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

[Daniel Borkmann]

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@xxxxxxxxxxxxx>
> > > CC: Kees Cook <keescook@xxxxxxxxxxxx>
> > > CC: Will Drewry <wad@xxxxxxxxxxxx>
> > > CC: Oleg Nesterov <oleg@xxxxxxxxxx>
> > > CC: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> > > CC: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
> > > CC: Serge E. Hallyn <serge.hallyn@xxxxxxxxxx>
> > > CC: Alexei Starovoitov <ast@xxxxxxxxxx>
> > > CC: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> > > ---
> > >   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.
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