Re: [PATCH v3 05/17] add support for Clang's Shadow Call Stack (SCS)

Kees Cook
2019-11-01T03:52:03Z

On Thu, Oct 31, 2019 at 09:46:25AM -0700, samitolvanen@google.com wrote:
> This change adds generic support for Clang's Shadow Call Stack,
> which uses a shadow stack to protect return addresses from being
> overwritten by an attacker. Details are available here:
>
> https://clang.llvm.org/docs/ShadowCallStack.html
>
> Note that security guarantees in the kernel differ from the
> ones documented for user space. The kernel must store addresses
> of shadow stacks used by other tasks and interrupt handlers in
> memory, which means an attacker capable reading and writing
> arbitrary memory may be able to locate them and hijack control
> flow by modifying shadow stacks that are not currently in use.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> ---
> Makefile | 6 ++
> arch/Kconfig | 33 +++++++
> include/linux/compiler-clang.h | 6 ++
> include/linux/compiler_types.h | 4 +
> include/linux/scs.h | 54 +++++++++++
> init/init_task.c | 8 ++
> kernel/Makefile | 1 +
> kernel/fork.c | 9 ++
> kernel/sched/core.c | 2 +
> kernel/sched/sched.h | 1 +
> kernel/scs.c | 169 +++++++++++++++++++++++++++++++++
> 11 files changed, 293 insertions(+)
> create mode 100644 include/linux/scs.h
> create mode 100644 kernel/scs.c
>
> diff --git a/Makefile b/Makefile
> index 79be70bf2899..e6337314f8fb 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -846,6 +846,12 @@ ifdef CONFIG_LIVEPATCH
> KBUILD_CFLAGS += $(call cc-option, -flive-patching=inline-clone)
> endif
>
> +ifdef CONFIG_SHADOW_CALL_STACK
> +CC_FLAGS_SCS := -fsanitize=shadow-call-stack
> +KBUILD_CFLAGS += $(CC_FLAGS_SCS)
> +export CC_FLAGS_SCS
> +endif
> +
> # arch Makefile may override CC so keep this after arch Makefile is included
> NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 5f8a5d84dbbe..5e34cbcd8d6a 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -521,6 +521,39 @@ config STACKPROTECTOR_STRONG
> about 20% of all kernel functions, which increases the kernel code
> size by about 2%.
>
> +config ARCH_SUPPORTS_SHADOW_CALL_STACK
> + bool
> + help
> + An architecture should select this if it supports Clang's Shadow
> + Call Stack, has asm/scs.h, and implements runtime support for shadow
> + stack switching.
> +
> +config SHADOW_CALL_STACK_VMAP
> + bool
> + depends on SHADOW_CALL_STACK
> + help
> + Use virtually mapped shadow call stacks. Selecting this option
> + provides better stack exhaustion protection, but increases per-thread
> + memory consumption as a full page is allocated for each shadow stack.
> +
> +config SHADOW_CALL_STACK
> + bool "Clang Shadow Call Stack"
> + depends on ARCH_SUPPORTS_SHADOW_CALL_STACK
> + help
> + This option enables Clang's Shadow Call Stack, which uses a
> + shadow stack to protect function return addresses from being
> + overwritten by an attacker. More information can be found from
> + Clang's documentation:
> +
> + https://clang.llvm.org/docs/ShadowCallStack.html
> +
> + Note that security guarantees in the kernel differ from the ones
> + documented for user space. The kernel must store addresses of shadow
> + stacks used by other tasks and interrupt handlers in memory, which
> + means an attacker capable reading and writing arbitrary memory may
> + be able to locate them and hijack control flow by modifying shadow
> + stacks that are not currently in use.
> +
> config HAVE_ARCH_WITHIN_STACK_FRAMES
> bool
> help
> diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
> index 333a6695a918..18fc4d29ef27 100644
> --- a/include/linux/compiler-clang.h
> +++ b/include/linux/compiler-clang.h
> @@ -42,3 +42,9 @@
> * compilers, like ICC.
> */
> #define barrier() __asm__ __volatile__("" : : : "memory")
> +
> +#if __has_feature(shadow_call_stack)
> +# define __noscs __attribute__((__no_sanitize__("shadow-call-stack")))
> +#else
> +# define __noscs
> +#endif
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 72393a8c1a6c..be5d5be4b1ae 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -202,6 +202,10 @@ struct ftrace_likely_data {
> # define randomized_struct_fields_end
> #endif
>
> +#ifndef __noscs
> +# define __noscs
> +#endif
> +
> #ifndef asm_volatile_goto
> #define asm_volatile_goto(x...) asm goto(x)
> #endif
> diff --git a/include/linux/scs.h b/include/linux/scs.h
> new file mode 100644
> index 000000000000..0b70aff3846a
> --- /dev/null
> +++ b/include/linux/scs.h
> @@ -0,0 +1,54 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Shadow Call Stack support.
> + *
> + * Copyright (C) 2019 Google LLC
> + */
> +
> +#ifndef _LINUX_SCS_H
> +#define _LINUX_SCS_H
> +
> +#include <linux/gfp.h>
> +#include <linux/sched.h>
> +#include <asm/page.h>
> +
> +#ifdef CONFIG_SHADOW_CALL_STACK
> +
> +/*
> + * In testing, 1 KiB shadow stack size (i.e. 128 stack frames on a 64-bit
> + * architecture) provided ~40% safety margin on stack usage while keeping
> + * memory allocation overhead reasonable.
> + */
> +#define SCS_SIZE 1024
> +#define GFP_SCS (GFP_KERNEL | __GFP_ZERO)
> +
> +/* A random number to mark the end of the shadow stack. */
> +#define SCS_END_MAGIC 0xaf0194819b1635f6UL

Is 0xaf.... non-canonical for arm64? While "random", it should also
likely be an "impossible" value to find on the call stack.

Otherwise, all looks great. :)

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> +
> +#define task_scs(tsk) (task_thread_info(tsk)->shadow_call_stack)
> +
> +static inline void task_set_scs(struct task_struct *tsk, void *s)
> +{
> + task_scs(tsk) = s;
> +}
> +
> +extern void scs_init(void);
> +extern void scs_task_reset(struct task_struct *tsk);
> +extern int scs_prepare(struct task_struct *tsk, int node);
> +extern bool scs_corrupted(struct task_struct *tsk);
> +extern void scs_release(struct task_struct *tsk);
> +
> +#else /* CONFIG_SHADOW_CALL_STACK */
> +
> +#define task_scs(tsk) NULL
> +
> +static inline void task_set_scs(struct task_struct *tsk, void *s) {}
> +static inline void scs_init(void) {}
> +static inline void scs_task_reset(struct task_struct *tsk) {}
> +static inline int scs_prepare(struct task_struct *tsk, int node) { return 0; }
> +static inline bool scs_corrupted(struct task_struct *tsk) { return false; }
> +static inline void scs_release(struct task_struct *tsk) {}
> +
> +#endif /* CONFIG_SHADOW_CALL_STACK */
> +
> +#endif /* _LINUX_SCS_H */
> diff --git a/init/init_task.c b/init/init_task.c
> index 9e5cbe5eab7b..cbd40460e903 100644
> --- a/init/init_task.c
> +++ b/init/init_task.c
> @@ -11,6 +11,7 @@
> #include <linux/mm.h>
> #include <linux/audit.h>
> #include <linux/numa.h>
> +#include <linux/scs.h>
>
> #include <asm/pgtable.h>
> #include <linux/uaccess.h>
> @@ -184,6 +185,13 @@ struct task_struct init_task
> };
> EXPORT_SYMBOL(init_task);
>
> +#ifdef CONFIG_SHADOW_CALL_STACK
> +unsigned long init_shadow_call_stack[SCS_SIZE / sizeof(long)] __init_task_data
> + __aligned(SCS_SIZE) = {
> + [(SCS_SIZE / sizeof(long)) - 1] = SCS_END_MAGIC
> +};
> +#endif
> +
> /*
> * Initial thread structure. Alignment of this is handled by a special
> * linker map entry.
> diff --git a/kernel/Makefile b/kernel/Makefile
> index daad787fb795..313dbd44d576 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -102,6 +102,7 @@ obj-$(CONFIG_TRACEPOINTS) += trace/
> obj-$(CONFIG_IRQ_WORK) += irq_work.o
> obj-$(CONFIG_CPU_PM) += cpu_pm.o
> obj-$(CONFIG_BPF) += bpf/
> +obj-$(CONFIG_SHADOW_CALL_STACK) += scs.o
>
> obj-$(CONFIG_PERF_EVENTS) += events/
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index bcdf53125210..3fa7ba64c62d 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -94,6 +94,7 @@
> #include <linux/livepatch.h>
> #include <linux/thread_info.h>
> #include <linux/stackleak.h>
> +#include <linux/scs.h>
>
> #include <asm/pgtable.h>
> #include <asm/pgalloc.h>
> @@ -451,6 +452,8 @@ void put_task_stack(struct task_struct *tsk)
>
> void free_task(struct task_struct *tsk)
> {
> + scs_release(tsk);
> +
> #ifndef CONFIG_THREAD_INFO_IN_TASK
> /*
> * The task is finally done with both the stack and thread_info,
> @@ -834,6 +837,8 @@ void __init fork_init(void)
> NULL, free_vm_stack_cache);
> #endif
>
> + scs_init();
> +
> lockdep_init_task(&init_task);
> uprobes_init();
> }
> @@ -893,6 +898,10 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
> if (err)
> goto free_stack;
>
> + err = scs_prepare(tsk, node);
> + if (err)
> + goto free_stack;
> +
> #ifdef CONFIG_SECCOMP
> /*
> * We must handle setting up seccomp filters once we're under
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index dd05a378631a..e7faeb383008 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6013,6 +6013,8 @@ void init_idle(struct task_struct *idle, int cpu)
> raw_spin_lock_irqsave(&idle->pi_lock, flags);
> raw_spin_lock(&rq->lock);
>
> + scs_task_reset(idle);
> +
> __sched_fork(0, idle);
> idle->state = TASK_RUNNING;
> idle->se.exec_start = sched_clock();
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 0db2c1b3361e..c153003a011c 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -58,6 +58,7 @@
> #include <linux/profile.h>
> #include <linux/psi.h>
> #include <linux/rcupdate_wait.h>
> +#include <linux/scs.h>
> #include <linux/security.h>
> #include <linux/stop_machine.h>
> #include <linux/suspend.h>
> diff --git a/kernel/scs.c b/kernel/scs.c
> new file mode 100644
> index 000000000000..7c1a40020754
> --- /dev/null
> +++ b/kernel/scs.c
> @@ -0,0 +1,169 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Shadow Call Stack support.
> + *
> + * Copyright (C) 2019 Google LLC
> + */
> +
> +#include <linux/cpuhotplug.h>
> +#include <linux/mm.h>
> +#include <linux/mmzone.h>
> +#include <linux/scs.h>
> +#include <linux/slab.h>
> +#include <linux/vmalloc.h>
> +#include <asm/scs.h>
> +
> +static inline void *__scs_base(struct task_struct *tsk)
> +{
> + /*
> + * We allow architectures to use the shadow_call_stack field in
> + * struct thread_info to store the current shadow stack pointer
> + * during context switches.
> + *
> + * This allows the implementation to also clear the field when
> + * the task is active to avoid keeping pointers to the current
> + * task's shadow stack in memory. This can make it harder for an
> + * attacker to locate the shadow stack, but also requires us to
> + * compute the base address when needed.
> + *
> + * We assume the stack is aligned to SCS_SIZE.
> + */
> + return (void *)((uintptr_t)task_scs(tsk) & ~(SCS_SIZE - 1));
> +}
> +
> +#ifdef CONFIG_SHADOW_CALL_STACK_VMAP
> +
> +/* Keep a cache of shadow stacks */
> +#define SCS_CACHE_SIZE 2
> +static DEFINE_PER_CPU(void *, scs_cache[SCS_CACHE_SIZE]);
> +
> +static void *scs_alloc(int node)
> +{
> + int i;
> +
> + for (i = 0; i < SCS_CACHE_SIZE; i++) {
> + void *s;
> +
> + s = this_cpu_xchg(scs_cache[i], NULL);
> + if (s) {
> + memset(s, 0, SCS_SIZE);
> + return s;
> + }
> + }
> +
> + /*
> + * We allocate a full page for the shadow stack, which should be
> + * more than we need. Check the assumption nevertheless.
> + */
> + BUILD_BUG_ON(SCS_SIZE > PAGE_SIZE);
> +
> + return __vmalloc_node_range(PAGE_SIZE, SCS_SIZE,
> + VMALLOC_START, VMALLOC_END,
> + GFP_SCS, PAGE_KERNEL, 0,
> + node, __builtin_return_address(0));
> +}
> +
> +static void scs_free(void *s)
> +{
> + int i;
> +
> + for (i = 0; i < SCS_CACHE_SIZE; i++)
> + if (this_cpu_cmpxchg(scs_cache[i], 0, s) == 0)
> + return;
> +
> + vfree_atomic(s);
> +}
> +
> +static int scs_cleanup(unsigned int cpu)
> +{
> + int i;
> + void **cache = per_cpu_ptr(scs_cache, cpu);
> +
> + for (i = 0; i < SCS_CACHE_SIZE; i++) {
> + vfree(cache[i]);
> + cache[i] = NULL;
> + }
> +
> + return 0;
> +}
> +
> +void __init scs_init(void)
> +{
> + cpuhp_setup_state(CPUHP_BP_PREPARE_DYN, "scs:scs_cache", NULL,
> + scs_cleanup);
> +}
> +
> +#else /* !CONFIG_SHADOW_CALL_STACK_VMAP */
> +
> +static struct kmem_cache *scs_cache;
> +
> +static inline void *scs_alloc(int node)
> +{
> + return kmem_cache_alloc_node(scs_cache, GFP_SCS, node);
> +}
> +
> +static inline void scs_free(void *s)
> +{
> + kmem_cache_free(scs_cache, s);
> +}
> +
> +void __init scs_init(void)
> +{
> + scs_cache = kmem_cache_create("scs_cache", SCS_SIZE, SCS_SIZE,
> + 0, NULL);
> + WARN_ON(!scs_cache);
> +}
> +
> +#endif /* CONFIG_SHADOW_CALL_STACK_VMAP */
> +
> +static inline unsigned long *scs_magic(struct task_struct *tsk)
> +{
> + return (unsigned long *)(__scs_base(tsk) + SCS_SIZE) - 1;
> +}
> +
> +static inline void scs_set_magic(struct task_struct *tsk)
> +{
> + *scs_magic(tsk) = SCS_END_MAGIC;
> +}
> +
> +void scs_task_reset(struct task_struct *tsk)
> +{
> + /*
> + * Reset the shadow stack to the base address in case the task
> + * is reused.
> + */
> + task_set_scs(tsk, __scs_base(tsk));
> +}
> +
> +int scs_prepare(struct task_struct *tsk, int node)
> +{
> + void *s;
> +
> + s = scs_alloc(node);
> + if (!s)
> + return -ENOMEM;
> +
> + task_set_scs(tsk, s);
> + scs_set_magic(tsk);
> +
> + return 0;
> +}
> +
> +bool scs_corrupted(struct task_struct *tsk)
> +{
> + return *scs_magic(tsk) != SCS_END_MAGIC;
> +}
> +
> +void scs_release(struct task_struct *tsk)
> +{
> + void *s;
> +
> + s = __scs_base(tsk);
> + if (!s)
> + return;
> +
> + WARN_ON(scs_corrupted(tsk));
> +
> + task_set_scs(tsk, NULL);
> + scs_free(s);
> +}
> --
> 2.24.0.rc0.303.g954a862665-goog
>

--
Kees Cook