x86/fpu: Avoid copying dynamic FP state from init_task in arch_dup_task_struct()

The init_task instance of struct task_struct is statically allocated and
may not contain the full FP state for userspace. As such, limit the copy
to the valid area of both init_task and 'dst' and ensure all memory is
initialized.

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

Fixes: 5aaeb5c01c ("x86/fpu, sched: Introduce CONFIG_ARCH_WANTS_DYNAMIC_TASK_STRUCT and use it on x86")
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20250226133136.816901-1-benjamin@sipsolutions.net
----

v2:
- Fix code if arch_task_struct_size < sizeof(init_task) by using
  memcpy_and_pad.

arch/x86/kernel/process.c

@@ -93,7 +93,12 @@ EXPORT_PER_CPU_SYMBOL_GPL(__tss_limit_invalid);
  */
 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 (incomplete FPU state) */
+	if (unlikely(src == &init_task))
+		memcpy_and_pad(dst, arch_task_struct_size, src, sizeof(init_task), 0);
+	else
+		memcpy(dst, src, arch_task_struct_size);
+
 #ifdef CONFIG_VM86
 	dst->thread.vm86 = NULL;
 #endif