[PATCH 4/4] perf sched: Fix comp_cpus heap overflow with cross-machine recordings

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

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

setup_map_cpus() allocates comp_cpus based on
sysconf(_SC_NPROCESSORS_CONF), the host machine's CPU count.  But
map_switch_event() indexes comp_cpus using cpus_nr derived from
bitmap_weight(comp_cpus_mask, MAX_CPUS), where comp_cpus_mask is
declared as DECLARE_BITMAP(..., MAX_CPUS) with MAX_CPUS=4096.

When analyzing a perf.data recording from a machine with more CPUs
than the analysis host (e.g. 128-CPU server recording analyzed on an
8-CPU laptop), cpus_nr exceeds the allocation size, causing a heap
buffer overflow.

Also fix a type mismatch: comp_cpus is 'struct perf_cpu *' (2 bytes
per element) but was allocated with sizeof(int) (4 bytes per element).

Allocate comp_cpus with MAX_CPUS entries using the correct element
size, matching the comp_cpus_mask bitmap bounds.  Remove the
sysconf(_SC_NPROCESSORS_CONF) initialization of max_cpu — its only
consumer was the comp_cpus allocation, and max_cpu is dynamically
updated from the recording's events during processing.  Fix the
non-compact path to use max_cpu.cpu + 1 as cpus_nr, converting from
0-based index to count — sysconf() returned a count which masked
this off-by-one.

Fixes: 99623c628f54 ("perf sched: Add compact display option")
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Cc: Jiri Olsa <jolsa@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 | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c
index 36da451447b5e59f..4aa7833cae6e36b8 100644
--- a/tools/perf/builtin-sched.c
+++ b/tools/perf/builtin-sched.c
@@ -1670,7 +1670,7 @@ static int map_switch_event(struct perf_sched *sched,  struct perf_sample *sampl
 			new_cpu = true;
 		}
 	} else
-		cpus_nr = sched->max_cpu.cpu;
+		cpus_nr = sched->max_cpu.cpu + 1;
 
 	timestamp0 = sched->cpu_last_switched[this_cpu.cpu];
 	sched->cpu_last_switched[this_cpu.cpu] = timestamp;
@@ -3573,10 +3573,8 @@ static int perf_sched__lat(struct perf_sched *sched)
 
 static int setup_map_cpus(struct perf_sched *sched)
 {
-	sched->max_cpu.cpu  = sysconf(_SC_NPROCESSORS_CONF);
-
 	if (sched->map.comp) {
-		sched->map.comp_cpus = calloc(sched->max_cpu.cpu, sizeof(int));
+		sched->map.comp_cpus = calloc(MAX_CPUS, sizeof(*sched->map.comp_cpus));
 		if (!sched->map.comp_cpus)
 			return -1;
 	}
-- 
2.54.0