um: avoid copying FP state from init_task

The init_task instance of struct task_struct is statically allocated and
does not contain the dynamic area for the userspace FP registers. As
such, limit the copy to the valid area of init_task and fill the rest
with zero.

Note that the FP state is only needed for userspace, and as such it is
entirely reasonable for init_task to not contain it.

Reported-by: Brian Norris <briannorris@chromium.org>
Closes: https://lore.kernel.org/Z1ySXmjZm-xOqk90@google.com
Fixes: 3f17fed ("um: switch to regset API and depend on XSTATE")
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20241217202745.1402932-3-benjamin@sipsolutions.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

Author: benzea

arch/um/kernel/process.c

@@ -191,7 +191,15 @@ void initial_thread_cb(void (*proc)(void *), void *arg)
 int arch_dup_task_struct(struct task_struct *dst,
 			 struct task_struct *src)
 {
-	memcpy(dst, src, arch_task_struct_size);
+	/* init_task is not dynamically sized (missing FPU state) */
+	if (unlikely(src == &init_task)) {
+		memcpy(dst, src, sizeof(init_task));
+		memset((void *)dst + sizeof(init_task), 0,
+		       arch_task_struct_size - sizeof(init_task));
+	} else {
+		memcpy(dst, src, arch_task_struct_size);
+	}
+
 	return 0;
 }