linux-kernel - Re: [PATCH] bpf: btf: Fix a missing-check bug

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Date:   Mon, 22 Oct 2018 08:57:46 -0700
From:   Y Song <ys114321@...il.com>
To:     wang6495@....edu
Cc:     kjlu@....edu, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bpf: btf: Fix a missing-check bug

On Fri, Oct 19, 2018 at 3:30 PM Wenwen Wang <wang6495@....edu> wrote:
>
> In btf_parse(), the header of the user-space btf data 'btf_data' is firstly
> parsed and verified through btf_parse_hdr(). In btf_parse_hdr(), the header
> is copied from user-space 'btf_data' to kernel-space 'btf->hdr' and then
> verified. If no error happens during the verification process, the whole
> data of 'btf_data', including the header, is then copied to 'data' in
> btf_parse(). It is obvious that the header is copied twice here. More
> importantly, no check is enforced after the second copy to make sure the
> headers obtained in these two copies are same. Given that 'btf_data'
> resides in the user space, a malicious user can race to modify the header
> between these two copies. By doing so, the user can inject inconsistent
> data, which can cause undefined behavior of the kernel and introduce
> potential security risk.
>
> To avoid the above issue, this patch rewrites the header after the second
> copy, using 'btf->hdr', which is obtained in the first copy.
>
> Signed-off-by: Wenwen Wang <wang6495@....edu>
> ---
>  kernel/bpf/btf.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 138f030..2a85f91 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -2202,6 +2202,9 @@ static struct btf *btf_parse(void __user *btf_data, u32 btf_data_size,
>                 goto errout;
>         }
>
> +       memcpy(data, &btf->hdr,
> +               min_t(u32, btf->hdr.hdr_len, sizeof(btf->hdr)));

Could you restructure the code to memcpy the header followed by copying
the rest of btf_data with copy_from_user? This way, each byte is only
copied once.
Could you add some comments right before memcpy so later people will know
why we implement this way?

> +
>         err = btf_parse_str_sec(env);
>         if (err)
>                 goto errout;
> --
> 2.7.4
>