x86/apm: Don't access __preempt_count with zeroed fs

x86/apm: Don't access __preempt_count with zeroed fs

commit 6f6060a5c9cc76fdbc22748264e6aa3779ec2427 upstream.

APM_DO_POP_SEGS does not restore fs/gs which were zeroed by
APM_DO_ZERO_SEGS. Trying to access __preempt_count with
zeroed fs doesn't really work.

Move the ibrs call outside the APM_DO_SAVE_SEGS/APM_DO_RESTORE_SEGS
invocations so that fs is actually restored before calling
preempt_enable().

Fixes the following sort of oopses:
[    0.313581] general protection fault: 0000 [#1] PREEMPT SMP
[    0.313803] Modules linked in:
[    0.314040] CPU: 0 PID: 268 Comm: kapmd Not tainted 4.16.0-rc1-triton-bisect-00090-gdd84441a7971 #19
[    0.316161] EIP: __apm_bios_call_simple+0xc8/0x170
[    0.316161] EFLAGS: 00210016 CPU: 0
[    0.316161] EAX: 00000102 EBX: 00000000 ECX: 00000102 EDX: 00000000
[    0.316161] ESI: 0000530e EDI: dea95f64 EBP: dea95f18 ESP: dea95ef0
[    0.316161]  DS: 007b ES: 007b FS: 0000 GS: 0000 SS: 0068
[    0.316161] CR0: 80050033 CR2: 00000000 CR3: 015d3000 CR4: 000006d0
[    0.316161] Call Trace:
[    0.316161]  ? cpumask_weight.constprop.15+0x20/0x20
[    0.316161]  on_cpu0+0x44/0x70
[    0.316161]  apm+0x54e/0x720
[    0.316161]  ? __switch_to_asm+0x26/0x40
[    0.316161]  ? __schedule+0x17d/0x590
[    0.316161]  kthread+0xc0/0xf0
[    0.316161]  ? proc_apm_show+0x150/0x150
[    0.316161]  ? kthread_create_worker_on_cpu+0x20/0x20
[    0.316161]  ret_from_fork+0x2e/0x38
[    0.316161] Code: da 8e c2 8e e2 8e ea 57 55 2e ff 1d e0 bb 5d b1 0f 92 c3 5d 5f 07 1f 89 47 0c 90 8d b4 26 00 00 00 00 90 8d b4 26 00 00 00 00 90 <64> ff 0d 84 16 5c b1 74 7f 8b 45 dc 8e e0 8b 45 d8 8e e8 8b 45
[    0.316161] EIP: __apm_bios_call_simple+0xc8/0x170 SS:ESP: 0068:dea95ef0
[    0.316161] ---[ end trace 656253db2deaa12c ]---

Fixes: dd84441a7971 ("x86/speculation: Use IBRS if available before calling into firmware")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Cc:  David Woodhouse <dwmw@amazon.co.uk>
Cc:  "H. Peter Anvin" <hpa@zytor.com>
Cc:  x86@kernel.org
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lkml.kernel.org/r/20180709133534.5963-1-ville.syrjala@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>