Re: [PATCH v2 2/5] seccomp: make underlying bpf ref counted as well
From: Daniel Borkmann <hidden>
Date: 2015-09-11 13:02:47
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lkml, netdev
On 09/11/2015 02:20 AM, Tycho Andersen wrote:
quoted hunk ↗ jump to hunk
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.
quoted hunk ↗ jump to hunk
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); } }