[PATCH 6/9] perf sched: Use is_idle_sample() for idle thread runtime cast guard

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

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

timehist_sched_change_event() uses thread__tid(thread) == 0 to decide
whether to cast thread_runtime to idle_thread_runtime.  However, a
crafted perf.data can set common_pid=0 and common_tid=0 (the perf_sample
fields) while prev_pid != 0 (the tracepoint field).  is_idle_sample()
returns false (it checks prev_pid for sched_switch), so
timehist_get_thread() goes through machine__findnew_thread() and returns
the machine's TID 0 thread — whose priv data is a regular thread_runtime,
not the larger idle_thread_runtime allocated by init_idle_thread().

The subsequent cast to idle_thread_runtime reads past the thread_runtime
allocation, accessing itr->last_thread, itr->cursor, and itr->callchain
from adjacent heap memory.  Writing to itr->last_thread corrupts the
heap; calling thread__put() on the OOB value frees an arbitrary pointer.

Replace the thread__tid() == 0 check with is_idle_sample(), which uses
the tracepoint-specific prev_pid field and correctly identifies whether
the sample originated from an idle thread with idle_thread_runtime priv.

Fixes: 5d8f17fb5822 ("perf sched timehist: Add -I/--idle-hist option")
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
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, 7 insertions(+), 1 deletion(-)

diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c
index e4378cc9ab3ed48b..4600d70b486104dd 100644
--- a/tools/perf/builtin-sched.c
+++ b/tools/perf/builtin-sched.c
@@ -2902,7 +2902,13 @@ static int timehist_sched_change_event(const struct perf_tool *tool,
 			t = ptime->end;
 	}
 
-	if (!sched->idle_hist || thread__tid(thread) == 0) {
+	/*
+	 * Use is_idle_sample() not thread__tid() == 0: a crafted perf.data
+	 * can set common_pid=0 with prev_pid!=0, giving us a machine thread
+	 * whose priv is thread_runtime, not idle_thread_runtime — the cast
+	 * below would read past the allocation.
+	 */
+	if (!sched->idle_hist || is_idle_sample(sample)) {
 		if (!cpu_list || (sample->cpu < MAX_NR_CPUS &&
 				 test_bit(sample->cpu, cpu_bitmap)))
 			timehist_update_runtime_stats(tr, t, tprev);
-- 
2.54.0