device-tree: add re-randomization helper function

When the system reboots, the rng-seed that the FDT has should be
re-randomized, so that the new boot gets a new seed. Several
architectures require this functionality, so export a function for
injecting a new seed into the given FDT.

Cc: Alistair Francis <alistair.francis@wdc.com>
Cc: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20221025004327.568476-3-Jason@zx2c4.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

include/sysemu/device_tree.h

@@ -197,6 +197,15 @@ int qemu_fdt_setprop_sized_cells_from_array(void *fdt,
                                                 qdt_tmp);                 \
     })
 
+
+/**
+ * qemu_fdt_randomize_seeds:
+ * @fdt: device tree blob
+ *
+ * Re-randomize all "rng-seed" properties with new seeds.
+ */
+void qemu_fdt_randomize_seeds(void *fdt);
+
 #define FDT_PCI_RANGE_RELOCATABLE          0x80000000
 #define FDT_PCI_RANGE_PREFETCHABLE         0x40000000
 #define FDT_PCI_RANGE_ALIASED              0x20000000

softmmu/device_tree.c

@@ -22,6 +22,7 @@
 #include "qemu/option.h"
 #include "qemu/bswap.h"
 #include "qemu/cutils.h"
+#include "qemu/guest-random.h"
 #include "sysemu/device_tree.h"
 #include "hw/loader.h"
 #include "hw/boards.h"
@@ -680,3 +681,23 @@ void hmp_dumpdtb(Monitor *mon, const QDict *qdict)
 
     info_report("dtb dumped to %s", filename);
 }
+
+void qemu_fdt_randomize_seeds(void *fdt)
+{
+    int noffset, poffset, len;
+    const char *name;
+    uint8_t *data;
+
+    for (noffset = fdt_next_node(fdt, 0, NULL);
+         noffset >= 0;
+         noffset = fdt_next_node(fdt, noffset, NULL)) {
+        for (poffset = fdt_first_property_offset(fdt, noffset);
+             poffset >= 0;
+             poffset = fdt_next_property_offset(fdt, poffset)) {
+            data = (uint8_t *)fdt_getprop_by_offset(fdt, poffset, &name, &len);
+            if (!data || strcmp(name, "rng-seed"))
+                continue;
+            qemu_guest_getrandom_nofail(data, len);
+        }
+    }
+}