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From: Kees Cook <keescook@chromium.org>
To: Paul Chaignon <paul.chaignon@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
	Will Drewry <wad@chromium.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] seccomp: allow BPF_MOD ALU instructions
Date: Mon, 12 Aug 2019 10:38:02 -0700	[thread overview]
Message-ID: <201908121035.06695C79F@keescook> (raw)
In-Reply-To: <CAO5pjwSe+U70tSPjKOgFsqqF=gCKXPDREzYF81NCZ03kGAyWww@mail.gmail.com>

On Sun, Aug 11, 2019 at 10:58:33AM +0200, Paul Chaignon wrote:
> On Fri, Aug 9, 2019 at 8:26 PM Paul Chaignon <paul.chaignon@orange.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...

No worries! Changing the dialect of seccomp BPF isn't something I'd like
to do without really good reason since it creates a split in the filter
correctness from userspace (i.e. a filter using BPF_MOD on an older
kernel will fail). So there would need to be a distinct flag set
somewhere, etc. So, if you do end up discovering later you really want
BPF_MOD, we can figure that out, but for now if you can get by with "&",
that would be best. :)

Thanks!

-Kees

> 
> >
> > Signed-off-by: Paul Chaignon <paul.chaignon@orange.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

-- 
Kees Cook

  reply	other threads:[~2019-08-12 18:06 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-09 18:26 [PATCH] seccomp: allow BPF_MOD ALU instructions Paul Chaignon
2019-08-11  8:58 ` Paul Chaignon
2019-08-12 17:38   ` Kees Cook [this message]
2020-03-16 16:36 Anton Protopopov
2020-03-16 21:23 ` Kees Cook
2020-03-16 22:17   ` Anton Protopopov
2020-03-17 20:20     ` Kees Cook
2020-03-18  1:11       ` Anton Protopopov
2020-03-18  4:06         ` Kees Cook
2020-03-18 15:23           ` Anton Protopopov

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