Re: [PATCH RFC 1/4] bpf: add cgroup device guard to flag a cgroup device prog
From: Alexander Mikhalitsyn
Date: Mon Aug 14 2023 - 11:55:17 EST
On Mon, Aug 14, 2023 at 4:32 PM Michael Weiß
<michael.weiss@xxxxxxxxxxxxxxxxxxx> wrote:
>
> Introduce the BPF_F_CGROUP_DEVICE_GUARD flag for BPF_PROG_LOAD
> which allows to set a cgroup device program to be a device guard.
> Later this may be used to guard actions on device nodes in
> non-initial userns. For this reason we provide the helper function
> cgroup_bpf_device_guard_enabled() to check if a task has a cgroups
> device program which is a device guard in its effective set of bpf
> programs.
>
> Signed-off-by: Michael Weiß <michael.weiss@xxxxxxxxxxxxxxxxxxx>
Hi Michael!
Thanks for working on this. It's also very useful for the LXC system
containers project.
> ---
> include/linux/bpf-cgroup.h | 7 +++++++
> include/linux/bpf.h | 1 +
> include/uapi/linux/bpf.h | 5 +++++
> kernel/bpf/cgroup.c | 30 ++++++++++++++++++++++++++++++
> kernel/bpf/syscall.c | 5 ++++-
> tools/include/uapi/linux/bpf.h | 5 +++++
> 6 files changed, 52 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/bpf-cgroup.h b/include/linux/bpf-cgroup.h
> index 57e9e109257e..112b6093f9fd 100644
> --- a/include/linux/bpf-cgroup.h
> +++ b/include/linux/bpf-cgroup.h
> @@ -184,6 +184,8 @@ static inline bool cgroup_bpf_sock_enabled(struct sock *sk,
> return array != &bpf_empty_prog_array.hdr;
> }
>
> +bool cgroup_bpf_device_guard_enabled(struct task_struct *task);
> +
> /* Wrappers for __cgroup_bpf_run_filter_skb() guarded by cgroup_bpf_enabled. */
> #define BPF_CGROUP_RUN_PROG_INET_INGRESS(sk, skb) \
> ({ \
> @@ -476,6 +478,11 @@ static inline int bpf_percpu_cgroup_storage_update(struct bpf_map *map,
> return 0;
> }
>
> +static bool cgroup_bpf_device_guard_enabled(struct task_struct *task)
> +{
> + return false;
> +}
> +
> #define cgroup_bpf_enabled(atype) (0)
> #define BPF_CGROUP_RUN_SA_PROG_LOCK(sk, uaddr, atype, t_ctx) ({ 0; })
> #define BPF_CGROUP_RUN_SA_PROG(sk, uaddr, atype) ({ 0; })
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index f58895830ada..313cce8aee05 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1384,6 +1384,7 @@ struct bpf_prog_aux {
> bool sleepable;
> bool tail_call_reachable;
> bool xdp_has_frags;
> + bool cgroup_device_guard;
> /* BTF_KIND_FUNC_PROTO for valid attach_btf_id */
> const struct btf_type *attach_func_proto;
> /* function name for valid attach_btf_id */
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 60a9d59beeab..3be57f7957b1 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -1165,6 +1165,11 @@ enum bpf_link_type {
> */
> #define BPF_F_XDP_DEV_BOUND_ONLY (1U << 6)
>
> +/* If BPF_F_CGROUP_DEVICE_GUARD is used in BPF_PROG_LOAD command, the loaded
> + * program will be allowed to guard device access inside user namespaces.
> + */
> +#define BPF_F_CGROUP_DEVICE_GUARD (1U << 7)
> +
> /* link_create.kprobe_multi.flags used in LINK_CREATE command for
> * BPF_TRACE_KPROBE_MULTI attach type to create return probe.
> */
> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> index 5b2741aa0d9b..230693ca4cdb 100644
> --- a/kernel/bpf/cgroup.c
> +++ b/kernel/bpf/cgroup.c
> @@ -2505,6 +2505,36 @@ const struct bpf_verifier_ops cg_sockopt_verifier_ops = {
> const struct bpf_prog_ops cg_sockopt_prog_ops = {
> };
>
> +bool
> +cgroup_bpf_device_guard_enabled(struct task_struct *task)
> +{
> + bool ret;
> + const struct bpf_prog_array *array;
> + const struct bpf_prog_array_item *item;
> + const struct bpf_prog *prog;
> + struct cgroup *cgrp = task_dfl_cgroup(task);
> +
> + ret = false;
> +
> + array = rcu_access_pointer(cgrp->bpf.effective[CGROUP_DEVICE]);
> + if (array == &bpf_empty_prog_array.hdr)
> + return ret;
> +
> + mutex_lock(&cgroup_mutex);
-> cgroup_lock();
> + array = rcu_dereference_protected(cgrp->bpf.effective[CGROUP_DEVICE],
> + lockdep_is_held(&cgroup_mutex));
> + item = &array->items[0];
> + while ((prog = READ_ONCE(item->prog))) {
> + if (prog->aux->cgroup_device_guard) {
> + ret = true;
> + break;
> + }
> + item++;
> + }
> + mutex_unlock(&cgroup_mutex);
Don't we want to make this function specific to "current" task? This
allows to make locking lighter like in
__cgroup_bpf_check_dev_permission
https://github.com/torvalds/linux/blob/2ccdd1b13c591d306f0401d98dedc4bdcd02b421/kernel/bpf/cgroup.c#L1531
Here we have only RCU read lock.
AFAIK, cgroup_mutex is a heavily-contended lock.
Kind regards,
Alex
> + return ret;
> +}
> +
> /* Common helpers for cgroup hooks. */
> const struct bpf_func_proto *
> cgroup_common_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index a2aef900519c..33ea67c702c1 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -2564,7 +2564,8 @@ static int bpf_prog_load(union bpf_attr *attr, bpfptr_t uattr, u32 uattr_size)
> BPF_F_SLEEPABLE |
> BPF_F_TEST_RND_HI32 |
> BPF_F_XDP_HAS_FRAGS |
> - BPF_F_XDP_DEV_BOUND_ONLY))
> + BPF_F_XDP_DEV_BOUND_ONLY |
> + BPF_F_CGROUP_DEVICE_GUARD))
> return -EINVAL;
>
> if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) &&
> @@ -2651,6 +2652,8 @@ static int bpf_prog_load(union bpf_attr *attr, bpfptr_t uattr, u32 uattr_size)
> prog->aux->dev_bound = !!attr->prog_ifindex;
> prog->aux->sleepable = attr->prog_flags & BPF_F_SLEEPABLE;
> prog->aux->xdp_has_frags = attr->prog_flags & BPF_F_XDP_HAS_FRAGS;
> + prog->aux->cgroup_device_guard =
> + attr->prog_flags & BPF_F_CGROUP_DEVICE_GUARD;
>
> err = security_bpf_prog_alloc(prog->aux);
> if (err)
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index 60a9d59beeab..3be57f7957b1 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -1165,6 +1165,11 @@ enum bpf_link_type {
> */
> #define BPF_F_XDP_DEV_BOUND_ONLY (1U << 6)
>
> +/* If BPF_F_CGROUP_DEVICE_GUARD is used in BPF_PROG_LOAD command, the loaded
> + * program will be allowed to guard device access inside user namespaces.
> + */
> +#define BPF_F_CGROUP_DEVICE_GUARD (1U << 7)
> +
> /* link_create.kprobe_multi.flags used in LINK_CREATE command for
> * BPF_TRACE_KPROBE_MULTI attach type to create return probe.
> */
>
> --
> 2.30.2
>