x86/entry/64: Stop initializing TSS.sp0 at boot

commit 20bb83443ea79087b5e5f8dab4e9d80bb9bf7acb upstream.

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.

The comment text mostly comes from Dave Hansen.  Thanks!

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/ee4a00540ad28c6cff475fbcc7769a4460acc861.1509609304.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 4e7fb9c3bfa5c170015ac5e67ba5a55f776dd3e8..cdf79ab628c244288a01561702e9c93d2d9ab348 100644 (file)
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1570,9 +1570,13 @@ void cpu_init(void)
        initialize_tlbstate_and_flush();
        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();
@@ -1594,7 +1598,6 @@ void cpu_init(void)
        int cpu = smp_processor_id();
        struct task_struct *curr = current;
        struct tss_struct *t = &per_cpu(cpu_tss, cpu);
-       struct thread_struct *thread = &curr->thread;
 
        wait_for_master_cpu(cpu);
 
@@ -1625,9 +1628,13 @@ void cpu_init(void)
        initialize_tlbstate_and_flush();
        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 c67685337c5ac33a5d5ddcec9a07a7949ae2922e..97fb3e5737f5d0b5d50f8d9232726923c2692e65 100644 (file)
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -49,7 +49,13 @@
  */
 __visible DEFINE_PER_CPU_SHARED_ALIGNED(struct tss_struct, cpu_tss) = {
        .x86_tss = {
-               .sp0 = TOP_OF_INIT_STACK,
+               /*
+                * .sp0 is only used when entering ring 0 from a lower
+                * privilege level.  Since the init task never runs anything
+                * but ring 0 code, there is no need for a valid value here.
+                * Poison it.
+                */
+               .sp0 = (1UL << (BITS_PER_LONG-1)) + 1,
 #ifdef CONFIG_X86_32
                .ss0 = __KERNEL_DS,
                .ss1 = __KERNEL_CS,