_btf.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/bpf/sysfs_btf.c b/kernel/bpf/sysfs_btf.c
> index 3b495773de5a..11b3380887fa 100644
> --- a/kernel/bpf/sysfs_btf.c
> +++ b/kernel/bpf/sysfs_btf.c
> @@ -30,15 +30,15 @@ static struct kobject *btf_kobj;
>
>  static int __init btf_vmlinux_init(void)
>  {
> -       if (!__start_BTF)
> +       bin_attr_btf_vmlinux.size = __stop_BTF - __start_BTF;
> +
> +       if (!__start_BTF || bin_attr_btf_vmlinux.size == 0)
>                 return 0;
>
>         btf_kobj = kobject_create_and_add("btf", kernel_kobj);
>         if (!btf_kobj)
>                 return -ENOMEM;
>
> -       bin_attr_btf_vmlinux.size = __stop_BTF - __start_BTF;
> -
>         return sysfs_create_bin_file(btf_kobj, &bin_attr_btf_vmlinux);
>  }
>
> --
> 2.25.1
>

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