11743 installboot: stage2 partition may be smaller than mboot buffer

Reviewed by: Yuri Pankov <yuri.pankov@nexenta.com>
Reviewed by: Andy Fiddaman <omnios@citrus-it.co.uk>
Approved by: Dan McDonald <danmcd@joyent.com>

usr/src/cmd/boot/installboot/i386/installboot.c

@@ -595,6 +595,7 @@ read_stage2_cb(struct partlist *plist)
 	uint32_t		buf_size;
 	uint32_t		mboot_off;
 	multiboot_header_t	*mboot;
+	size_t			scan_size;
 
 	bblock = calloc(1, sizeof (ib_bootblock_t));
 	if (bblock == NULL)
@@ -608,8 +609,10 @@ read_stage2_cb(struct partlist *plist)
 	device = plist->pl_device;
 	plist->pl_stage = bblock;
 	offset = device->stage.offset * SECTOR_SIZE;
+	scan_size = MIN(sizeof (mboot_scan),
+	    (device->stage.size - device->stage.offset) * sector_size);
 
-	if (read_in(fd, mboot_scan, sizeof (mboot_scan), offset)
+	if (read_in(fd, mboot_scan, scan_size, offset)
 	    != BC_SUCCESS) {
 		BOOT_DEBUG("Error reading bootblock area\n");
 		perror("read");
@@ -618,7 +621,7 @@ read_stage2_cb(struct partlist *plist)
 	}
 
 	/* No multiboot means no chance of knowing bootblock size */
-	if (find_multiboot(mboot_scan, sizeof (mboot_scan), &mboot_off)
+	if (find_multiboot(mboot_scan, scan_size, &mboot_off)
 	    != BC_SUCCESS) {
 		BOOT_DEBUG("Unable to find multiboot header\n");
 		(void) close(fd);