Messages in this thread | | | Date | Fri, 11 Sep 2015 15:02:36 +0200 | From | Daniel Borkmann <> | Subject | Re: [PATCH v2 2/5] seccomp: make underlying bpf ref counted as well |
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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@canonical.com> > CC: Kees Cook <keescook@chromium.org> > CC: Will Drewry <wad@chromium.org> > CC: Oleg Nesterov <oleg@redhat.com> > CC: Andy Lutomirski <luto@amacapital.net> > CC: Pavel Emelyanov <xemul@parallels.com> > CC: Serge E. Hallyn <serge.hallyn@ubuntu.com> > CC: Alexei Starovoitov <ast@kernel.org> > CC: Daniel Borkmann <daniel@iogearbox.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.
> return sfilter; > } > @@ -470,7 +472,7 @@ void get_seccomp_filter(struct task_struct *tsk) > static inline void seccomp_filter_free(struct seccomp_filter *filter) > { > if (filter) { > - bpf_prog_free(filter->prog); > + bpf_prog_put(filter->prog); > kfree(filter); > } > } >
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