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@xxxxxxxxxx> --- 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) + 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; + 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; + 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) 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
