--- /dev/null
+.. 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 causes 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
+----------
+
+VMAP_STACK bool configuration option when enabled 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, 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(). Leaving a few hints for searching
+the code base to understand when and how 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.
projects / kernel.git / commitdiff