[PATCH 27/28] perf kwork: Bounds check work->cpu before indexing cpus_runtime[]

[Arnaldo Carvalho de Melo <acme@kernel.org>]

From: Arnaldo Carvalho de Melo <acme@redhat.com>

work->cpu comes from sample->cpu which is (u32)-1 when
PERF_SAMPLE_CPU is absent.  Stored as int, this becomes -1
which passes the signed BUG_ON(work->cpu >= MAX_NR_CPUS) but
causes an out-of-bounds access on cpus_runtime[-1].

Replace the BUG_ON with an unsigned bounds check that skips
entries with invalid CPU values, and guard the idle and irq
runtime accumulators the same way.

Reported-by: sashiko-bot@kernel.org # Running on a local machine
Cc: Yang Jihong <yangjihong@bytedance.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Assisted-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
---
 tools/perf/builtin-kwork.c | 49 +++++++++++++++++++++++++++++++++-----
 tools/perf/util/kwork.h    |  1 +
 2 files changed, 44 insertions(+), 6 deletions(-)

diff --git a/tools/perf/builtin-kwork.c b/tools/perf/builtin-kwork.c
index 9d3a4c779a41e383..6e124a0f277c8294 100644
--- a/tools/perf/builtin-kwork.c
+++ b/tools/perf/builtin-kwork.c
@@ -424,7 +424,9 @@ static bool profile_event_match(struct perf_kwork *kwork,
 	u64 time = sample->time;
 	struct perf_time_interval *ptime = &kwork->ptime;
 
-	if ((kwork->cpu_list != NULL) && !test_bit(cpu, kwork->cpu_bitmap))
+	/* Guard test_bit: cpu == -1 (absent PERF_SAMPLE_CPU) would index past the bitmap */
+	if ((kwork->cpu_list != NULL) &&
+	    ((unsigned int)cpu >= MAX_NR_CPUS || !test_bit(cpu, kwork->cpu_bitmap)))
 		return false;
 
 	if (((ptime->start != 0) && (ptime->start > time)) ||
@@ -2008,7 +2010,18 @@ static void top_calc_total_runtime(struct perf_kwork *kwork)
 	next = rb_first_cached(&class->work_root);
 	while (next) {
 		work = rb_entry(next, struct kwork_work, node);
-		BUG_ON(work->cpu >= MAX_NR_CPUS);
+		/*
+		 * work->cpu comes from sample->cpu which is -1 when
+		 * PERF_SAMPLE_CPU is absent.  As int that's -1, but as
+		 * unsigned it exceeds MAX_NR_CPUS — skip to avoid OOB
+		 * on cpus_runtime[].
+		 */
+		/* Counted and reported in perf_kwork__top_report() */
+		if ((unsigned int)work->cpu >= MAX_NR_CPUS) {
+			stat->nr_skipped_cpu++;
+			next = rb_next(next);
+			continue;
+		}
 		stat->cpus_runtime[work->cpu].total += work->total_runtime;
 		stat->cpus_runtime[MAX_NR_CPUS].total += work->total_runtime;
 		next = rb_next(next);
@@ -2020,7 +2033,8 @@ static void top_calc_idle_time(struct perf_kwork *kwork,
 {
 	struct kwork_top_stat *stat = &kwork->top_stat;
 
-	if (work->id == 0) {
+	/* See comment in top_calc_total_runtime() */
+	if (work->id == 0 && (unsigned int)work->cpu < MAX_NR_CPUS) {
 		stat->cpus_runtime[work->cpu].idle += work->total_runtime;
 		stat->cpus_runtime[MAX_NR_CPUS].idle += work->total_runtime;
 	}
@@ -2032,6 +2046,12 @@ static void top_calc_irq_runtime(struct perf_kwork *kwork,
 {
 	struct kwork_top_stat *stat = &kwork->top_stat;
 
+	/* See comment in top_calc_total_runtime() */
+	if ((unsigned int)work->cpu >= MAX_NR_CPUS) {
+		stat->nr_skipped_cpu++;
+		return;
+	}
+
 	if (type == KWORK_CLASS_IRQ) {
 		stat->cpus_runtime[work->cpu].irq += work->total_runtime;
 		stat->cpus_runtime[MAX_NR_CPUS].irq += work->total_runtime;
@@ -2084,12 +2104,21 @@ static void top_calc_cpu_usage(struct perf_kwork *kwork)
 		if (work->total_runtime == 0)
 			goto next;
 
+		/* See comment in top_calc_total_runtime() */
+		if ((unsigned int)work->cpu >= MAX_NR_CPUS) {
+			stat->nr_skipped_cpu++;
+			goto next;
+		}
+
 		__set_bit(work->cpu, stat->all_cpus_bitmap);
 
 		top_subtract_irq_runtime(kwork, work);
 
-		work->cpu_usage = work->total_runtime * 10000 /
-			stat->cpus_runtime[work->cpu].total;
+		/* Guard against division by zero if no runtime was accumulated */
+		if (stat->cpus_runtime[work->cpu].total) {
+			work->cpu_usage = work->total_runtime * 10000 /
+				stat->cpus_runtime[work->cpu].total;
+		}
 
 		top_calc_idle_time(kwork, work);
 next:
@@ -2102,7 +2131,8 @@ static void top_calc_load_runtime(struct perf_kwork *kwork,
 {
 	struct kwork_top_stat *stat = &kwork->top_stat;
 
-	if (work->id != 0) {
+	/* See comment in top_calc_total_runtime() */
+	if (work->id != 0 && (unsigned int)work->cpu < MAX_NR_CPUS) {
 		stat->cpus_runtime[work->cpu].load += work->total_runtime;
 		stat->cpus_runtime[MAX_NR_CPUS].load += work->total_runtime;
 	}
@@ -2170,6 +2200,13 @@ static void perf_kwork__top_report(struct perf_kwork *kwork)
 		next = rb_next(next);
 	}
 
+	if (kwork->top_stat.nr_skipped_cpu) {
+		printf("  Warning: %u work entries with invalid CPU were excluded from totals.\n"
+		       "  Task runtimes may appear inflated (IRQ time not subtracted).\n"
+		       "  Consider re-recording with PERF_SAMPLE_CPU enabled.\n",
+		       kwork->top_stat.nr_skipped_cpu);
+	}
+
 	printf("\n");
 }
 
diff --git a/tools/perf/util/kwork.h b/tools/perf/util/kwork.h
index db00269b73f24c66..10290cd779402f9d 100644
--- a/tools/perf/util/kwork.h
+++ b/tools/perf/util/kwork.h
@@ -194,6 +194,7 @@ struct __top_cpus_runtime {
 struct kwork_top_stat {
 	DECLARE_BITMAP(all_cpus_bitmap, MAX_NR_CPUS);
 	struct __top_cpus_runtime *cpus_runtime;
+	unsigned int nr_skipped_cpu;
 };
 
 struct perf_kwork {
-- 
2.54.0