[PATCH bpf-next v2 5/6] bpf: teach the verifier to enforce css_iter and process_iter in RCU CS

Chuyi Zhou
2023-09-12T07:02:17Z

css_iter and process_iter should be used in rcu section. Specifically, in
sleepable progs explicit bpf_rcu_read_lock() is needed before use these
iters. In normal bpf progs that have implicit rcu_read_lock(), it's OK to
use them directly.

This patch checks whether we are in rcu cs before we want to invoke
bpf_iter_process_new and bpf_iter_css_{pre, post}_new in
mark_stack_slots_iter(). If the rcu protection is guaranteed, we would
let st->type = PTR_TO_STACK | MEM_RCU. is_iter_reg_valid_init() will
reject if reg->type is UNTRUSTED.

Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
---
kernel/bpf/verifier.c | 30 ++++++++++++++++++++++++++++--
1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 2367483bf4c2..6a6827ba7a18 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1172,7 +1172,13 @@ static bool is_dynptr_type_expected(struct bpf_verifier_env *env, struct bpf_reg

static void __mark_reg_known_zero(struct bpf_reg_state *reg);

+static bool in_rcu_cs(struct bpf_verifier_env *env);
+
+/* check whether we are using bpf_iter_process_*() or bpf_iter_css_*() */
+static bool is_iter_need_rcu(struct bpf_kfunc_call_arg_meta *meta);
+
static int mark_stack_slots_iter(struct bpf_verifier_env *env,
+ struct bpf_kfunc_call_arg_meta *meta,
struct bpf_reg_state *reg, int insn_idx,
struct btf *btf, u32 btf_id, int nr_slots)
{
@@ -1193,6 +1199,12 @@ static int mark_stack_slots_iter(struct bpf_verifier_env *env,

__mark_reg_known_zero(st);
st->type = PTR_TO_STACK; /* we don't have dedicated reg type */
+ if (is_iter_need_rcu(meta)) {
+ if (in_rcu_cs(env))
+ st->type |= MEM_RCU;
+ else
+ st->type |= PTR_UNTRUSTED;
+ }
st->live |= REG_LIVE_WRITTEN;
st->ref_obj_id = i == 0 ? id : 0;
st->iter.btf = btf;
@@ -1281,6 +1293,8 @@ static bool is_iter_reg_valid_init(struct bpf_verifier_env *env, struct bpf_reg_
struct bpf_stack_state *slot = &state->stack[spi - i];
struct bpf_reg_state *st = &slot->spilled_ptr;

+ if (st->type & PTR_UNTRUSTED)
+ return false;
/* only main (first) slot has ref_obj_id set */
if (i == 0 && !st->ref_obj_id)
return false;
@@ -7503,13 +7517,13 @@ static int process_iter_arg(struct bpf_verifier_env *env, int regno, int insn_id
return err;
}

- err = mark_stack_slots_iter(env, reg, insn_idx, meta->btf, btf_id, nr_slots);
+ err = mark_stack_slots_iter(env, meta, reg, insn_idx, meta->btf, btf_id, nr_slots);
if (err)
return err;
} else {
/* iter_next() or iter_destroy() expect initialized iter state*/
if (!is_iter_reg_valid_init(env, reg, meta->btf, btf_id, nr_slots)) {
- verbose(env, "expected an initialized iter_%s as arg #%d\n",
+ verbose(env, "expected an initialized iter_%s as arg #%d or without bpf_rcu_read_lock()\n",
iter_type_str(meta->btf, btf_id), regno);
return -EINVAL;
}
@@ -10382,6 +10396,18 @@ BTF_ID(func, bpf_percpu_obj_new_impl)
BTF_ID(func, bpf_percpu_obj_drop_impl)
BTF_ID(func, bpf_iter_css_task_new)

+BTF_SET_START(rcu_protect_kfuns_set)
+BTF_ID(func, bpf_iter_process_new)
+BTF_ID(func, bpf_iter_css_pre_new)
+BTF_ID(func, bpf_iter_css_post_new)
+BTF_SET_END(rcu_protect_kfuns_set)
+
+static inline bool is_iter_need_rcu(struct bpf_kfunc_call_arg_meta *meta)
+{
+ return btf_id_set_contains(&rcu_protect_kfuns_set, meta->func_id);
+}
+
+
static bool is_kfunc_ret_null(struct bpf_kfunc_call_arg_meta *meta)
{
if (meta->func_id == special_kfunc_list[KF_bpf_refcount_acquire_impl] &&
--
2.20.1