.. 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 patchseries 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 kernelsusceptible to exploits. Problems could show up at a later time makingit difficult to isolate and root-cause. Virtually mapped kernel stacks with guard pages cause kernel stackoverflows to be caught immediately rather than causing difficult todiagnose corruptions. HAVE_ARCH_VMAP_STACK and VMAP_STACK configuration options enablesupport for virtually mapped stacks with guard pages. This featurecauses reliable faults when the stack overflows. The usability ofthe stack trace after overflow and response to the overflow itselfis 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 shouldenable 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 virtuallymapped 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 tothe 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 fromvirtually contiguous memory pages from the page level allocator. Thesepages are mapped into contiguous kernel virtual space with PAGE_KERNELprotections. alloc_thread_stack_node() calls __vmalloc_node_range() to allocate stackwith 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 searchingthe 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 allocatedstack and a non-null stack_vm_area pointer serves as an indication that thevirtually mapped kernel stacks are enabled. ::         struct vm_struct *stack_vm_area; Stack overflow handling----------------------- Leading and trailing guard pages help detect stack overflows. When the stackoverflows into the guard pages, handlers have to be careful not to overflowthe stack again. When handlers are called, it is likely that very littlestack space is left. On x86, this is done by handling the page fault indicating the kernelstack overflow on the double-fault stack. Testing VMAP allocation with guard pages---------------------------------------- How do we ensure that VMAP_STACK is actually allocating with a leadingand trailing guard page? The following lkdtm tests can help detect anyregressions. ::         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 freed 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.