perf sched: Fix register_pid() overflow, strcpy, and BUG_ON

register_pid() has several issues when processing untrusted perf.data:

1. Integer overflow: (pid + 1) * sizeof(struct task_desc *) can wrap
   to a small value on 32-bit systems when pid is large (e.g.
   0x40000000), causing realloc to return a tiny buffer followed by
   out-of-bounds writes in the initialization loop.

2. Heap buffer overflow: strcpy(task->comm, comm) copies the
   untrusted comm string into a fixed 20-byte COMM_LEN buffer with
   no length check.

3. BUG_ON on allocation failure: perf.data is untrusted input, so
   allocation failures should be handled gracefully rather than
   killing the process.

4. Realloc of sched->tasks assigned directly back, leaking the old
   pointer on failure; nr_tasks incremented before the realloc,
   leaving corrupted state on failure.

Cap pid at PID_MAX_LIMIT (4194304, matching the kernel's maximum
on 64-bit), replace strcpy with strlcpy, guard against NULL comm,
replace BUG_ON with NULL returns using safe realloc patterns, and
add NULL checks in callers that dereference the result.

Fixes: ec156764d4 ("perf sched: Import schedbench.c")
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Cc: Ingo Molnar <mingo@elte.hu>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-sched.c

@@ -55,6 +55,7 @@
 #define COMM_LEN		20
 #define SYM_LEN			129
 #define MAX_PID			1024000
+#define PID_MAX_LIMIT		4194304 /* kernel limit on 64-bit */
 #define MAX_PRIO		140
 #define SEP_LEN			100
 
@@ -448,17 +449,28 @@ static void add_sched_event_sleep(struct perf_sched *sched, struct task_desc *ta
 static struct task_desc *register_pid(struct perf_sched *sched,
 				      unsigned long pid, const char *comm)
 {
-	struct task_desc *task;
+	struct task_desc *task, **tasks_p;
 	static int pid_max;
 
+	/* perf.data is untrusted — cap pid to prevent overflow in size calculations */
+	if (pid >= PID_MAX_LIMIT) {
+		pr_err("pid %lu exceeds limit %d, skipping\n", pid, PID_MAX_LIMIT);
+		return NULL;
+	}
+
 	if (sched->pid_to_task == NULL) {
 		if (sysctl__read_int("kernel/pid_max", &pid_max) < 0)
 			pid_max = MAX_PID;
-		BUG_ON((sched->pid_to_task = calloc(pid_max, sizeof(struct task_desc *))) == NULL);
+		sched->pid_to_task = calloc(pid_max, sizeof(struct task_desc *));
+		if (sched->pid_to_task == NULL)
+			return NULL;
 	}
 	if (pid >= (unsigned long)pid_max) {
-		BUG_ON((sched->pid_to_task = realloc(sched->pid_to_task, (pid + 1) *
-			sizeof(struct task_desc *))) == NULL);
+		void *p = realloc(sched->pid_to_task, (pid + 1) * sizeof(struct task_desc *));
+
+		if (p == NULL)
+			return NULL;
+		sched->pid_to_task = p;
 		while (pid >= (unsigned long)pid_max)
 			sched->pid_to_task[pid_max++] = NULL;
 	}
@@ -469,9 +481,11 @@ static struct task_desc *register_pid(struct perf_sched *sched,
 		return task;
 
 	task = zalloc(sizeof(*task));
+	if (task == NULL)
+		return NULL;
 	task->pid = pid;
-	task->nr = sched->nr_tasks;
-	strcpy(task->comm, comm);
+	if (comm)
+		strlcpy(task->comm, comm, sizeof(task->comm));
 	/*
 	 * every task starts in sleeping state - this gets ignored
 	 * if there's no wakeup pointing to this sleep state:
@@ -479,10 +493,12 @@ static struct task_desc *register_pid(struct perf_sched *sched,
 	add_sched_event_sleep(sched, task, 0);
 
 	sched->pid_to_task[pid] = task;
-	sched->nr_tasks++;
-	sched->tasks = realloc(sched->tasks, sched->nr_tasks * sizeof(struct task_desc *));
-	BUG_ON(!sched->tasks);
-	sched->tasks[task->nr] = task;
+	tasks_p = realloc(sched->tasks, (sched->nr_tasks + 1) * sizeof(struct task_desc *));
+	if (!tasks_p)
+		return NULL;
+	sched->tasks = tasks_p;
+	sched->tasks[sched->nr_tasks] = task;
+	task->nr = sched->nr_tasks++;
 
 	if (verbose > 0)
 		printf("registered task #%ld, PID %ld (%s)\n", sched->nr_tasks, pid, comm);
@@ -841,6 +857,8 @@ replay_wakeup_event(struct perf_sched *sched,
 
 	waker = register_pid(sched, sample->tid, "<unknown>");
 	wakee = register_pid(sched, pid, comm);
+	if (waker == NULL || wakee == NULL)
+		return -1;
 
 	add_sched_event_wakeup(sched, waker, sample->time, wakee);
 	return 0;
@@ -881,6 +899,8 @@ static int replay_switch_event(struct perf_sched *sched,
 
 	prev = register_pid(sched, prev_pid, prev_comm);
 	next = register_pid(sched, next_pid, next_comm);
+	if (prev == NULL || next == NULL)
+		return -1;
 
 	sched->cpu_last_switched[cpu] = timestamp;