linux-kernel - Re: [PATCH v2 2/5] seccomp: make underlying bpf ref counted as well

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Message-ID: <20150914160030.GC31864@smitten>
Date:	Mon, 14 Sep 2015 10:00:30 -0600
From:	Tycho Andersen <tycho.andersen@...onical.com>
To:	Daniel Borkmann <daniel@...earbox.net>
Cc:	Kees Cook <keescook@...omium.org>,
	Alexei Starovoitov <ast@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Will Drewry <wad@...omium.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Pavel Emelyanov <xemul@...allels.com>,
	"Serge E. Hallyn" <serge.hallyn@...ntu.com>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	linux-api@...r.kernel.org
Subject: Re: [PATCH v2 2/5] seccomp: make underlying bpf ref counted as well

Hi Daniel,

On Fri, Sep 11, 2015 at 08:28:19PM +0200, Daniel Borkmann wrote:
> I think due to the given insns restrictions on classic seccomp, this
> could work for "most cases" (see below) for the time being until pointer
> sanitation is resolved and that seccomp-only restriction from the dump
> could be removed,

Ok, thanks.

> BUT there's one more stone in the road which you still
> need to take care of with this whole 'giving classic seccomp-BPF -> eBPF
> transforms an fd, dumping and restoring that via bpf(2)' approach:
> 
> If you have JIT enabled on ARM32, and add a classic seccomp-BPF filter,
> and dump that via your bpf(2) interface based on the current patches, what
> you'll get is not eBPF opcodes but classic (!) BPF opcodes as ARM32 classic
> JIT supports compilation of seccomp, since commit 24e737c1ebac ("ARM: net:
> add JIT support for loads from struct seccomp_data.").
> 
> So in that case, bpf_prepare_filter() will not call into bpf_migrate_filter()
> as there's simply no need for it, because the classic code could already
> be JITed there. I guess other archs where JIT support for eBPF in not yet
> within near sight might sooner or later support this insn for their classic
> JITs, too ...

Thanks for pointing this out.

What if we legislate that the output of bpf(BPF_PROG_DUMP, ...) is
always eBPF? As near as I can tell there is no way to determine if a
struct bpf_prog is classic or eBPF, so we'd need to add a bit to
indicate whether or not the prog has been converted so that
BPF_PROG_DUMP knows when to convert it.

Tycho
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