diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 449b9a5d3fe3..54abc67c48c7 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -826,7 +826,8 @@ int bpf_bprintf_prepare(char *fmt, u32 fmt_size, const u64 *raw_args,
 	u64 cur_arg;
 	char fmt_ptype, cur_ip[16], ip_spec[] = "%pXX";
 
-	fmt_end = strnchr(fmt, fmt_size, 0);
+	kmsan_unpoison_memory(fmt, fmt_size);
+	fmt_end = strnchrnul(fmt, fmt_size, 0);
 	if (!fmt_end)
 		return -EINVAL;
 	fmt_size = fmt_end - fmt;