linux-kernel - [RFC 13/17] x86/boot/64: Stop initializing TSS.sp0 at boot

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Date:   Wed,  6 Sep 2017 14:36:58 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: [RFC 13/17] x86/boot/64: Stop initializing TSS.sp0 at boot

In my quest to get rid of thread_struct::sp0, I want to clean up or
remove all of its readers.  Two of them are in cpu_init() (32-bit and
64-bit), and they aren't needed.  This is because we never enter
userspace at all on the threads that CPUs are initialized in.

Poison the initial TSS.sp0 and stop initializing it on CPU init.

Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/kernel/cpu/common.c | 12 ++++++++++--
 arch/x86/kernel/process.c    |  3 ++-
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 40312b3ef9de..b3c621272e6b 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1554,9 +1554,13 @@ void cpu_init(void)
 	BUG_ON(me->mm);
 	enter_lazy_tlb(&init_mm, me);
 
-	load_sp0(current->thread.sp0);
+	/*
+	 * Initialize the TSS.  Don't bother initializing sp0, as the initial
+	 * task never enters user mode.
+	 */
 	set_tss_desc(cpu, t);
 	load_TR_desc();
+
 	load_mm_ldt(&init_mm);
 
 	clear_all_debug_regs();
@@ -1608,9 +1612,13 @@ void cpu_init(void)
 	BUG_ON(curr->mm);
 	enter_lazy_tlb(&init_mm, curr);
 
-	load_sp0(thread->sp0);
+	/*
+	 * Initialize the TSS.  Don't bother initializing sp0, as the initial
+	 * task never enters user mode.
+	 */
 	set_tss_desc(cpu, t);
 	load_TR_desc();
+
 	load_mm_ldt(&init_mm);
 
 	t->x86_tss.io_bitmap_base = offsetof(struct tss_struct, io_bitmap);
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 3ca198080ea9..df478002b07d 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -48,7 +48,8 @@
  */
 __visible DEFINE_PER_CPU_SHARED_ALIGNED(struct tss_struct, cpu_tss) = {
 	.x86_tss = {
-		.sp0 = TOP_OF_INIT_STACK,
+		/* Initialize sp0 to a value that is definitely invalid. */
+		.sp0 = 0x8000000000000001,
 #ifdef CONFIG_X86_32
 		.ss0 = __KERNEL_DS,
 		.ss1 = __KERNEL_CS,
-- 
2.13.5