Re: [PATCH bpf-next v9 06/10] bpf: expose bpf_{g,s}etsockopt to lsm cgroup

[Martin KaFai Lau <kafai@fb.com>]

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.
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.

> 
> Signed-off-by: Stanislav Fomichev <sdf@google.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?

> +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?

> +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)
> +