Create 2 kfuncs exposing control over Landlock functionality to BPF callers. Export an opaque struct bpf_landlock_ruleset preventing callers from accessing unstable internal Landlock fields. 1) bpf_landlock_put_ruleset releases a reference on a bpf_landlock_ruleset. This is properly passed to the verifier with the KF_RELEASE annotation. 2) bpf_landlock_restrict_binprm alters the pre-committed credentials in the linux_binprm struct, ensuring the program will start with the specified landlock ruleset. Normal domain inheritance, for existing and future domains apply as normal. To enable proper reference counting and destruction, a destructor is registered for the bpf_landlock_ruleset. Additionally, both kfuncs are restricted to LSM programs attached to bprm_creds_for_exec or bprm_creds_from_file, and only sleepable varients of these hooks. Landlock may block because a ruleset is protected by a lock, so both of the above kfuncs may sleep and are KF_SLEEPABLE. If RESTRICT_FLAGS_NO_NEW_PRIVS is set, and the task doesn't have CAP_SYS_ADMIN or is not already running with no_new_privs, we set the set_nnp_on_point_of_no_return to ensure that the next execution transition (but not the current one) will be subject to no_new_privs. Running task_set_no_new_privs directly is unsafe in this path, as a failed execution will result in a lingering side effect of no_new_privs being set on the original thread. Signed-off-by: Justin Suess --- include/linux/bpf_lsm.h | 15 +++++ kernel/bpf/bpf_lsm.c | 145 ++++++++++++++++++++++++++++++++++++++++ 2 files changed, 160 insertions(+) diff --git a/include/linux/bpf_lsm.h b/include/linux/bpf_lsm.h index 643809cc78c3..1fc019c0db44 100644 --- a/include/linux/bpf_lsm.h +++ b/include/linux/bpf_lsm.h @@ -31,6 +31,21 @@ int bpf_lsm_verify_prog(struct bpf_verifier_log *vlog, bool bpf_lsm_is_sleepable_hook(u32 btf_id); bool bpf_lsm_is_trusted(const struct bpf_prog *prog); +/* + * Opaque type for BPF landlock ruleset. This is used to prevent BPF programs + * from directly accessing the landlock_ruleset structure, which is not designed + * for external use and may change in the future. + */ +struct bpf_landlock_ruleset {}; +BTF_ID_LIST_SINGLE(bpf_landlock_ruleset_btf_ids, struct, bpf_landlock_ruleset) +__bpf_kfunc void +bpf_landlock_put_ruleset(const struct bpf_landlock_ruleset *ruleset); +__bpf_kfunc int +bpf_landlock_restrict_binprm(struct linux_binprm *bprm, + const struct bpf_landlock_ruleset *ruleset, + u32 flags); +__bpf_kfunc void bpf_landlock_put_ruleset_dtor(void *ruleset); + static inline struct bpf_storage_blob *bpf_inode( const struct inode *inode) { diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c index 0c4a0c8e6f70..5da9950aa555 100644 --- a/kernel/bpf/bpf_lsm.c +++ b/kernel/bpf/bpf_lsm.c @@ -16,6 +16,7 @@ #include #include #include +#include /* For every LSM hook that allows attachment of BPF programs, declare a nop * function where a BPF program can be attached. Notably, we qualify each with @@ -447,3 +448,147 @@ int bpf_lsm_get_retval_range(const struct bpf_prog *prog, } return 0; } + +BTF_SET_START(bpf_landlock_kfunc_hooks) +BTF_ID(func, bpf_lsm_bprm_creds_for_exec) +BTF_ID(func, bpf_lsm_bprm_creds_from_file) +BTF_SET_END(bpf_landlock_kfunc_hooks) + +BTF_KFUNCS_START(bpf_landlock_kfunc_btf_ids) +BTF_ID_FLAGS(func, bpf_landlock_put_ruleset, KF_RELEASE | KF_SLEEPABLE) +BTF_ID_FLAGS(func, bpf_landlock_restrict_binprm, KF_SLEEPABLE) +BTF_KFUNCS_END(bpf_landlock_kfunc_btf_ids) + +BTF_ID_LIST(bpf_landlock_dtor_ids) +BTF_ID(struct, bpf_landlock_ruleset) +BTF_ID(func, bpf_landlock_put_ruleset_dtor) + +static int bpf_landlock_kfunc_filter(const struct bpf_prog *prog, u32 kfunc_id) +{ + if (!btf_id_set8_contains(&bpf_landlock_kfunc_btf_ids, kfunc_id)) + return 0; + + /* BPF_LSM_CGROUP programs run under classic RCU and cannot sleep. */ + if (prog->expected_attach_type == BPF_LSM_CGROUP) + return -EACCES; + + if (!btf_id_set_contains(&bpf_landlock_kfunc_hooks, + prog->aux->attach_btf_id)) + return -EACCES; + + return 0; +} + +static const struct btf_kfunc_id_set bpf_landlock_kfunc_set = { + .owner = THIS_MODULE, + .set = &bpf_landlock_kfunc_btf_ids, + .filter = bpf_landlock_kfunc_filter, +}; + +static int __init bpf_landlock_kfunc_init(void) +{ + const struct btf_id_dtor_kfunc bpf_landlock_dtors[] = { + { + .btf_id = bpf_landlock_dtor_ids[0], + .kfunc_btf_id = bpf_landlock_dtor_ids[1], + }, + }; + int ret; + + ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_LSM, + &bpf_landlock_kfunc_set); + if (ret) + return ret; + + return register_btf_id_dtor_kfuncs(bpf_landlock_dtors, + ARRAY_SIZE(bpf_landlock_dtors), + THIS_MODULE); +} + +late_initcall(bpf_landlock_kfunc_init); + +__bpf_kfunc_start_defs(); + +#if IS_ENABLED(CONFIG_SECURITY_LANDLOCK) + +/** + * bpf_landlock_put_ruleset - put a Landlock ruleset + * @ruleset: Landlock ruleset to put + */ +__bpf_kfunc void +bpf_landlock_put_ruleset(const struct bpf_landlock_ruleset *ruleset) +{ + landlock_put_ruleset((struct landlock_ruleset *)ruleset); +} + +/** + * bpf_landlock_restrict_binprm - enforce a Landlock ruleset on exec credentials + * @bprm: execution context providing the prepared credentials to restrict + * @ruleset: Landlock ruleset to enforce, may be NULL only with + * LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF + * @flags: landlock_restrict_self() flags + * + * When @flags contains LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS, the request is + * staged through @bprm and committed only after exec reaches point-of-no-return. + * This guarantees that the resulting task cannot gain more privileges through + * later exec transitions, including when called from bprm_creds_from_file. + * The current execution is unaffected, and may escalate as usual until the next + * exec. + */ +__bpf_kfunc int +bpf_landlock_restrict_binprm(struct linux_binprm *bprm, + const struct bpf_landlock_ruleset *ruleset, + u32 flags) +{ + int err = landlock_restrict_cred_precheck(flags, false); + + if (err) + return err; + + err = landlock_restrict_cred(bprm->cred, + (struct landlock_ruleset *)ruleset, + flags); + + if (err) + return err; + /* + * Stage no_new_privs through @bprm so exec can honor it without + * mutating the current task before point-of-no-return. + */ + if ((flags & LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS) + && !task_no_new_privs(current) + && !ns_capable_noaudit(current_user_ns(), CAP_SYS_ADMIN)) + bprm->set_nnp_on_point_of_no_return = 1; + + return err; +} + +/* We define stubs for these to allow ebpf programs using landlock kfuncs to load + * even when CONFIG_SECURITY_LANDLOCK is not enabled. + */ +#else /* IS_ENABLED(CONFIG_SECURITY_LANDLOCK) */ + +__bpf_kfunc void +bpf_landlock_put_ruleset(const struct bpf_landlock_ruleset *ruleset) +{ +} + +__bpf_kfunc int +bpf_landlock_restrict_binprm(struct linux_binprm *bprm, + const struct bpf_landlock_ruleset *ruleset, + u32 flags) +{ + return -EOPNOTSUPP; +} + +#endif /* IS_ENABLED(CONFIG_SECURITY_LANDLOCK) */ + +/* Destructor does nothing when Landlock is not enabled */ +__bpf_kfunc void bpf_landlock_put_ruleset_dtor(void *ruleset) +{ + bpf_landlock_put_ruleset(ruleset); +} + +CFI_NOSEAL(bpf_landlock_put_ruleset_dtor); + +__bpf_kfunc_end_defs(); -- 2.53.0