[PATCH] arm: re-randomize rng-seed on reboot

[Jason A. Donenfeld]

When the system reboots, the rng-seed that the FDT has should be
re-randomized, so that the new boot gets a new seed. Since the FDT is in
the ROM region at this point, we add a hook right after the ROM has been
added, so that we have a pointer to that copy of the FDT. When the
reboot happens, we then look for RNG seeds and replace their contents
with new random data.

Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 hw/arm/boot.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/hw/arm/boot.c b/hw/arm/boot.c
index ada2717f76..2836db4abb 100644
--- a/hw/arm/boot.c
+++ b/hw/arm/boot.c
@@ -25,6 +25,7 @@
 #include "qemu/config-file.h"
 #include "qemu/option.h"
 #include "qemu/units.h"
+#include "qemu/guest-random.h"
 
 /* Kernel boot protocol is specified in the kernel docs
  * Documentation/arm/Booting and Documentation/arm64/booting.txt
@@ -529,6 +530,26 @@ static void fdt_add_psci_node(void *fdt)
     qemu_fdt_setprop_cell(fdt, "/psci", "migrate", migrate_fn);
 }
 
+static void rerandomize_fdt_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);
+        }
+    }
+}
+
 int arm_load_dtb(hwaddr addr, const struct arm_boot_info *binfo,
                  hwaddr addr_limit, AddressSpace *as, MachineState *ms)
 {
@@ -683,6 +704,7 @@ int arm_load_dtb(hwaddr addr, const struct arm_boot_info 
*binfo,
      * the DTB is copied again upon reset, even if addr points into RAM.
      */
     rom_add_blob_fixed_as("dtb", fdt, size, addr, as);
+    qemu_register_reset(rerandomize_fdt_seeds, rom_ptr_for_as(as, addr, size));
 
     g_free(fdt);
 
-- 
2.37.3