This provides the ability for architectures to enable kernel stack base address offset randomization. This feature is controlled by the boot param "randomize_kstack_offset=on/off", with its default value set by CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT. This feature is based on the original idea from the last public release of PaX's RANDKSTACK feature: https://pax.grsecurity.net/docs/randkstack.txt All the credit for the original idea goes to the PaX team. Note that the design and implementation of this upstream randomize_kstack_offset feature differs greatly from the RANDKSTACK feature (see below). Reasoning for the feature: This feature aims to make harder the various stack-based attacks that rely on deterministic stack structure. We have had many such attacks in past (just to name few): https://jon.oberheide.org/files/infiltrate12-thestackisback.pdf https://jon.oberheide.org/files/stackjacking-infiltrate11.pdf https://googleprojectzero.blogspot.com/2016/06/exploiting-recursion-in-linux-kernel_20.html As Linux kernel stack protections have been constantly improving (vmap-based stack allocation with guard pages, removal of thread_info, STACKLEAK), attackers have to find new ways for their exploits to work. They have done so, continuing to rely on the kernel's stack determinism, in situations where VMAP_STACK and THREAD_INFO_IN_TASK_STRUCT were not relevant. For example, the following recent attacks would have been hampered if the stack offset was non-deterministic between syscalls: https://repositorio-aberto.up.pt/bitstream/10216/125357/2/374717.pdf https://a13xp0p0v.github.io/2020/02/15/CVE-2019-18683.html The main idea is that since the stack offset is randomized upon each system call, it is hard for an attack to reliably land in any particular place on the thread stack, even with address exposures, as the stack base will change on the next syscall. Also, since randomization is performed after placing pt_regs, the ptrace-based approach[1] to discover the randomized offset during a long-running syscall should not be possible. Design description: During most of the kernel's execution, it runs on the "thread stack", which is allocated at fork.c/dup_task_struct() and stored in a per-task variable (tsk->stack). Since stack is growing downward, the stack top can be always calculated using task_top_of_stack(tsk) function, which essentially returns an address of tsk->stack + stack size. When VMAP_STACK is enabled, the thread stack is allocated from vmalloc space. The thread stack is pretty deterministic in its structure -- fixed in size, and upon every entry from a userspace to kernel on a syscall the thread stack is started to be constructed from an address fetched from a per-cpu cpu_current_top_of_stack variable. The first element to be pushed to the thread stack is the pt_regs struct that stores all required CPU registers and syscall parameters. The goal of randomize_kstack_offset feature is to add a random offset after the pt_regs has been pushed to the stack and the rest of thread stack (used during the syscall processing) every time a process issues a syscall. The source of randomness is currently arch-defined (but x86 is using the low byte of rdtsc()). Future improvements for different entropy sources is possible, but out of scope for this patch. The offset is added using alloca() call since it helps avoiding changes in assembly syscall entry code and unwinder, and provides correct stack alignment as defined by the compiler. In order to make this available by default with zero performance impact for those that don't want it, now it is selectable with static branches. This way, if the overhead is not wanted, it can just be turned off. Using the per-cpu variable as the entropy source and __builtin_alloc() for stack adjustment and alignment, the generated assembly for x86_64 with GCC looks like this: ... ffffffff81003977: 65 8b 05 02 ea 00 7f mov %gs:0x7f00ea02(%rip),%eax # 12380 <kstack_offset> ffffffff8100397e: 25 ff 03 00 00 and $0x3ff,%eax ffffffff81003983: 48 83 c0 0f add $0xf,%rax ffffffff81003987: 25 f8 07 00 00 and $0x7f8,%eax ffffffff8100398c: 48 29 c4 sub %rax,%rsp ffffffff8100398f: 48 8d 44 24 0f lea 0xf(%rsp),%rax ffffffff81003994: 48 83 e0 f0 and $0xfffffffffffffff0,%rax ... As a result of the above stack alignment, this patch introduces about 5 bits of randomness after pt_regs is spilled to the thread stack on x86_64, and 6 bits on x86_32 (since its has 1 fewer bits required for stack alignment). The amount of entropy could be adjusted based on how much of the stack space we wish to trade for security. My measure of syscall performance overhead (on x86_64): lmbench: /usr/lib/lmbench/bin/x86_64-linux-gnu/lat_syscall -N 10000 null randomize_kstack_offset=y Simple syscall: 0.7082 microseconds randomize_kstack_offset=n Simple syscall: 0.7016 microseconds So, roughly 0.9% overhead growth for a no-op syscall, which is very manageable. And for people that don't want this, it's off by default. Comparison to PaX RANDKSTACK feature: The RANDKSTACK feature randomizes the location of the stack start (cpu_current_top_of_stack), i.e. including the location of pt_regs structure itself on the stack. Initially this patch followed the same approach, but during the recent discussions[2], it has been determined to be of a little value since, if ptrace functionality is available for an attacker, they can use PTRACE_PEEKUSR/PTRACE_POKEUSR to read/write different offsets in the pt_regs struct, observe the cache behavior of the pt_regs accesses, and figure out the random stack offset. Another difference is that the random offset is stored in a per-cpu variable, rather than having it be per-thread. As a result, these implementations differ a fair bit in their implementation details and results, though obviously the intent is similar. [1] https://lore.kernel.org/kernel-hardening/2236FBA76BA1254E88B949DDB74E612BA4BC57C1@xxxxxxxxxxxxxxxxxxxxxxxxxxxx/ [2] https://lore.kernel.org/kernel-hardening/20190329081358.30497-1-elena.reshetova@xxxxxxxxx/ Co-developed-by: Elena Reshetova <elena.reshetova@xxxxxxxxx> Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx> Link: https://lore.kernel.org/r/20190415060918.3766-1-elena.reshetova@xxxxxxxxx Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> --- v2: - move to per-cpu rdtsc() saved on syscall exit - add static branches for zero-cost dynamic enabling - Kconfig just selects the default state of static branch - __builtin_alloca() produces ugly asm without -fno-stack-clash-protection - made arch agnostic rfc: https://lore.kernel.org/kernel-hardening/20190329081358.30497-1-elena.reshetova@xxxxxxxxx/ --- Makefile | 4 ++++ arch/Kconfig | 19 +++++++++++++++ include/linux/randomize_kstack.h | 40 ++++++++++++++++++++++++++++++++ init/main.c | 23 ++++++++++++++++++ 4 files changed, 86 insertions(+) create mode 100644 include/linux/randomize_kstack.h diff --git a/Makefile b/Makefile index 171f2b004c8a..c99463406522 100644 --- a/Makefile +++ b/Makefile @@ -779,6 +779,10 @@ ifdef CONFIG_INIT_STACK_ALL KBUILD_CFLAGS += -ftrivial-auto-var-init=pattern endif +# While VLAs have been removed, GCC produces unreachable stack probes +# for the random_kstack_offset feature. Disable it for all compilers. +KBUILD_CFLAGS += $(call cc-option,-fno-stack-clash-protection,) + DEBUG_CFLAGS := $(call cc-option, -fno-var-tracking-assignments) ifdef CONFIG_DEBUG_INFO diff --git a/arch/Kconfig b/arch/Kconfig index 17fe351cdde0..619a56da4b76 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -854,6 +854,25 @@ config VMAP_STACK virtual mappings with real shadow memory, and KASAN_VMALLOC must be enabled. +config HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET + def_bool n + help + An arch should select this symbol if it can support kernel stack + offset randomization with calls to add_random_kstack_offset() + during syscall entry and choose_random_kstack_offset() during + syscall exit. + +config RANDOMIZE_KSTACK_OFFSET_DEFAULT + bool "Randomize kernel stack offset on syscall entry" + depends on HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET + help + The kernel stack offset can be randomized (after pt_regs) by + roughly 5 bits of entropy, frustrating memory corruption + attacks that depend on stack address determinism or + cross-syscall address exposures. This feature is controlled + by kernel boot param "randomize_kstack_offset=on/off", and this + config chooses the default boot state. + config ARCH_OPTIONAL_KERNEL_RWX def_bool n diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h new file mode 100644 index 000000000000..651ba9504568 --- /dev/null +++ b/include/linux/randomize_kstack.h @@ -0,0 +1,40 @@ +/* SPDX-License-Identifier: GPL-2.0-only */ +#ifndef _LINUX_RANDOMIZE_KSTACK_H +#define _LINUX_RANDOMIZE_KSTACK_H + +#include <linux/kernel.h> +#include <linux/jump_label.h> +#include <linux/percpu-defs.h> + +DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, + randomize_kstack_offset); +DECLARE_PER_CPU(u32, kstack_offset); + +/* + * Do not use this anywhere else in the kernel. This is used here because + * it provides an arch-agnostic way to grow the stack with correct + * alignment. Also, since this use is being explicitly masked to a max of + * 10 bits, stack-clash style attacks are unlikely. For more details see + * "VLAs" in Documentation/process/deprecated.rst + */ +void *__builtin_alloca(size_t size); + +#define add_random_kstack_offset() do { \ + if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ + &randomize_kstack_offset)) { \ + u32 offset = this_cpu_read(kstack_offset); \ + char *ptr = __builtin_alloca(offset & 0x3FF); \ + asm volatile("" : "=m"(*ptr)); \ + } \ +} while (0) + +#define choose_random_kstack_offset(rand) do { \ + if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ + &randomize_kstack_offset)) { \ + u32 offset = this_cpu_read(kstack_offset); \ + offset ^= (rand); \ + this_cpu_write(kstack_offset, offset); \ + } \ +} while (0) + +#endif diff --git a/init/main.c b/init/main.c index ee4947af823f..78fe3aea00b0 100644 --- a/init/main.c +++ b/init/main.c @@ -777,6 +777,29 @@ static void __init mm_init(void) pti_init(); } +#ifdef CONFIG_HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET +DEFINE_STATIC_KEY_MAYBE_RO(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, + randomize_kstack_offset); +DEFINE_PER_CPU(u32, kstack_offset); + +static int __init early_randomize_kstack_offset(char *buf) +{ + int ret; + bool bool_result; + + ret = kstrtobool(buf, &bool_result); + if (ret) + return ret; + + if (bool_result) + static_branch_enable(&randomize_kstack_offset); + else + static_branch_disable(&randomize_kstack_offset); + return 0; +} +early_param("randomize_kstack_offset", early_randomize_kstack_offset); +#endif + void __init __weak arch_call_rest_init(void) { rest_init(); -- 2.20.1
