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Message-ID: <CAEf4Bzb43AzO1+tg8n8u7KSxe1De3y4Sau4cy+=NQ-m6RCU_Gg@mail.gmail.com>
Date: Thu, 13 Jun 2019 16:04:22 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Alexei Starovoitov <ast@...nel.org>
Cc: "David S. Miller" <davem@...emloft.net>,
Daniel Borkmann <daniel@...earbox.net>,
Jakub Kicinski <jakub.kicinski@...ronome.com>,
Edward Cree <ecree@...arflare.com>,
john fastabend <john.fastabend@...il.com>,
Andrii Nakryiko <andriin@...com>, Jann Horn <jannh@...gle.com>,
Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 4/9] bpf: introduce bounded loops
On Thu, Jun 13, 2019 at 9:49 AM Alexei Starovoitov <ast@...nel.org> wrote:
>
> Allow the verifier to validate the loops by simulating their execution.
> Exisiting programs have used '#pragma unroll' to unroll the loops
> by the compiler. Instead let the verifier simulate all iterations
> of the loop.
> In order to do that introduce parentage chain of bpf_verifier_state and
> 'branches' counter for the number of branches left to explore.
> See more detailed algorithm description in bpf_verifier.h
>
> This algorithm borrows the key idea from Edward Cree approach:
> https://patchwork.ozlabs.org/patch/877222/
> Additional state pruning heuristics make such brute force loop walk
> practical even for large loops.
>
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> ---
LGTM, few suggestions below.
Acked-by: Andrii Nakryiko <andriin@...com>
> include/linux/bpf_verifier.h | 51 +++++++++++++-
> kernel/bpf/verifier.c | 133 ++++++++++++++++++++++++++++++++---
> 2 files changed, 175 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> index 704ed7971472..03037373b447 100644
> --- a/include/linux/bpf_verifier.h
> +++ b/include/linux/bpf_verifier.h
> @@ -194,6 +194,53 @@ struct bpf_func_state {
> struct bpf_verifier_state {
> /* call stack tracking */
> struct bpf_func_state *frame[MAX_CALL_FRAMES];
> + struct bpf_verifier_state *parent;
> + /*
> + * 'branches' field is the number of branches left to explore:
> + * 0 - all possible paths from this state reached bpf_exit or
> + * were safely pruned
> + * 1 - at least one path is being explored.
> + * This state hasn't reached bpf_exit
> + * 2 - at least two paths are being explored.
> + * This state is an immediate parent of two children.
> + * One is fallthrough branch with branches==1 and another
> + * state is pushed into stack (to be explored later) also with
> + * branches==1. The parent of this state has branches==1.
> + * The verifier state tree connected via 'parent' pointer looks like:
> + * 1
> + * 1
> + * 2 -> 1 (first 'if' pushed into stack)
> + * 1
> + * 2 -> 1 (second 'if' pushed into stack)
> + * 1
> + * 1
> + * 1 bpf_exit.
> + *
> + * Once do_check() reaches bpf_exit, it calls update_branch_counts()
> + * and the verifier state tree will look:
> + * 1
> + * 1
> + * 2 -> 1 (first 'if' pushed into stack)
> + * 1
> + * 1 -> 1 (second 'if' pushed into stack)
> + * 0
> + * 0
> + * 0 bpf_exit.
> + * After pop_stack() the do_check() will resume at second 'if'.
> + *
> + * If is_state_visited() sees a state with branches > 0 it means
> + * there is a loop. If such state is exactly equal to the current state
> + * it's an infinite loop. Note states_equal() checks for states
> + * equvalency, so two states being 'states_equal' does not mean
> + * infinite loop. The exact comparison is provided by
> + * states_maybe_looping() function. It's a stronger pre-check and
> + * much faster than states_equal().
> + *
> + * This algorithm may not find all possible infinite loops or
> + * loop iteration count may be too high.
> + * In such cases BPF_COMPLEXITY_LIMIT_INSNS limit kicks in.
> + */
> + u32 branches;
> u32 insn_idx;
> u32 curframe;
> u32 active_spin_lock;
> @@ -312,7 +359,9 @@ struct bpf_verifier_env {
> } cfg;
> u32 subprog_cnt;
> /* number of instructions analyzed by the verifier */
> - u32 insn_processed;
> + u32 prev_insn_processed, insn_processed;
> + /* number of jmps, calls, exits analyzed so far */
> + u32 prev_jmps_processed, jmps_processed;
> /* total verification time */
> u64 verification_time;
> /* maximum number of verifier states kept in 'branching' instructions */
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index c79c09586a9e..55d5ab4ab83e 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -721,6 +721,8 @@ static int copy_verifier_state(struct bpf_verifier_state *dst_state,
> dst_state->speculative = src->speculative;
> dst_state->curframe = src->curframe;
> dst_state->active_spin_lock = src->active_spin_lock;
> + dst_state->branches = src->branches;
> + dst_state->parent = src->parent;
> for (i = 0; i <= src->curframe; i++) {
> dst = dst_state->frame[i];
> if (!dst) {
> @@ -736,6 +738,23 @@ static int copy_verifier_state(struct bpf_verifier_state *dst_state,
> return 0;
> }
>
> +static void update_branch_counts(struct bpf_verifier_env *env, struct bpf_verifier_state *st)
> +{
> + while (st) {
> + u32 br = --st->branches;
> +
> + /* WARN_ON(br > 1) technically makes sense here,
> + * but see comment in push_stack(), hence:
> + */
> + WARN_ONCE((int)br < 0,
> + "BUG update_branch_counts:branches_to_explore=%d\n",
> + br);
> + if (br)
> + break;
> + st = st->parent;
> + }
> +}
> +
> static int pop_stack(struct bpf_verifier_env *env, int *prev_insn_idx,
> int *insn_idx)
> {
> @@ -789,6 +808,18 @@ static struct bpf_verifier_state *push_stack(struct bpf_verifier_env *env,
> env->stack_size);
> goto err;
> }
> + if (elem->st.parent) {
> + ++elem->st.parent->branches;
> + /* WARN_ON(branches > 2) technically makes sense here,
> + * but
> + * 1. speculative states will bump 'branches' for non-branch
> + * instructions
> + * 2. is_state_visited() heuristics may decide not to create
> + * a new state for a sequence of branches and all such current
> + * and cloned states will be pointing to a single parent state
> + * which might have large 'branches' count.
> + */
> + }
> return &elem->st;
> err:
> free_verifier_state(env->cur_state, true);
> @@ -5685,7 +5716,8 @@ static void init_explored_state(struct bpf_verifier_env *env, int idx)
> * w - next instruction
> * e - edge
> */
> -static int push_insn(int t, int w, int e, struct bpf_verifier_env *env)
> +static int push_insn(int t, int w, int e, struct bpf_verifier_env *env,
> + bool loop_ok)
> {
> int *insn_stack = env->cfg.insn_stack;
> int *insn_state = env->cfg.insn_state;
> @@ -5715,6 +5747,8 @@ static int push_insn(int t, int w, int e, struct bpf_verifier_env *env)
> insn_stack[env->cfg.cur_stack++] = w;
> return 1;
> } else if ((insn_state[w] & 0xF0) == DISCOVERED) {
> + if (loop_ok && env->allow_ptr_leaks)
allow_ptr_leaks is used as a proxy for SYS_CAP_ADMIN, right? Maybe
have explicit env->allow_loops instead. That would make more sense
when reading code. Plus, it would decouple loop enablement from
SYS_CAP_ADMIN, so would allow more fine-grained control, if necessary.
> + return 0;
> verbose_linfo(env, t, "%d: ", t);
> verbose_linfo(env, w, "%d: ", w);
> verbose(env, "back-edge from insn %d to %d\n", t, w);
> @@ -5766,7 +5800,7 @@ static int check_cfg(struct bpf_verifier_env *env)
> if (opcode == BPF_EXIT) {
> goto mark_explored;
> } else if (opcode == BPF_CALL) {
> - ret = push_insn(t, t + 1, FALLTHROUGH, env);
> + ret = push_insn(t, t + 1, FALLTHROUGH, env, false);
> if (ret == 1)
> goto peek_stack;
> else if (ret < 0)
> @@ -5775,7 +5809,8 @@ static int check_cfg(struct bpf_verifier_env *env)
> init_explored_state(env, t + 1);
> if (insns[t].src_reg == BPF_PSEUDO_CALL) {
> init_explored_state(env, t);
> - ret = push_insn(t, t + insns[t].imm + 1, BRANCH, env);
> + ret = push_insn(t, t + insns[t].imm + 1, BRANCH,
> + env, false);
> if (ret == 1)
> goto peek_stack;
> else if (ret < 0)
> @@ -5788,7 +5823,7 @@ static int check_cfg(struct bpf_verifier_env *env)
> }
> /* unconditional jump with single edge */
> ret = push_insn(t, t + insns[t].off + 1,
> - FALLTHROUGH, env);
> + FALLTHROUGH, env, true);
> if (ret == 1)
> goto peek_stack;
> else if (ret < 0)
> @@ -5801,13 +5836,13 @@ static int check_cfg(struct bpf_verifier_env *env)
> } else {
> /* conditional jump with two edges */
> init_explored_state(env, t);
> - ret = push_insn(t, t + 1, FALLTHROUGH, env);
> + ret = push_insn(t, t + 1, FALLTHROUGH, env, true);
> if (ret == 1)
> goto peek_stack;
> else if (ret < 0)
> goto err_free;
>
> - ret = push_insn(t, t + insns[t].off + 1, BRANCH, env);
> + ret = push_insn(t, t + insns[t].off + 1, BRANCH, env, true);
> if (ret == 1)
> goto peek_stack;
> else if (ret < 0)
> @@ -5817,7 +5852,7 @@ static int check_cfg(struct bpf_verifier_env *env)
> /* all other non-branch instructions with single
> * fall-through edge
> */
> - ret = push_insn(t, t + 1, FALLTHROUGH, env);
> + ret = push_insn(t, t + 1, FALLTHROUGH, env, false);
> if (ret == 1)
> goto peek_stack;
> else if (ret < 0)
> @@ -6250,6 +6285,8 @@ static void clean_live_states(struct bpf_verifier_env *env, int insn,
>
> sl = *explored_state(env, insn);
> while (sl) {
> + if (sl->state.branches)
> + goto next;
> if (sl->state.insn_idx != insn ||
> sl->state.curframe != cur->curframe)
> goto next;
> @@ -6614,12 +6651,32 @@ static int propagate_liveness(struct bpf_verifier_env *env,
> return 0;
> }
>
> +static bool states_maybe_looping(struct bpf_verifier_state *old,
> + struct bpf_verifier_state *cur)
> +{
> + struct bpf_func_state *fold, *fcur;
> + int i, fr = cur->curframe;
> +
> + if (old->curframe != fr)
> + return false;
> +
> + fold = old->frame[fr];
> + fcur = cur->frame[fr];
> + for (i = 0; i < MAX_BPF_REG; i++)
> + if (memcmp(&fold->regs[i], &fcur->regs[i],
> + offsetof(struct bpf_reg_state, parent)))
> + return false;
> + return true;
> +}
> +
> +
> static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
> {
> struct bpf_verifier_state_list *new_sl;
> struct bpf_verifier_state_list *sl, **pprev;
> struct bpf_verifier_state *cur = env->cur_state, *new;
> int i, j, err, states_cnt = 0;
> + bool add_new_state = false;
>
> if (!env->insn_aux_data[insn_idx].prune_point)
> /* this 'insn_idx' instruction wasn't marked, so we will not
> @@ -6627,6 +6684,18 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
> */
> return 0;
>
> + /* bpf progs typically have pruning point every 4 instructions
> + * http://vger.kernel.org/bpfconf2019.html#session-1
> + * Do not add new state for future pruning if the verifier hasn't seen
> + * at least 2 jumps and at least 8 instructions.
> + * This heuristics helps decrease 'total_states' and 'peak_states' metric.
> + * In tests that amounts to up to 50% reduction into total verifier
> + * memory consumption and 20% verifier time speedup.
> + */
> + if (env->jmps_processed - env->prev_jmps_processed >= 2 &&
> + env->insn_processed - env->prev_insn_processed >= 8)
> + add_new_state = true;
nit: trivial if, why not:
add_new_state = env->jmps_processed - env->prev_jmps_processed >= 2 &&
env->insn_processed - env->prev_insn_processed >= 8;
> +
> pprev = explored_state(env, insn_idx);
> sl = *pprev;
>
> @@ -6636,6 +6705,30 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
> states_cnt++;
> if (sl->state.insn_idx != insn_idx)
> goto next;
> + if (sl->state.branches) {
> + if (states_maybe_looping(&sl->state, cur) &&
> + states_equal(env, &sl->state, cur)) {
> + verbose_linfo(env, insn_idx, "; ");
> + verbose(env, "infinite loop detected at insn %d\n", insn_idx);
> + return -EINVAL;
> + }
> + /* if the verifier is processing a loop, avoid adding new state
> + * too often, since different loop iterations have distinct
> + * states and may not help future pruning.
> + * This threshold shouldn't be too low to make sure that
> + * a loop with large bound will be rejected quickly.
> + * The most abusive loop will be:
> + * r1 += 1
> + * if r1 < 1000000 goto pc-2
> + * 1M insn_procssed limit / 100 == 10k peak states.
> + * This threshold shouldn't be too high either, since states
> + * at the end of the loop are likely to be useful in pruning.
> + */
> + if (env->jmps_processed - env->prev_jmps_processed < 20 &&
> + env->insn_processed - env->prev_insn_processed < 100)
> + add_new_state = false;
same as above
> + goto miss;
> + }
> if (states_equal(env, &sl->state, cur)) {
> sl->hit_cnt++;
> /* reached equivalent register/stack state,
> @@ -6653,7 +6746,15 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
> return err;
> return 1;
> }
> - sl->miss_cnt++;
> +miss:
> + /* when new state is not going to be added do not increase miss count.
> + * Otherwise several loop iterations will remove the state
> + * recorded earlier. The goal of these heuristics is to have
> + * states from some iterations of the loop (some in the beginning
> + * and some at the end) to help pruning.
> + */
> + if (add_new_state)
> + sl->miss_cnt++;
> /* heuristic to determine whether this state is beneficial
> * to keep checking from state equivalence point of view.
> * Higher numbers increase max_states_per_insn and verification time,
> @@ -6665,6 +6766,11 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
> */
> *pprev = sl->next;
> if (sl->state.frame[0]->regs[0].live & REG_LIVE_DONE) {
> + u32 br = sl->state.branches;
> +
> + WARN_ONCE(br,
> + "BUG live_done but branches_to_explore %d\n",
> + br);
> free_verifier_state(&sl->state, false);
> kfree(sl);
> env->peak_states--;
> @@ -6690,6 +6796,9 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
> if (!env->allow_ptr_leaks && states_cnt > BPF_COMPLEXITY_LIMIT_STATES)
> return 0;
>
> + if (!add_new_state)
> + return 0;
> +
> /* there were no equivalent states, remember current one.
> * technically the current state is not proven to be safe yet,
> * but it will either reach outer most bpf_exit (which means it's safe)
> @@ -6702,6 +6811,8 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
> return -ENOMEM;
> env->total_states++;
> env->peak_states++;
> + env->prev_jmps_processed = env->jmps_processed;
> + env->prev_insn_processed = env->insn_processed;
>
> /* add new state to the head of linked list */
> new = &new_sl->state;
> @@ -6712,6 +6823,9 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
Few lines above this there is comment stating " Since there are no
loops ...", can you please fix it?
> return err;
> }
> new->insn_idx = insn_idx;
> + WARN_ONCE(new->branches != 1,
> + "BUG is_state_visited:branches_to_explore=%d insn %d\n", new->branches, insn_idx);
> + cur->parent = new;
> new_sl->next = *explored_state(env, insn_idx);
> *explored_state(env, insn_idx) = new_sl;
> /* connect new state to parentage chain. Current frame needs all
> @@ -6798,6 +6912,7 @@ static int do_check(struct bpf_verifier_env *env)
> return -ENOMEM;
> state->curframe = 0;
> state->speculative = false;
> + state->branches = 1;
> state->frame[0] = kzalloc(sizeof(struct bpf_func_state), GFP_KERNEL);
> if (!state->frame[0]) {
> kfree(state);
> @@ -7004,6 +7119,7 @@ static int do_check(struct bpf_verifier_env *env)
> } else if (class == BPF_JMP || class == BPF_JMP32) {
> u8 opcode = BPF_OP(insn->code);
>
> + env->jmps_processed++;
> if (opcode == BPF_CALL) {
> if (BPF_SRC(insn->code) != BPF_K ||
> insn->off != 0 ||
> @@ -7089,6 +7205,7 @@ static int do_check(struct bpf_verifier_env *env)
> if (err)
> return err;
> process_bpf_exit:
> + update_branch_counts(env, env->cur_state);
> err = pop_stack(env, &env->prev_insn_idx,
> &env->insn_idx);
> if (err < 0) {
> --
> 2.20.0
>
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