.. SPDX-License-Identifier: GPL-2.0

=====================================
Virtually Mapped Kernel Stack Support
=====================================

:Author: Shuah Khan <skhan@linuxfoundation.org>

.. contents:: :local:

Overview
--------

This is a compilation of information from the code and original patch
series that introduced the `Virtually Mapped Kernel Stacks feature
<https://lwn.net/Articles/694348/>`

Introduction
------------

Kernel stack overflows are often hard to debug and make the kernel
susceptible to exploits. Problems could show up at a later time making
it difficult to isolate and root-cause.

Virtually mapped kernel stacks with guard pages cause kernel stack
overflows to be caught immediately rather than causing difficult to
diagnose corruptions.

HAVE_ARCH_VMAP_STACK and VMAP_STACK configuration options enable
support for virtually mapped stacks with guard pages. This feature
causes reliable faults when the stack overflows. The usability of
the stack trace after overflow and response to the overflow itself
is architecture dependent.

.. note::
        As of this writing, arm64, powerpc, riscv, s390, um, and x86 have
        support for VMAP_STACK.

HAVE_ARCH_VMAP_STACK
--------------------

Architectures that can support Virtually Mapped Kernel Stacks should
enable this bool configuration option. The requirements are:

- vmalloc space must be large enough to hold many kernel stacks. This
  may rule out many 32-bit architectures.
- Stacks in vmalloc space need to work reliably.  For example, if
  vmap page tables are created on demand, either this mechanism
  needs to work while the stack points to a virtual address with
  unpopulated page tables or arch code (switch_to() and switch_mm(),
  most likely) needs to ensure that the stack's page table entries
  are populated before running on a possibly unpopulated stack.
- If the stack overflows into a guard page, something reasonable
  should happen. The definition of "reasonable" is flexible, but
  instantly rebooting without logging anything would be unfriendly.

VMAP_STACK
----------

When enabled, the VMAP_STACK bool configuration option allocates virtually
mapped task stacks. This option depends on HAVE_ARCH_VMAP_STACK.

- Enable this if you want the use virtually-mapped kernel stacks
  with guard pages. This causes kernel stack overflows to be caught
  immediately rather than causing difficult-to-diagnose corruption.

.. note::

        Using this feature with KASAN requires architecture support
        for backing virtual mappings with real shadow memory, and
        KASAN_VMALLOC must be enabled.

.. note::

        VMAP_STACK is enabled, it is not possible to run DMA on stack
        allocated data.

Kernel configuration options and dependencies keep changing. Refer to
the latest code base:

`Kconfig <https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/Kconfig>`

Allocation
-----------

When a new kernel thread is created, a thread stack is allocated from
virtually contiguous memory pages from the page level allocator. These
pages are mapped into contiguous kernel virtual space with PAGE_KERNEL
protections.

alloc_thread_stack_node() calls __vmalloc_node_range() to allocate stack
with PAGE_KERNEL protections.

- Allocated stacks are cached and later reused by new threads, so memcg
  accounting is performed manually on assigning/releasing stacks to tasks.
  Hence, __vmalloc_node_range is called without __GFP_ACCOUNT.
- vm_struct is cached to be able to find when thread free is initiated
  in interrupt context. free_thread_stack() can be called in interrupt
  context.
- On arm64, all VMAP's stacks need to have the same alignment to ensure
  that VMAP'd stack overflow detection works correctly. Arch specific
  vmap stack allocator takes care of this detail.
- This does not address interrupt stacks - according to the original patch

Thread stack allocation is initiated from clone(), fork(), vfork(),
kernel_thread() via kernel_clone(). These are a few hints for searching
the code base to understand when and how a thread stack is allocated.

Bulk of the code is in:
`kernel/fork.c <https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/fork.c>`.

stack_vm_area pointer in task_struct keeps track of the virtually allocated
stack and a non-null stack_vm_area pointer serves as a indication that the
virtually mapped kernel stacks are enabled.

::

        struct vm_struct *stack_vm_area;

Stack overflow handling
-----------------------

Leading and trailing guard pages help detect stack overflows. When stack
overflows into the guard pages, handlers have to be careful not overflow
the stack again. When handlers are called, it is likely that very little
stack space is left.

On x86, this is done by handling the page fault indicating the kernel
stack overflow on the double-fault stack.

Testing VMAP allocation with guard pages
----------------------------------------

How do we ensure that VMAP_STACK is actually allocating with a leading
and trailing guard page? The following lkdtm tests can help detect any
regressions.

::

        void lkdtm_STACK_GUARD_PAGE_LEADING()
        void lkdtm_STACK_GUARD_PAGE_TRAILING()

Conclusions
-----------

- A percpu cache of vmalloced stacks appears to be a bit faster than a
  high-order stack allocation, at least when the cache hits.
- THREAD_INFO_IN_TASK gets rid of arch-specific thread_info entirely and
  simply embed the thread_info (containing only flags) and 'int cpu' into
  task_struct.
- The thread stack can be free'ed as soon as the task is dead (without
  waiting for RCU) and then, if vmapped stacks are in use, cache the
  entire stack for reuse on the same cpu.