bpf: per-cgroup lsm flavor

Allow attaching to lsm hooks in the cgroup context.

Attaching to per-cgroup LSM works exactly like attaching
to other per-cgroup hooks. New BPF_LSM_CGROUP is added
to trigger new mode; the actual lsm hook we attach to is
signaled via existing attach_btf_id.

For the hooks that have 'struct socket' or 'struct sock' as its first
argument, we use the cgroup associated with that socket. For the rest,
we use 'current' cgroup (this is all on default hierarchy == v2 only).
Note that for some hooks that work on 'struct sock' we still
take the cgroup from 'current' because some of them work on the socket
that hasn't been properly initialized yet.

Behind the scenes, we allocate a shim program that is attached
to the trampoline and runs cgroup effective BPF programs array.
This shim has some rudimentary ref counting and can be shared
between several programs attaching to the same lsm hook from
different cgroups.

Note that this patch bloats cgroup size because we add 211
cgroup_bpf_attach_type(s) for simplicity sake. This will be
addressed in the subsequent patch.

Also note that we only add non-sleepable flavor for now. To enable
sleepable use-cases, bpf_prog_run_array_cg has to grab trace rcu,
shim programs have to be freed via trace rcu, cgroup_bpf.effective
should be also trace-rcu-managed + maybe some other changes that
I'm not aware of.

Reviewed-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20220628174314.1216643-4-sdf@google.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 32 insertions, 0 deletions

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 4938477912cd..df3ec6b05f05 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7322,6 +7322,18 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
 				reg_type_str(env, regs[BPF_REG_1].type));
 			return -EACCES;
 		}
+		break;
+	case BPF_FUNC_set_retval:
+		if (env->prog->expected_attach_type == BPF_LSM_CGROUP) {
+			if (!env->prog->aux->attach_func_proto->type) {
+				/* Make sure programs that attach to void
+				 * hooks don't try to modify return value.
+				 */
+				verbose(env, "BPF_LSM_CGROUP that attach to void LSM hooks can't modify return value!\n");
+				return -EINVAL;
+			}
+		}
+		break;
 	}
 
 	if (err)
@@ -10527,6 +10539,22 @@ static int check_return_code(struct bpf_verifier_env *env)
 	case BPF_PROG_TYPE_SK_LOOKUP:
 		range = tnum_range(SK_DROP, SK_PASS);
 		break;
+
+	case BPF_PROG_TYPE_LSM:
+		if (env->prog->expected_attach_type != BPF_LSM_CGROUP) {
+			/* Regular BPF_PROG_TYPE_LSM programs can return
+			 * any value.
+			 */
+			return 0;
+		}
+		if (!env->prog->aux->attach_func_proto->type) {
+			/* Make sure programs that attach to void
+			 * hooks don't try to modify return value.
+			 */
+			range = tnum_range(1, 1);
+		}
+		break;
+
 	case BPF_PROG_TYPE_EXT:
 		/* freplace program can return anything as its return value
 		 * depends on the to-be-replaced kernel func or bpf program.
@@ -10543,6 +10571,9 @@ static int check_return_code(struct bpf_verifier_env *env)
 
 	if (!tnum_in(range, reg->var_off)) {
 		verbose_invalid_scalar(env, reg, &range, "program exit", "R0");
+		if (prog->expected_attach_type == BPF_LSM_CGROUP &&
+		    !prog->aux->attach_func_proto->type)
+			verbose(env, "Note, BPF_LSM_CGROUP that attach to void LSM hooks can't modify return value!\n");
 		return -EINVAL;
 	}
 
@@ -14902,6 +14933,7 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
 		fallthrough;
 	case BPF_MODIFY_RETURN:
 	case BPF_LSM_MAC:
+	case BPF_LSM_CGROUP:
 	case BPF_TRACE_FENTRY:
 	case BPF_TRACE_FEXIT:
 		if (!btf_type_is_func(t)) {