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

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Date:	Fri, 11 Sep 2015 11:33:11 -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

On Fri, Sep 11, 2015 at 06:03:59PM +0200, Daniel Borkmann wrote:
> On 09/11/2015 04:44 PM, Tycho Andersen wrote:
> >On Fri, Sep 11, 2015 at 03:02:36PM +0200, Daniel Borkmann wrote:
> >>On 09/11/2015 02:20 AM, Tycho Andersen wrote:
> >>>In the next patch, we're going to add a way to access the underlying
> >>>filters via bpf fds. This means that we need to ref-count both the
> >>>struct seccomp_filter objects and the struct bpf_prog objects separately,
> >>>in case a process dies but a filter is still referred to by another
> >>>process.
> >>>
> >>>Additionally, we mark classic converted seccomp filters as seccomp eBPF
> >>>programs, since they are a subset of what is supported in seccomp eBPF.
> >>>
> >>>Signed-off-by: Tycho Andersen <tycho.andersen@...onical.com>
> >>>CC: Kees Cook <keescook@...omium.org>
> >>>CC: Will Drewry <wad@...omium.org>
> >>>CC: Oleg Nesterov <oleg@...hat.com>
> >>>CC: Andy Lutomirski <luto@...capital.net>
> >>>CC: Pavel Emelyanov <xemul@...allels.com>
> >>>CC: Serge E. Hallyn <serge.hallyn@...ntu.com>
> >>>CC: Alexei Starovoitov <ast@...nel.org>
> >>>CC: Daniel Borkmann <daniel@...earbox.net>
> >>>---
> >>>  kernel/seccomp.c | 4 +++-
> >>>  1 file changed, 3 insertions(+), 1 deletion(-)
> >>>
> >>>diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> >>>index 245df6b..afaeddf 100644
> >>>--- a/kernel/seccomp.c
> >>>+++ b/kernel/seccomp.c
> >>>@@ -378,6 +378,8 @@ static struct seccomp_filter *seccomp_prepare_filter(struct sock_fprog *fprog)
> >>>  	}
> >>>
> >>>  	atomic_set(&sfilter->usage, 1);
> >>>+	atomic_set(&sfilter->prog->aux->refcnt, 1);
> >>>+	sfilter->prog->type = BPF_PROG_TYPE_SECCOMP;
> >>
> >>So, if you do this, then this breaks the assumption of eBPF JITs
> >>that, currently, all classic converted BPF programs always have a
> >>prog->type of BPF_PROG_TYPE_UNSPEC (see: bpf_prog_was_classic()).
> >>
> >>Currently, JITs make use of this information to determine whether
> >>A and X mappings for such programs should or should not be cleared
> >>in the prologue (s390 currently).
> >>
> >>In the seccomp_prepare_filter() stage, we're already past that, so
> >>it will not cause an issue, but we certainly would need to be very
> >>careful in future, if bpf_prog_was_classic() is then used at a later
> >>stage when we already have a generated bpf_prog somewhere, as then
> >>this assumption will break.
> >
> >The only reason we need to do this is to allow BPF_DUMP_PROG to work,
> >since we were restricting it to only allow dumping of seccomp
> >programs, since those don't have maps. Instead, perhaps we could allow
> >dumping of BPF_PROG_TYPE_SECCOMP and BPF_PROG_TYPE_UNSPEC?
> 
> There are possibilities that BPF_PROG_TYPE_UNSPEC is calling helpers
> already today, at least in networking case, not seccomp. So, since
> you want to export [classic -> eBPF] only for seccomp, put fds on them
> and dump these via bpf(2), you could allow that (with a big comment
> stating why it's safe), but mid-term we really need to sanitize all
> this stuff properly as this is needed for other types, too.

Sorry, just to be clear, you're suggesting that the patch is ok modulo
a comment describing the jit issues?

Tycho
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