x86/boot/32: Defer resyncing initial_page_table until per-cpu is set up

The x86 smpboot trampoline expects initial_page_table to have the
GDT mapped. If the GDT ends up in a virtually mapped per-cpu page,
then it won't be in the page tables at all until perc-pu areas are
set up. The result will be a triple fault the first time that the
CPU attempts to access the GDT after LGDT loads the perc-pu GDT.

This appears to be an old bug, but somehow the GDT fixmap rework
is triggering it. This seems to have something to do with the
memory layout.

Signed-off-by: Andy Lutomirski <[e-mail]>
Cc: Ard Biesheuvel <[e-mail]>
Cc: Boris Ostrovsky <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Brian Gerst <[e-mail]>
Cc: Denys Vlasenko <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Josh Poimboeuf <[e-mail]>
Cc: Juergen Gross <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Matt Fleming <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Garnier <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/kernel/setup.c

···
 
 	kasan_init();
 
-#ifdef CONFIG_X86_32
-	/* sync back kernel address range */
-	clone_pgd_range(initial_page_table + KERNEL_PGD_BOUNDARY,
-			swapper_pg_dir     + KERNEL_PGD_BOUNDARY,
-			KERNEL_PGD_PTRS);
-
-	/*
-	 * sync back low identity map too.  It is used for example
-	 * in the 32-bit EFI stub.
-	 */
-	clone_pgd_range(initial_page_table,
-			swapper_pg_dir     + KERNEL_PGD_BOUNDARY,
-			min(KERNEL_PGD_PTRS, KERNEL_PGD_BOUNDARY));
-#endif
-
 	tboot_probe();
 
 	map_vsyscall();

arch/x86/kernel/setup_percpu.c

···
 
 	/* Setup cpu initialized, callin, callout masks */
 	setup_cpu_local_masks();
+
+#ifdef CONFIG_X86_32
+	/*
+	 * Sync back kernel address range.  We want to make sure that
+	 * all kernel mappings, including percpu mappings, are available
+	 * in the smpboot asm.  We can't reliably pick up percpu
+	 * mappings using vmalloc_fault(), because exception dispatch
+	 * needs percpu data.
+	 */
+	clone_pgd_range(initial_page_table + KERNEL_PGD_BOUNDARY,
+			swapper_pg_dir     + KERNEL_PGD_BOUNDARY,
+			KERNEL_PGD_PTRS);
+
+	/*
+	 * sync back low identity map too.  It is used for example
+	 * in the 32-bit EFI stub.
+	 */
+	clone_pgd_range(initial_page_table,
+			swapper_pg_dir     + KERNEL_PGD_BOUNDARY,
+			min(KERNEL_PGD_PTRS, KERNEL_PGD_BOUNDARY));
+#endif
 }