[PATCH] mm/page_frag: reject invalid CPUs in page_frag_test

[Samuel Moelius <sam.moelius@trailofbits.com>]

The page_frag selftest module accepts test_push_cpu and test_pop_cpu as
signed module parameters, then validates them by passing them directly
to cpu_active().

That validation is itself unsafe for negative or out-of-range CPU
numbers. For example, test_push_cpu=-1 is converted to a very large
unsigned CPU number before cpu_active() reaches cpumask_test_cpu(),
which trips the cpumask range check with CONFIG_DEBUG_PER_CPU_MAPS
enabled.

Reject CPU values outside [0, nr_cpu_ids) before asking whether the CPU
is active.

Assisted-by: Codex:gpt-5.5-cyber-preview
Signed-off-by: Samuel Moelius <sam.moelius@trailofbits.com>
---
 tools/testing/selftests/mm/page_frag/page_frag_test.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tools/testing/selftests/mm/page_frag/page_frag_test.c b/tools/testing/selftests/mm/page_frag/page_frag_test.c
index e806c1866e36..c8584d0fdeab 100644
--- a/tools/testing/selftests/mm/page_frag/page_frag_test.c
+++ b/tools/testing/selftests/mm/page_frag/page_frag_test.c
@@ -131,6 +131,8 @@ static int __init page_frag_test_init(void)
 	init_completion(&wait);
 
 	if (test_alloc_len > PAGE_SIZE || test_alloc_len <= 0 ||
+	    test_push_cpu < 0 || test_push_cpu >= nr_cpu_ids ||
+	    test_pop_cpu < 0 || test_pop_cpu >= nr_cpu_ids ||
 	    !cpu_active(test_push_cpu) || !cpu_active(test_pop_cpu))
 		return -EINVAL;
 
-- 
2.43.0