[ Upstream commit 2a0992829ea3 ]
It turns out that .branches > 0 in is_state_visited() is not a sufficient condition to identify if two verifier states form a loop when iterators convergence is computed. This commit adds logic to distinguish situations like below:
(I) initial (II) initial | | V V .---------> hdr .. | | | | V V | .------... .------.. | | | | | | V V V V | ... ... .-> hdr .. | | | | | | | V V | V V | succ <- cur | succ <- cur | | | | | V | V | ... | ... | | | | '----' '----'
For both (I) and (II) successor 'succ' of the current state 'cur' was previously explored and has branches count at 0. However, loop entry 'hdr' corresponding to 'succ' might be a part of current DFS path. If that is the case 'succ' and 'cur' are members of the same loop and have to be compared exactly.
Co-developed-by: Andrii Nakryiko andrii.nakryiko@gmail.com Co-developed-by: Alexei Starovoitov alexei.starovoitov@gmail.com Reviewed-by: Andrii Nakryiko andrii@kernel.org Signed-off-by: Eduard Zingerman eddyz87@gmail.com Link: https://lore.kernel.org/r/20231024000917.12153-6-eddyz87@gmail.com Signed-off-by: Alexei Starovoitov ast@kernel.org --- include/linux/bpf_verifier.h | 15 +++ kernel/bpf/verifier.c | 207 ++++++++++++++++++++++++++++++++++- 2 files changed, 218 insertions(+), 4 deletions(-)
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h index 57d76ef9036f..33992ea42124 100644 --- a/include/linux/bpf_verifier.h +++ b/include/linux/bpf_verifier.h @@ -372,10 +372,25 @@ struct bpf_verifier_state { struct bpf_active_lock active_lock; bool speculative; bool active_rcu_lock; + /* If this state was ever pointed-to by other state's loop_entry field + * this flag would be set to true. Used to avoid freeing such states + * while they are still in use. + */ + bool used_as_loop_entry;
/* first and last insn idx of this verifier state */ u32 first_insn_idx; u32 last_insn_idx; + /* If this state is a part of states loop this field points to some + * parent of this state such that: + * - it is also a member of the same states loop; + * - DFS states traversal starting from initial state visits loop_entry + * state before this state. + * Used to compute topmost loop entry for state loops. + * State loops might appear because of open coded iterators logic. + * See get_loop_entry() for more information. + */ + struct bpf_verifier_state *loop_entry; /* jmp history recorded from first to last. * backtracking is using it to go from last to first. * For most states jmp_history_cnt is [0-3]. diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 175d611d7d80..6658f6750715 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -1764,6 +1764,7 @@ static int copy_verifier_state(struct bpf_verifier_state *dst_state, dst_state->first_insn_idx = src->first_insn_idx; dst_state->last_insn_idx = src->last_insn_idx; dst_state->dfs_depth = src->dfs_depth; + dst_state->used_as_loop_entry = src->used_as_loop_entry; for (i = 0; i <= src->curframe; i++) { dst = dst_state->frame[i]; if (!dst) { @@ -1806,11 +1807,176 @@ static bool same_callsites(struct bpf_verifier_state *a, struct bpf_verifier_sta return true; }
+/* Open coded iterators allow back-edges in the state graph in order to + * check unbounded loops that iterators. + * + * In is_state_visited() it is necessary to know if explored states are + * part of some loops in order to decide whether non-exact states + * comparison could be used: + * - non-exact states comparison establishes sub-state relation and uses + * read and precision marks to do so, these marks are propagated from + * children states and thus are not guaranteed to be final in a loop; + * - exact states comparison just checks if current and explored states + * are identical (and thus form a back-edge). + * + * Paper "A New Algorithm for Identifying Loops in Decompilation" + * by Tao Wei, Jian Mao, Wei Zou and Yu Chen [1] presents a convenient + * algorithm for loop structure detection and gives an overview of + * relevant terminology. It also has helpful illustrations. + * + * [1] https://api.semanticscholar.org/CorpusID:15784067 + * + * We use a similar algorithm but because loop nested structure is + * irrelevant for verifier ours is significantly simpler and resembles + * strongly connected components algorithm from Sedgewick's textbook. + * + * Define topmost loop entry as a first node of the loop traversed in a + * depth first search starting from initial state. The goal of the loop + * tracking algorithm is to associate topmost loop entries with states + * derived from these entries. + * + * For each step in the DFS states traversal algorithm needs to identify + * the following situations: + * + * initial initial initial + * | | | + * V V V + * ... ... .---------> hdr + * | | | | + * V V | V + * cur .-> succ | .------... + * | | | | | | + * V | V | V V + * succ '-- cur | ... ... + * | | | + * | V V + * | succ <- cur + * | | + * | V + * | ... + * | | + * '----' + * + * (A) successor state of cur (B) successor state of cur or it's entry + * not yet traversed are in current DFS path, thus cur and succ + * are members of the same outermost loop + * + * initial initial + * | | + * V V + * ... ... + * | | + * V V + * .------... .------... + * | | | | + * V V V V + * .-> hdr ... ... ... + * | | | | | + * | V V V V + * | succ <- cur succ <- cur + * | | | + * | V V + * | ... ... + * | | | + * '----' exit + * + * (C) successor state of cur is a part of some loop but this loop + * does not include cur or successor state is not in a loop at all. + * + * Algorithm could be described as the following python code: + * + * traversed = set() # Set of traversed nodes + * entries = {} # Mapping from node to loop entry + * depths = {} # Depth level assigned to graph node + * path = set() # Current DFS path + * + * # Find outermost loop entry known for n + * def get_loop_entry(n): + * h = entries.get(n, None) + * while h in entries and entries[h] != h: + * h = entries[h] + * return h + * + * # Update n's loop entry if h's outermost entry comes + * # before n's outermost entry in current DFS path. + * def update_loop_entry(n, h): + * n1 = get_loop_entry(n) or n + * h1 = get_loop_entry(h) or h + * if h1 in path and depths[h1] <= depths[n1]: + * entries[n] = h1 + * + * def dfs(n, depth): + * traversed.add(n) + * path.add(n) + * depths[n] = depth + * for succ in G.successors(n): + * if succ not in traversed: + * # Case A: explore succ and update cur's loop entry + * # only if succ's entry is in current DFS path. + * dfs(succ, depth + 1) + * h = get_loop_entry(succ) + * update_loop_entry(n, h) + * else: + * # Case B or C depending on `h1 in path` check in update_loop_entry(). + * update_loop_entry(n, succ) + * path.remove(n) + * + * To adapt this algorithm for use with verifier: + * - use st->branch == 0 as a signal that DFS of succ had been finished + * and cur's loop entry has to be updated (case A), handle this in + * update_branch_counts(); + * - use st->branch > 0 as a signal that st is in the current DFS path; + * - handle cases B and C in is_state_visited(); + * - update topmost loop entry for intermediate states in get_loop_entry(). + */ +static struct bpf_verifier_state *get_loop_entry(struct bpf_verifier_state *st) +{ + struct bpf_verifier_state *topmost = st->loop_entry, *old; + + while (topmost && topmost->loop_entry && topmost != topmost->loop_entry) + topmost = topmost->loop_entry; + /* Update loop entries for intermediate states to avoid this + * traversal in future get_loop_entry() calls. + */ + while (st && st->loop_entry != topmost) { + old = st->loop_entry; + st->loop_entry = topmost; + st = old; + } + return topmost; +} + +static void update_loop_entry(struct bpf_verifier_state *cur, struct bpf_verifier_state *hdr) +{ + struct bpf_verifier_state *cur1, *hdr1; + + cur1 = get_loop_entry(cur) ?: cur; + hdr1 = get_loop_entry(hdr) ?: hdr; + /* The head1->branches check decides between cases B and C in + * comment for get_loop_entry(). If hdr1->branches == 0 then + * head's topmost loop entry is not in current DFS path, + * hence 'cur' and 'hdr' are not in the same loop and there is + * no need to update cur->loop_entry. + */ + if (hdr1->branches && hdr1->dfs_depth <= cur1->dfs_depth) { + cur->loop_entry = hdr; + hdr->used_as_loop_entry = true; + } +} + static void update_branch_counts(struct bpf_verifier_env *env, struct bpf_verifier_state *st) { while (st) { u32 br = --st->branches;
+ /* br == 0 signals that DFS exploration for 'st' is finished, + * thus it is necessary to update parent's loop entry if it + * turned out that st is a part of some loop. + * This is a part of 'case A' in get_loop_entry() comment. + */ + if (br == 0 && st->parent && st->loop_entry) + update_loop_entry(st->parent, st->loop_entry); + /* WARN_ON(br > 1) technically makes sense here, * but see comment in push_stack(), hence: */ @@ -16270,10 +16436,11 @@ 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; + struct bpf_verifier_state *cur = env->cur_state, *new, *loop_entry; int i, j, n, err, states_cnt = 0; bool force_new_state = env->test_state_freq || is_force_checkpoint(env, insn_idx); bool add_new_state = force_new_state; + bool force_exact;
/* bpf progs typically have pruning point every 4 instructions * http://vger.kernel.org/bpfconf2019.html#session-1 @@ -16368,8 +16535,10 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx) */ spi = __get_spi(iter_reg->off + iter_reg->var_off.value); iter_state = &func(env, iter_reg)->stack[spi].spilled_ptr; - if (iter_state->iter.state == BPF_ITER_STATE_ACTIVE) + if (iter_state->iter.state == BPF_ITER_STATE_ACTIVE) { + update_loop_entry(cur, &sl->state); goto hit; + } } goto skip_inf_loop_check; } @@ -16400,7 +16569,36 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx) add_new_state = false; goto miss; } - if (states_equal(env, &sl->state, cur, false)) { + /* If sl->state is a part of a loop and this loop's entry is a part of + * current verification path then states have to be compared exactly. + * 'force_exact' is needed to catch the following case: + * + * initial Here state 'succ' was processed first, + * | it was eventually tracked to produce a + * V state identical to 'hdr'. + * .---------> hdr All branches from 'succ' had been explored + * | | and thus 'succ' has its .branches == 0. + * | V + * | .------... Suppose states 'cur' and 'succ' correspond + * | | | to the same instruction + callsites. + * | V V In such case it is necessary to check + * | ... ... if 'succ' and 'cur' are states_equal(). + * | | | If 'succ' and 'cur' are a part of the + * | V V same loop exact flag has to be set. + * | succ <- cur To check if that is the case, verify + * | | if loop entry of 'succ' is in current + * | V DFS path. + * | ... + * | | + * '----' + * + * Additional details are in the comment before get_loop_entry(). + */ + loop_entry = get_loop_entry(&sl->state); + force_exact = loop_entry && loop_entry->branches > 0; + if (states_equal(env, &sl->state, cur, force_exact)) { + if (force_exact) + update_loop_entry(cur, loop_entry); hit: sl->hit_cnt++; /* reached equivalent register/stack state, @@ -16449,7 +16647,8 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx) * speed up verification */ *pprev = sl->next; - if (sl->state.frame[0]->regs[0].live & REG_LIVE_DONE) { + if (sl->state.frame[0]->regs[0].live & REG_LIVE_DONE && + !sl->state.used_as_loop_entry) { u32 br = sl->state.branches;
WARN_ONCE(br,