scs: Release kasan vmalloc poison in scs_free process

scs: Release kasan vmalloc poison in scs_free process

Since scs allocation is moved to vmalloc region, the
shadow stack is protected by kasan_posion_vmalloc.
However, the vfree_atomic operation needs to access
its context for scs_free process and causes kasan error
as the dump info below.

This patch Adds kasan_unpoison_vmalloc() before vfree_atomic,
which aligns to the prior flow as using kmem_cache.
The vmalloc region will go back posioned in the following
vumap() operations.

 ==================================================================
 BUG: KASAN: vmalloc-out-of-bounds in llist_add_batch+0x60/0xd4
 Write of size 8 at addr ffff8000100b9000 by task kthreadd/2

 CPU: 0 PID: 2 Comm: kthreadd Not tainted 5.15.0-rc2-11681-g92477dd1faa6-dirty #1
 Hardware name: linux,dummy-virt (DT)
 Call trace:
  dump_backtrace+0x0/0x43c
  show_stack+0x1c/0x2c
  dump_stack_lvl+0x68/0x84
  print_address_description+0x80/0x394
  kasan_report+0x180/0x1dc
  __asan_report_store8_noabort+0x48/0x58
  llist_add_batch+0x60/0xd4
  vfree_atomic+0x60/0xe0
  scs_free+0x1dc/0x1fc
  scs_release+0xa4/0xd4
  free_task+0x30/0xe4
  __put_task_struct+0x1ec/0x2e0
  delayed_put_task_struct+0x5c/0xa0
  rcu_do_batch+0x62c/0x8a0
  rcu_core+0x60c/0xc14
  rcu_core_si+0x14/0x24
  __do_softirq+0x19c/0x68c
  irq_exit+0x118/0x2dc
  handle_domain_irq+0xcc/0x134
  gic_handle_irq+0x7c/0x1bc
  call_on_irq_stack+0x40/0x70
  do_interrupt_handler+0x78/0x9c
  el1_interrupt+0x34/0x60
  el1h_64_irq_handler+0x1c/0x2c
  el1h_64_irq+0x78/0x7c
  _raw_spin_unlock_irqrestore+0x40/0xcc
  sched_fork+0x4f0/0xb00
  copy_process+0xacc/0x3648
  kernel_clone+0x168/0x534
  kernel_thread+0x13c/0x1b0
  kthreadd+0x2bc/0x400
  ret_from_fork+0x10/0x20

 Memory state around the buggy address:
  ffff8000100b8f00: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
  ffff8000100b8f80: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
 >ffff8000100b9000: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
                    ^
  ffff8000100b9080: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
  ffff8000100b9100: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
 ==================================================================

Suggested-by: Kuan-Ying Lee <kuan-ying.lee@mediatek.com>
Acked-by: Will Deacon <will@kernel.org>
Tested-by: Will Deacon <will@kernel.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Yee Lee <yee.lee@mediatek.com>
Fixes: a2abe7cbd8fe ("scs: switch to vmapped shadow stacks")
Link: https://lore.kernel.org/r/20210930081619.30091-1-yee.lee@mediatek.com
Signed-off-by: Will Deacon <will@kernel.org>