Re: [PATCH bpf-next v3 5/7] bpf: teach the verifier to enforce css_iter and task_iter in RCU CS

Chuyi Zhou
2023-09-27T10:17:07Z

在 2023/9/27 18:00, Yafang Shao 写道:
> On Mon, Sep 25, 2023 at 6:56 PM Chuyi Zhou <zhouchuyi@bytedance.com> wrote:
>>
>> css_iter and task_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 adds a new a KF flag KF_RCU_PROTECTED for bpf_iter_task_new and
>> bpf_iter_css_new. It means the kfunc should be used in RCU CS. We check
>> whether we are in rcu cs before we want to invoke this kfunc. If the rcu
>> protection is guaranteed, we would let st->type = PTR_TO_STACK | MEM_RCU.
>> Once user do rcu_unlock during the iteration, state MEM_RCU of regs would
>> be cleared. is_iter_reg_valid_init() will reject if reg->type is UNTRUSTED.
>>
>> It is worth noting that currently, bpf_rcu_read_unlock does not
>> clear the state of the STACK_ITER reg, since bpf_for_each_spilled_reg
>> only considers STACK_SPILL. This patch also let bpf_for_each_spilled_reg
>> search STACK_ITER.
>>
>> Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
>
> This patch should be ahead of patch #2 and you introduce
> KF_RCU_PROTECTED in it then use this flag in later patches.
> BTW, I can't apply your series on bpf-next. I think you should rebase
> it on the latest bpf-next, otherwise the BPF CI can't be triggered.
>

Sorry for the mistake, will rebase in v4.

>> ---
>> include/linux/bpf_verifier.h | 19 ++++++++------
>> include/linux/btf.h | 1 +
>> kernel/bpf/helpers.c | 4 +--
>> kernel/bpf/verifier.c | 48 +++++++++++++++++++++++++++---------
>> 4 files changed, 50 insertions(+), 22 deletions(-)
>>
>> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
>> index a3236651ec64..b5cdcc332b0a 100644
>> --- a/include/linux/bpf_verifier.h
>> +++ b/include/linux/bpf_verifier.h
>> @@ -385,19 +385,18 @@ struct bpf_verifier_state {
>> u32 jmp_history_cnt;
>> };
>>
>> -#define bpf_get_spilled_reg(slot, frame) \
>> +#define bpf_get_spilled_reg(slot, frame, mask) \
>> (((slot < frame->allocated_stack / BPF_REG_SIZE) && \
>> - (frame->stack[slot].slot_type[0] == STACK_SPILL)) \
>> + ((1 << frame->stack[slot].slot_type[0]) & (mask))) \
>> ? &frame->stack[slot].spilled_ptr : NULL)
>>
>> /* Iterate over 'frame', setting 'reg' to either NULL or a spilled register. */
>> -#define bpf_for_each_spilled_reg(iter, frame, reg) \
>> - for (iter = 0, reg = bpf_get_spilled_reg(iter, frame); \
>> +#define bpf_for_each_spilled_reg(iter, frame, reg, mask) \
>> + for (iter = 0, reg = bpf_get_spilled_reg(iter, frame, mask); \
>> iter < frame->allocated_stack / BPF_REG_SIZE; \
>> - iter++, reg = bpf_get_spilled_reg(iter, frame))
>> + iter++, reg = bpf_get_spilled_reg(iter, frame, mask))
>>
>> -/* Invoke __expr over regsiters in __vst, setting __state and __reg */
>> -#define bpf_for_each_reg_in_vstate(__vst, __state, __reg, __expr) \
>> +#define bpf_for_each_reg_in_vstate_mask(__vst, __state, __reg, __mask, __expr) \
>> ({ \
>> struct bpf_verifier_state *___vstate = __vst; \
>> int ___i, ___j; \
>> @@ -409,7 +408,7 @@ struct bpf_verifier_state {
>> __reg = &___regs[___j]; \
>> (void)(__expr); \
>> } \
>> - bpf_for_each_spilled_reg(___j, __state, __reg) { \
>> + bpf_for_each_spilled_reg(___j, __state, __reg, __mask) { \
>> if (!__reg) \
>> continue; \
>> (void)(__expr); \
>> @@ -417,6 +416,10 @@ struct bpf_verifier_state {
>> } \
>> })
>>
>> +/* Invoke __expr over regsiters in __vst, setting __state and __reg */
>> +#define bpf_for_each_reg_in_vstate(__vst, __state, __reg, __expr) \
>> + bpf_for_each_reg_in_vstate_mask(__vst, __state, __reg, 1 << STACK_SPILL, __expr)
>> +
>> /* linked list of verifier states used to prune search */
>> struct bpf_verifier_state_list {
>> struct bpf_verifier_state state;
>> diff --git a/include/linux/btf.h b/include/linux/btf.h
>> index 928113a80a95..c2231c64d60b 100644
>> --- a/include/linux/btf.h
>> +++ b/include/linux/btf.h
>> @@ -74,6 +74,7 @@
>> #define KF_ITER_NEW (1 << 8) /* kfunc implements BPF iter constructor */
>> #define KF_ITER_NEXT (1 << 9) /* kfunc implements BPF iter next method */
>> #define KF_ITER_DESTROY (1 << 10) /* kfunc implements BPF iter destructor */
>> +#define KF_RCU_PROTECTED (1 << 11) /* kfunc should be protected by rcu cs when they are invoked */
>>
>> /*
>> * Tag marking a kernel function as a kfunc. This is meant to minimize the
>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
>> index 9c3af36249a2..aa9e03fbfe1a 100644
>> --- a/kernel/bpf/helpers.c
>> +++ b/kernel/bpf/helpers.c
>> @@ -2507,10 +2507,10 @@ BTF_ID_FLAGS(func, bpf_iter_num_destroy, KF_ITER_DESTROY)
>> BTF_ID_FLAGS(func, bpf_iter_css_task_new, KF_ITER_NEW | KF_TRUSTED_ARGS)
>> BTF_ID_FLAGS(func, bpf_iter_css_task_next, KF_ITER_NEXT | KF_RET_NULL)
>> BTF_ID_FLAGS(func, bpf_iter_css_task_destroy, KF_ITER_DESTROY)
>> -BTF_ID_FLAGS(func, bpf_iter_task_new, KF_ITER_NEW | KF_TRUSTED_ARGS)
>> +BTF_ID_FLAGS(func, bpf_iter_task_new, KF_ITER_NEW | KF_TRUSTED_ARGS | KF_RCU_PROTECTED)
>> BTF_ID_FLAGS(func, bpf_iter_task_next, KF_ITER_NEXT | KF_RET_NULL)
>> BTF_ID_FLAGS(func, bpf_iter_task_destroy, KF_ITER_DESTROY)
>> -BTF_ID_FLAGS(func, bpf_iter_css_new, KF_ITER_NEW | KF_TRUSTED_ARGS)
>> +BTF_ID_FLAGS(func, bpf_iter_css_new, KF_ITER_NEW | KF_TRUSTED_ARGS | KF_RCU_PROTECTED)
>> BTF_ID_FLAGS(func, bpf_iter_css_next, KF_ITER_NEXT | KF_RET_NULL)
>> BTF_ID_FLAGS(func, bpf_iter_css_destroy, KF_ITER_DESTROY)
>> BTF_ID_FLAGS(func, bpf_dynptr_adjust)
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 2367483bf4c2..a065e18a0b3a 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -1172,7 +1172,12 @@ 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);
>> +
>> +static bool is_kfunc_rcu_protected(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 +1198,12 @@ 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_kfunc_rcu_protected(meta)) {
>> + if (in_rcu_cs(env))
>> + st->type |= MEM_RCU;
>
> I think this change is incorrect. The type of st->type is enum
> bpf_reg_type, but MEM_RCU is enum bpf_type_flag.
> Or am I missing something?
Looking at is_rcu_reg(), It seems OK to add MEM_RCU flag to st->type.

static bool is_rcu_reg(const struct bpf_reg_state *reg)
{
return reg->type & MEM_RCU;
}

Here is the previous discussion link:
https://lore.kernel.org/lkml/CAADnVQKu+a6MKKfJy8NVmwtpEw1ae-_8opsGjdvvfoUjwE1sog@mail.gmail.com/

Thanks.

>
>> + else
>> + st->type |= PTR_UNTRUSTED;
>> + }
>> st->live |= REG_LIVE_WRITTEN;
>> st->ref_obj_id = i == 0 ? id : 0;
>> st->iter.btf = btf;
>> @@ -1267,7 +1278,7 @@ static bool is_iter_reg_valid_uninit(struct bpf_verifier_env *env,
>> return true;
>> }
>>
>> -static bool is_iter_reg_valid_init(struct bpf_verifier_env *env, struct bpf_reg_state *reg,
>> +static int is_iter_reg_valid_init(struct bpf_verifier_env *env, struct bpf_reg_state *reg,
>> struct btf *btf, u32 btf_id, int nr_slots)
>> {
>> struct bpf_func_state *state = func(env, reg);
>> @@ -1275,26 +1286,28 @@ static bool is_iter_reg_valid_init(struct bpf_verifier_env *env, struct bpf_reg_
>>
>> spi = iter_get_spi(env, reg, nr_slots);
>> if (spi < 0)
>> - return false;
>> + return -EINVAL;
>>
>> for (i = 0; i < nr_slots; i++) {
>> struct bpf_stack_state *slot = &state->stack[spi - i];
>> struct bpf_reg_state *st = &slot->spilled_ptr;
>>
>> + if (st->type & PTR_UNTRUSTED)
>> + return -EPERM;
>> /* only main (first) slot has ref_obj_id set */
>> if (i == 0 && !st->ref_obj_id)
>> - return false;
>> + return -EINVAL;
>> if (i != 0 && st->ref_obj_id)
>> - return false;
>> + return -EINVAL;
>> if (st->iter.btf != btf || st->iter.btf_id != btf_id)
>> - return false;
>> + return -EINVAL;
>>
>> for (j = 0; j < BPF_REG_SIZE; j++)
>> if (slot->slot_type[j] != STACK_ITER)
>> - return false;
>> + return -EINVAL;
>> }
>>
>> - return true;
>> + return 0;
>> }
>>
>> /* Check if given stack slot is "special":
>> @@ -7503,15 +7516,20 @@ 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",
>> + err = is_iter_reg_valid_init(env, reg, meta->btf, btf_id, nr_slots);
>> + switch (err) {
>> + case -EINVAL:
>> + 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;
>> + return err;
>> + case -EPERM:
>> + verbose(env, "expected an RCU CS when using %s\n", meta->func_name);
>> + return err;
>> }
>>
>> spi = iter_get_spi(env, reg, nr_slots);
>> @@ -10092,6 +10110,11 @@ static bool is_kfunc_rcu(struct bpf_kfunc_call_arg_meta *meta)
>> return meta->kfunc_flags & KF_RCU;
>> }
>>
>> +static bool is_kfunc_rcu_protected(struct bpf_kfunc_call_arg_meta *meta)
>> +{
>> + return meta->kfunc_flags & KF_RCU_PROTECTED;
>> +}
>> +
>> static bool __kfunc_param_match_suffix(const struct btf *btf,
>> const struct btf_param *arg,
>> const char *suffix)
>> @@ -11428,6 +11451,7 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>> if (env->cur_state->active_rcu_lock) {
>> struct bpf_func_state *state;
>> struct bpf_reg_state *reg;
>> + u32 clear_mask = (1 << STACK_SPILL) | (1 << STACK_ITER);
>>
>> if (in_rbtree_lock_required_cb(env) && (rcu_lock || rcu_unlock)) {
>> verbose(env, "Calling bpf_rcu_read_{lock,unlock} in unnecessary rbtree callback\n");
>> @@ -11438,7 +11462,7 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>> verbose(env, "nested rcu read lock (kernel function %s)\n", func_name);
>> return -EINVAL;
>> } else if (rcu_unlock) {
>> - bpf_for_each_reg_in_vstate(env->cur_state, state, reg, ({
>> + bpf_for_each_reg_in_vstate_mask(env->cur_state, state, reg, clear_mask, ({
>> if (reg->type & MEM_RCU) {
>> reg->type &= ~(MEM_RCU | PTR_MAYBE_NULL);
>> reg->type |= PTR_UNTRUSTED;
>> --
>> 2.20.1
>>
>>
>
>