From c1ce66357f8f7d73ff70353ec15f1fba164bc7e1 Mon Sep 17 00:00:00 2001 From: Eduard Zingerman Date: Sat, 15 Feb 2025 03:03:56 -0800 Subject: [PATCH] bpf: detect infinite loop in get_loop_entry() Tejun Heo reported an infinite loop in get_loop_entry(), when verifying a sched_ext program layered_dispatch in [1]. After some investigation I'm sure that root cause is fixed by patches 1,3 in this patch-set. To err on the safe side, this commit modifies get_loop_entry() to detect infinite loops and abort verification in such cases. The number of steps get_loop_entry(S) can make while moving along the bpf_verifier_state->loop_entry chain is bounded by the DFS depth of state S. This fact is exploited to implement the check. To avoid dealing with the potential error code returned from get_loop_entry() in update_loop_entry(), remove the get_loop_entry() calls there: - This change does not affect correctness. Loop entries would still be updated during the backward DFS move in update_branch_counts(). - This change does not affect performance. Measurements show that get_loop_entry() performs at most 1 step on selftests and at most 2 steps on sched_ext programs (1 step in 17 cases, 2 steps in 3 cases, measured using "do-not-submit" patches from [2]). [1] https://github.com/sched-ext/scx/ commit f0b27038ea10 ("XXX - kernel stall") [2] https://github.com/eddyz87/bpf/tree/get-loop-entry-hungup Reported-by: Tejun Heo Signed-off-by: Eduard Zingerman Link: https://lore.kernel.org/r/20250215110411.3236773-6-eddyz87@gmail.com Signed-off-by: Alexei Starovoitov --- kernel/bpf/verifier.c | 39 +++++++++++++++++++++------------------ 1 file changed, 21 insertions(+), 18 deletions(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 8963ec7e2032..ddf6ed9f3482 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -1803,12 +1803,9 @@ static bool same_callsites(struct bpf_verifier_state *a, struct bpf_verifier_sta * 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. + * # Update n's loop entry if h comes before n 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]: + * if h in path and depths[entries.get(n, n)] < depths[n]: * entries[n] = h1 * * def dfs(n, depth): @@ -1820,7 +1817,7 @@ static bool same_callsites(struct bpf_verifier_state *a, struct bpf_verifier_sta * # 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) + * h = entries.get(succ, None) * update_loop_entry(n, h) * else: * # Case B or C depending on `h1 in path` check in update_loop_entry(). @@ -1835,12 +1832,20 @@ static bool same_callsites(struct bpf_verifier_state *a, struct bpf_verifier_sta * - 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) +static struct bpf_verifier_state *get_loop_entry(struct bpf_verifier_env *env, + struct bpf_verifier_state *st) { struct bpf_verifier_state *topmost = st->loop_entry, *old; + u32 steps = 0; - while (topmost && topmost->loop_entry && topmost != topmost->loop_entry) + while (topmost && topmost->loop_entry && topmost != topmost->loop_entry) { + if (steps++ > st->dfs_depth) { + WARN_ONCE(true, "verifier bug: infinite loop in get_loop_entry\n"); + verbose(env, "verifier bug: infinite loop in get_loop_entry()\n"); + return ERR_PTR(-EFAULT); + } topmost = topmost->loop_entry; + } /* Update loop entries for intermediate states to avoid this * traversal in future get_loop_entry() calls. */ @@ -1854,17 +1859,13 @@ static struct bpf_verifier_state *get_loop_entry(struct bpf_verifier_state *st) 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 + /* The hdr->branches check decides between cases B and C in + * comment for get_loop_entry(). If hdr->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) { + if (hdr->branches && hdr->dfs_depth <= (cur->loop_entry ?: cur)->dfs_depth) { cur->loop_entry = hdr; hdr->used_as_loop_entry = true; } @@ -17870,8 +17871,8 @@ static void clean_live_states(struct bpf_verifier_env *env, int insn, while (sl) { if (sl->state.branches) goto next; - loop_entry = get_loop_entry(&sl->state); - if (loop_entry && loop_entry->branches) + loop_entry = get_loop_entry(env, &sl->state); + if (!IS_ERR_OR_NULL(loop_entry) && loop_entry->branches) goto next; if (sl->state.insn_idx != insn || !same_callsites(&sl->state, cur)) @@ -18749,7 +18750,9 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx) * * Additional details are in the comment before get_loop_entry(). */ - loop_entry = get_loop_entry(&sl->state); + loop_entry = get_loop_entry(env, &sl->state); + if (IS_ERR(loop_entry)) + return PTR_ERR(loop_entry); force_exact = loop_entry && loop_entry->branches > 0; if (states_equal(env, &sl->state, cur, force_exact ? RANGE_WITHIN : NOT_EXACT)) { if (force_exact)