On Fri, Sep 11, 2015 at 03:02:36PM +0200, Daniel Borkmann wrote:

On 09/11/2015 02:20 AM, Tycho Andersen wrote:

quoted

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 <redacted>
CC: Kees Cook <redacted>
CC: Will Drewry <redacted>
CC: Oleg Nesterov <redacted>
CC: Andy Lutomirski <redacted>
CC: Pavel Emelyanov <redacted>
CC: Serge E. Hallyn <serge.hallyn-GeWIH/nMZzLQT0dZR+AlfA@public.gmane.org>
CC: Alexei Starovoitov <redacted>
CC: Daniel Borkmann <redacted>
---
 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?

Tycho