linux-kernel - Re: [PATCH] seccomp: allow BPF

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Message-ID: <CAO5pjwSe+U70tSPjKOgFsqqF=gCKXPDREzYF81NCZ03kGAyWww@mail.gmail.com>
Date:   Sun, 11 Aug 2019 10:58:33 +0200
From:   Paul Chaignon <paul.chaignon@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] seccomp: allow BPF_MOD ALU instructions

On Fri, Aug 9, 2019 at 8:26 PM Paul Chaignon <paul.chaignon@...nge.com> wrote:
>
> We need BPF_MOD to match system calls against whitelists encoded as 32-bit
> bit arrays.  The selection of the syscall's bit in the appropriate bit
> array requires a modulo operation such that X = 1 << nr % 32.

Of course, X = 1 << nr & 0x1F, and we can do without BPF_MOD in our case.
I'll put that on a lack of sleep...

>
> Signed-off-by: Paul Chaignon <paul.chaignon@...nge.com>
> ---
>  kernel/seccomp.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 811b4a86cdf6..87de6532ff6d 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -205,6 +205,8 @@ static int seccomp_check_filter(struct sock_filter *filter, unsigned int flen)
>                 case BPF_ALU | BPF_MUL | BPF_X:
>                 case BPF_ALU | BPF_DIV | BPF_K:
>                 case BPF_ALU | BPF_DIV | BPF_X:
> +               case BPF_ALU | BPF_MOD | BPF_K:
> +               case BPF_ALU | BPF_MOD | BPF_X:
>                 case BPF_ALU | BPF_AND | BPF_K:
>                 case BPF_ALU | BPF_AND | BPF_X:
>                 case BPF_ALU | BPF_OR | BPF_K:
> --
> 2.17.1