--- a/sys/platform/pc64/x86_64/bios.c
+++ b/sys/platform/pc64/x86_64/bios.c
@@ -66,8 +66,14 @@ bios_sigsearch(u_int32_t start, u_char *sig, int siglen, int paralen, int sigofs)
 {
     u_char	*sp, *end;
 
+    /*
+     * Sanity-check the scan parameters.  A zero or negative paragraph
+     * step would loop forever or walk below the BIOS window, and a
+     * negative sigofs / non-positive siglen would read outside it.
+     */
+    if (sig == NULL || siglen <= 0 || paralen <= 0 || sigofs < 0)
+	return 0;
+
     /* compute the starting address */
     if ((start >= BIOS_START) && (start <= (BIOS_START + BIOS_SIZE))) {
 	sp = (char *)BIOS_PADDRTOVADDR(start);