hw/arm: do not free machine->fdt in arm_load_dtb()

At this moment, arm_load_dtb() can free machine->fdt when
binfo->dtb_filename is NULL. If there's no 'dtb_filename', 'fdt' will be
retrieved by binfo->get_dtb(). If get_dtb() returns machine->fdt, as is
the case of machvirt_dtb() from hw/arm/virt.c, fdt now has a pointer to
machine->fdt. And, in that case, the existing g_free(fdt) at the end of
arm_load_dtb() will make machine->fdt point to an invalid memory region.

Since monitor command 'dumpdtb' was introduced a couple of releases
ago, running it with any ARM machine that uses arm_load_dtb() will
crash QEMU.

Let's enable all arm_load_dtb() callers to use dumpdtb properly. Instead
of freeing 'fdt', assign it back to ms->fdt.

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-arm@nongnu.org
Fixes: bf353ad555 ("qmp/hmp, device_tree.c: introduce dumpdtb")
Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-id: 20230328165935.1512846-1-armbru@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
(cherry picked from commit 12148d442ec3f4386c8624ffcf44c61a8b344018)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/arm/boot.c

@@ -686,7 +686,10 @@ int arm_load_dtb(hwaddr addr, const struct arm_boot_info *binfo,
     qemu_register_reset_nosnapshotload(qemu_fdt_randomize_seeds,
                                        rom_ptr_for_as(as, addr, size));
 
-    g_free(fdt);
+    if (fdt != ms->fdt) {
+        g_free(ms->fdt);
+        ms->fdt = fdt;
+    }
 
     return size;