[PATCH 4/5] perf sched: Cap max_cpu at MAX_CPUS in timehist sample processing

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

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

perf_timehist__process_sample() updates sched->max_cpu from the
sample CPU without bounds checking.  Later code uses max_cpu + 1 as
an iteration count over arrays allocated with MAX_CPUS entries
(curr_thread, cpu_last_switched).  A recording with CPU IDs >= MAX_CPUS
causes out-of-bounds array accesses.

Also cap the env->nr_cpus_online initialization of max_cpu in
perf_sched__timehist(), which could exceed MAX_CPUS on very large
systems.

Add bounds checks before both max_cpu updates, matching the pattern
already used in map_switch_event().

Fixes: 49394a2a24c7 ("perf sched timehist: Introduce timehist command")
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Assisted-by: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
---
 tools/perf/builtin-sched.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c
index 7bd61028327b39db..87a1f4cf8760e1e9 100644
--- a/tools/perf/builtin-sched.c
+++ b/tools/perf/builtin-sched.c
@@ -3215,7 +3215,9 @@ static int perf_timehist__process_sample(const struct perf_tool *tool,
 		.cpu = sample->cpu,
 	};
 
-	if (this_cpu.cpu > sched->max_cpu.cpu)
+	/* max_cpu indexes arrays allocated with MAX_CPUS entries */
+	if (this_cpu.cpu >= 0 && this_cpu.cpu < MAX_CPUS &&
+	    this_cpu.cpu > sched->max_cpu.cpu)
 		sched->max_cpu = this_cpu;
 
 	if (evsel->handler != NULL) {
@@ -3385,8 +3387,8 @@ static int perf_sched__timehist(struct perf_sched *sched)
 		perf_session__set_tracepoints_handlers(session, migrate_handlers))
 		goto out;
 
-	/* pre-allocate struct for per-CPU idle stats */
-	sched->max_cpu.cpu = env->nr_cpus_online;
+	/* pre-allocate struct for per-CPU idle stats; cap to array bounds */
+	sched->max_cpu.cpu = min(env->nr_cpus_online, MAX_CPUS);
 	if (sched->max_cpu.cpu == 0)
 		sched->max_cpu.cpu = 4;
 	if (init_idle_threads(sched->max_cpu.cpu))
-- 
2.54.0