x86: re-initialize RNG seed when selecting kernel

We don't want it to be possible to re-read the RNG seed after ingesting
it, because this ruins forward secrecy. Currently, however, the setup
data section can just be re-read. Since the kernel is always read after
the setup data, use the selection of the kernel as a trigger to
re-initialize the RNG seed, just like we do on reboot, to preserve
forward secrecy.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Message-Id: <20220922152847.3670513-1-Jason@zx2c4.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/i386/x86.c

@@ -1112,11 +1112,14 @@ void x86_load_linux(X86MachineState *x86ms,
         setup_data->len = cpu_to_le32(RNG_SEED_LENGTH);
         qemu_guest_getrandom_nofail(setup_data->data, RNG_SEED_LENGTH);
         qemu_register_reset(reset_rng_seed, setup_data);
+        fw_cfg_add_bytes_callback(fw_cfg, FW_CFG_KERNEL_DATA, reset_rng_seed, NULL,
+                                  setup_data, kernel, kernel_size, true);
+    } else {
+        fw_cfg_add_bytes(fw_cfg, FW_CFG_KERNEL_DATA, kernel, kernel_size);
     }
 
     fw_cfg_add_i32(fw_cfg, FW_CFG_KERNEL_ADDR, prot_addr);
     fw_cfg_add_i32(fw_cfg, FW_CFG_KERNEL_SIZE, kernel_size);
-    fw_cfg_add_bytes(fw_cfg, FW_CFG_KERNEL_DATA, kernel, kernel_size);
     sev_load_ctx.kernel_data = (char *)kernel;
     sev_load_ctx.kernel_size = kernel_size;