From c1ce66357f8f7d73ff70353ec15f1fba164bc7e1 Mon Sep 17 00:00:00 2001
From: Eduard Zingerman <eddyz87@gmail.com>
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 <tj@kernel.org>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250215110411.3236773-6-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
---
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)