netdev - Re: [PATCH bpf-next v9 06/10] bpf: expose bpf

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Date:   Fri, 17 Jun 2022 11:28:24 -0700
From:   Stanislav Fomichev <sdf@...gle.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org
Subject: Re: [PATCH bpf-next v9 06/10] bpf: expose bpf_{g,s}etsockopt to lsm cgroup

On Thu, Jun 16, 2022 at 10:42 PM Martin KaFai Lau <kafai@...com> wrote:
>
> On Fri, Jun 10, 2022 at 09:57:59AM -0700, Stanislav Fomichev wrote:
> > I don't see how to make it nice without introducing btf id lists
> > for the hooks where these helpers are allowed. Some LSM hooks
> > work on the locked sockets, some are triggering early and
> > don't grab any locks, so have two lists for now:
> >
> > 1. LSM hooks which trigger under socket lock - minority of the hooks,
> >    but ideal case for us, we can expose existing BTF-based helpers
> > 2. LSM hooks which trigger without socket lock, but they trigger
> >    early in the socket creation path where it should be safe to
> >    do setsockopt without any locks
> > 3. The rest are prohibited. I'm thinking that this use-case might
> >    be a good gateway to sleeping lsm cgroup hooks in the future.
> >    We can either expose lock/unlock operations (and add tracking
> >    to the verifier) or have another set of bpf_setsockopt
> >    wrapper that grab the locks and might sleep.
> Another possibility is to acquire/release the sk lock in
> __bpf_prog_{enter,exit}_lsm_cgroup().  However, it will unnecessarily
> acquire it even the prog is not doing any get/setsockopt.
> It probably can make some checking to avoid the lock...etc. :/
>
> sleepable bpf-prog is a cleaner way out.  From a quick look,
> cgroup_storage is not safe for sleepable bpf-prog.

Is it because it's using non-trace-flavor of rcu?

> All other BPF_MAP_TYPE_{SK,INODE,TASK}_STORAGE is already
> safe once their common infra in bpf_local_storage.c was made
> sleepable-safe.

That might be another argument in favor of replacing the internal
implementation for cgroup_storage with the generic framework we use
for sk/inode/task.

> > Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> > ---
> >  include/linux/bpf.h  |  2 ++
> >  kernel/bpf/bpf_lsm.c | 40 +++++++++++++++++++++++++++++
> >  net/core/filter.c    | 60 ++++++++++++++++++++++++++++++++++++++------
> >  3 files changed, 95 insertions(+), 7 deletions(-)
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index 503f28fa66d2..c0a269269882 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -2282,6 +2282,8 @@ extern const struct bpf_func_proto bpf_for_each_map_elem_proto;
> >  extern const struct bpf_func_proto bpf_btf_find_by_name_kind_proto;
> >  extern const struct bpf_func_proto bpf_sk_setsockopt_proto;
> >  extern const struct bpf_func_proto bpf_sk_getsockopt_proto;
> > +extern const struct bpf_func_proto bpf_unlocked_sk_setsockopt_proto;
> > +extern const struct bpf_func_proto bpf_unlocked_sk_getsockopt_proto;
> >  extern const struct bpf_func_proto bpf_kallsyms_lookup_name_proto;
> >  extern const struct bpf_func_proto bpf_find_vma_proto;
> >  extern const struct bpf_func_proto bpf_loop_proto;
> > diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
> > index 83aa431dd52e..52b6e3067986 100644
> > --- a/kernel/bpf/bpf_lsm.c
> > +++ b/kernel/bpf/bpf_lsm.c
> > @@ -45,6 +45,26 @@ BTF_ID(func, bpf_lsm_sk_alloc_security)
> >  BTF_ID(func, bpf_lsm_sk_free_security)
> >  BTF_SET_END(bpf_lsm_current_hooks)
> >
> > +/* List of LSM hooks that trigger while the socket is properly locked.
> > + */
> > +BTF_SET_START(bpf_lsm_locked_sockopt_hooks)
> > +BTF_ID(func, bpf_lsm_socket_sock_rcv_skb)
> > +BTF_ID(func, bpf_lsm_sk_clone_security)
> From looking how security_sk_clone() is used at sock_copy(),
> it has two sk args, one is listen sk and one is the clone.
> I think both of them are not locked.
>
> The bpf_lsm_inet_csk_clone below should be enough to
> do setsockopt in the new clone?

Hm, good point, let me drop this one.

I wonder if long term, instead of those lists, we can annotate the
arguments with __locked or __unlocked (the way we do with __user
pointers)? That might be more scalable and we can let sleepable bpf
deal with __unlocked cases. Just thinking out loud...

> > +BTF_ID(func, bpf_lsm_sock_graft)
> > +BTF_ID(func, bpf_lsm_inet_csk_clone)
> > +BTF_ID(func, bpf_lsm_inet_conn_established)
> > +BTF_ID(func, bpf_lsm_sctp_bind_connect)
> I didn't look at this one, so I can't comment.
> Do you have a use case?

No, let's drop as well. I didn't want those lists to contain only the
cases I want, otherwise it doesn't feel generic. But sctp seems dead
anyway.


> > +BTF_SET_END(bpf_lsm_locked_sockopt_hooks)
> > +
> > +/* List of LSM hooks that trigger while the socket is _not_ locked,
> > + * but it's ok to call bpf_{g,s}etsockopt because the socket is still
> > + * in the early init phase.
> > + */
> > +BTF_SET_START(bpf_lsm_unlocked_sockopt_hooks)
> > +BTF_ID(func, bpf_lsm_socket_post_create)
> > +BTF_ID(func, bpf_lsm_socket_socketpair)
> > +BTF_SET_END(bpf_lsm_unlocked_sockopt_hooks)
> > +