Re: [RFC PATCH v9 17/27] x86/cet/shstk: User-mode Shadow Stack support

Kees Cook
2020-02-25T21:07:51Z

On Wed, Feb 05, 2020 at 10:19:25AM -0800, Yu-cheng Yu wrote:
> This patch adds basic Shadow Stack (SHSTK) enabling/disabling routines.
> A task's SHSTK is allocated from memory with VM_SHSTK flag and read-only
> protection. It has a fixed size of RLIMIT_STACK.
>
> v9:
> - Change cpu_feature_enabled() to static_cpu_has().
> - Merge cet_disable_shstk to cet_disable_free_shstk.
> - Remove the empty slot at the top of the SHSTK, as it is not needed.
> - Move do_mmap_locked() to alloc_shstk(), which is a static function.
>
> v6:
> - Create a function do_mmap_locked() for SHSTK allocation.
>
> v2:
> - Change noshstk to no_cet_shstk.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> ---
> arch/x86/include/asm/cet.h | 31 +++++
> arch/x86/include/asm/disabled-features.h | 8 +-
> arch/x86/include/asm/processor.h | 5 +
> arch/x86/kernel/Makefile | 2 +
> arch/x86/kernel/cet.c | 121 ++++++++++++++++++
> arch/x86/kernel/cpu/common.c | 25 ++++
> arch/x86/kernel/process.c | 1 +
> .../arch/x86/include/asm/disabled-features.h | 8 +-
> 8 files changed, 199 insertions(+), 2 deletions(-)
> create mode 100644 arch/x86/include/asm/cet.h
> create mode 100644 arch/x86/kernel/cet.c
>
> diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
> new file mode 100644
> index 000000000000..c44c991ca91f
> --- /dev/null
> +++ b/arch/x86/include/asm/cet.h
> @@ -0,0 +1,31 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_X86_CET_H
> +#define _ASM_X86_CET_H
> +
> +#ifndef __ASSEMBLY__
> +#include <linux/types.h>
> +
> +struct task_struct;
> +/*
> + * Per-thread CET status
> + */
> +struct cet_status {
> + unsigned long shstk_base;
> + unsigned long shstk_size;
> + unsigned int shstk_enabled:1;
> +};
> +
> +#ifdef CONFIG_X86_INTEL_CET
> +int cet_setup_shstk(void);
> +void cet_disable_free_shstk(struct task_struct *p);
> +#else
> +static inline void cet_disable_free_shstk(struct task_struct *p) {}
> +#endif
> +
> +#define cpu_x86_cet_enabled() \
> + (static_cpu_has(X86_FEATURE_SHSTK) || \
> + static_cpu_has(X86_FEATURE_IBT))
> +
> +#endif /* __ASSEMBLY__ */
> +
> +#endif /* _ASM_X86_CET_H */
> diff --git a/arch/x86/include/asm/disabled-features.h b/arch/x86/include/asm/disabled-features.h
> index 8e1d0bb46361..e1454509ad83 100644
> --- a/arch/x86/include/asm/disabled-features.h
> +++ b/arch/x86/include/asm/disabled-features.h
> @@ -62,6 +62,12 @@
> # define DISABLE_PTI (1 << (X86_FEATURE_PTI & 31))
> #endif
>
> +#ifdef CONFIG_X86_INTEL_SHADOW_STACK_USER
> +#define DISABLE_SHSTK 0
> +#else
> +#define DISABLE_SHSTK (1<<(X86_FEATURE_SHSTK & 31))
> +#endif
> +
> /*
> * Make sure to add features to the correct mask
> */
> @@ -81,7 +87,7 @@
> #define DISABLED_MASK13 0
> #define DISABLED_MASK14 0
> #define DISABLED_MASK15 0
> -#define DISABLED_MASK16 (DISABLE_PKU|DISABLE_OSPKE|DISABLE_LA57|DISABLE_UMIP)
> +#define DISABLED_MASK16 (DISABLE_PKU|DISABLE_OSPKE|DISABLE_LA57|DISABLE_UMIP|DISABLE_SHSTK)
> #define DISABLED_MASK17 0
> #define DISABLED_MASK18 0
> #define DISABLED_MASK_CHECK BUILD_BUG_ON_ZERO(NCAPINTS != 19)
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index 0340aad3f2fc..793d210e64da 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -25,6 +25,7 @@ struct vm86;
> #include <asm/special_insns.h>
> #include <asm/fpu/types.h>
> #include <asm/unwind_hints.h>
> +#include <asm/cet.h>
>
> #include <linux/personality.h>
> #include <linux/cache.h>
> @@ -539,6 +540,10 @@ struct thread_struct {
> unsigned int sig_on_uaccess_err:1;
> unsigned int uaccess_err:1; /* uaccess failed */
>
> +#ifdef CONFIG_X86_INTEL_CET
> + struct cet_status cet;
> +#endif
> +
> /* Floating point and extended processor state */
> struct fpu fpu;
> /*
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index 6175e370ee4a..b8c1ea4ab7eb 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -142,6 +142,8 @@ obj-$(CONFIG_UNWINDER_ORC) += unwind_orc.o
> obj-$(CONFIG_UNWINDER_FRAME_POINTER) += unwind_frame.o
> obj-$(CONFIG_UNWINDER_GUESS) += unwind_guess.o
>
> +obj-$(CONFIG_X86_INTEL_CET) += cet.o
> +
> ###
> # 64 bit specific files
> ifeq ($(CONFIG_X86_64),y)
> diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
> new file mode 100644
> index 000000000000..b4c7d88e9a8f
> --- /dev/null
> +++ b/arch/x86/kernel/cet.c
> @@ -0,0 +1,121 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * cet.c - Control-flow Enforcement (CET)
> + *
> + * Copyright (c) 2019, Intel Corporation.
> + * Yu-cheng Yu <yu-cheng.yu@intel.com>
> + */
> +
> +#include <linux/types.h>
> +#include <linux/mm.h>
> +#include <linux/mman.h>
> +#include <linux/slab.h>
> +#include <linux/uaccess.h>
> +#include <linux/sched/signal.h>
> +#include <linux/compat.h>
> +#include <asm/msr.h>
> +#include <asm/user.h>
> +#include <asm/fpu/internal.h>
> +#include <asm/fpu/xstate.h>
> +#include <asm/fpu/types.h>
> +#include <asm/cet.h>
> +
> +static void start_update_msrs(void)
> +{
> + fpregs_lock();
> + if (test_thread_flag(TIF_NEED_FPU_LOAD))
> + __fpregs_load_activate();
> +}
> +
> +static void end_update_msrs(void)
> +{
> + fpregs_unlock();
> +}
> +
> +static unsigned long cet_get_shstk_addr(void)
> +{
> + struct fpu *fpu = &current->thread.fpu;
> + unsigned long ssp = 0;
> +
> + fpregs_lock();
> +
> + if (fpregs_state_valid(fpu, smp_processor_id())) {
> + rdmsrl(MSR_IA32_PL3_SSP, ssp);
> + } else {
> + struct cet_user_state *p;
> +
> + p = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER);
> + if (p)
> + ssp = p->user_ssp;
> + }
> +
> + fpregs_unlock();
> + return ssp;
> +}
> +
> +static unsigned long alloc_shstk(unsigned long size)
> +{
> + struct mm_struct *mm = current->mm;
> + unsigned long addr, populate;
> +
> + down_write(&mm->mmap_sem);
> + addr = do_mmap(NULL, 0, size, PROT_READ, MAP_ANONYMOUS | MAP_PRIVATE,
> + VM_SHSTK, 0, &populate, NULL);
> + up_write(&mm->mmap_sem);
> +
> + if (populate)
> + mm_populate(addr, populate);
> +
> + return addr;
> +}
> +
> +int cet_setup_shstk(void)
> +{
> + unsigned long addr, size;
> + struct cet_status *cet = &current->thread.cet;
> +
> + if (!static_cpu_has(X86_FEATURE_SHSTK))
> + return -EOPNOTSUPP;
> +
> + size = rlimit(RLIMIT_STACK);
> + addr = alloc_shstk(size);
> +
> + if (IS_ERR((void *)addr))
> + return PTR_ERR((void *)addr);
> +
> + cet->shstk_base = addr;
> + cet->shstk_size = size;
> + cet->shstk_enabled = 1;
> +
> + start_update_msrs();
> + wrmsrl(MSR_IA32_PL3_SSP, addr + size);
> + wrmsrl(MSR_IA32_U_CET, MSR_IA32_CET_SHSTK_EN);
> + end_update_msrs();
> + return 0;
> +}
> +
> +void cet_disable_free_shstk(struct task_struct *tsk)
> +{
> + struct cet_status *cet = &tsk->thread.cet;
> +
> + if (!static_cpu_has(X86_FEATURE_SHSTK) ||
> + !cet->shstk_enabled || !cet->shstk_base)
> + return;
> +
> + if (!tsk->mm || (tsk->mm != current->mm))
> + return;
> +
> + if (tsk == current) {
> + u64 msr_val;
> +
> + start_update_msrs();
> + rdmsrl(MSR_IA32_U_CET, msr_val);
> + wrmsrl(MSR_IA32_U_CET, msr_val & ~MSR_IA32_CET_SHSTK_EN);
> + end_update_msrs();
> + }
> +
> + vm_munmap(cet->shstk_base, cet->shstk_size);
> + cet->shstk_base = 0;
> + cet->shstk_size = 0;
> + cet->shstk_enabled = 0;
> +}
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 2e4d90294fe6..40498ec72fda 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -54,6 +54,7 @@
> #include <asm/microcode_intel.h>
> #include <asm/intel-family.h>
> #include <asm/cpu_device_id.h>
> +#include <asm/cet.h>
> #include <asm/uv/uv.h>
>
> #include "cpu.h"
> @@ -486,6 +487,29 @@ static __init int setup_disable_pku(char *arg)
> __setup("nopku", setup_disable_pku);
> #endif /* CONFIG_X86_64 */
>
> +static __always_inline void setup_cet(struct cpuinfo_x86 *c)
> +{
> + if (cpu_x86_cet_enabled())
> + cr4_set_bits(X86_CR4_CET);
> +}
> +
> +#ifdef CONFIG_X86_INTEL_SHADOW_STACK_USER
> +static __init int setup_disable_shstk(char *s)
> +{
> + /* require an exact match without trailing characters */
> + if (s[0] != '\0')
> + return 0;
> +
> + if (!boot_cpu_has(X86_FEATURE_SHSTK))
> + return 1;
> +
> + setup_clear_cpu_cap(X86_FEATURE_SHSTK);
> + pr_info("x86: 'no_cet_shstk' specified, disabling Shadow Stack\n");
> + return 1;
> +}
> +__setup("no_cet_shstk", setup_disable_shstk);
> +#endif

I wonder if this should be "cet_shstk=..." instead? Will it always be a
giant knob like this? Will we want to disable it for userspace but keep
it for kernel space, etc?

> +
> /*
> * Some CPU features depend on higher CPUID levels, which may not always
> * be available due to CPUID level capping or broken virtualization
> @@ -1510,6 +1534,7 @@ static void identify_cpu(struct cpuinfo_x86 *c)
> x86_init_rdrand(c);
> x86_init_cache_qos(c);
> setup_pku(c);
> + setup_cet(c);
>
> /*
> * Clear/Set all flags overridden by options, need do it
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index 8d0b9442202e..e102e63de641 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -43,6 +43,7 @@
> #include <asm/spec-ctrl.h>
> #include <asm/io_bitmap.h>
> #include <asm/proto.h>
> +#include <asm/cet.h>
>
> #include "process.h"
>
> diff --git a/tools/arch/x86/include/asm/disabled-features.h b/tools/arch/x86/include/asm/disabled-features.h
> index 8e1d0bb46361..e1454509ad83 100644
> --- a/tools/arch/x86/include/asm/disabled-features.h
> +++ b/tools/arch/x86/include/asm/disabled-features.h
> @@ -62,6 +62,12 @@
> # define DISABLE_PTI (1 << (X86_FEATURE_PTI & 31))
> #endif
>
> +#ifdef CONFIG_X86_INTEL_SHADOW_STACK_USER
> +#define DISABLE_SHSTK 0
> +#else
> +#define DISABLE_SHSTK (1<<(X86_FEATURE_SHSTK & 31))
> +#endif
> +
> /*
> * Make sure to add features to the correct mask
> */
> @@ -81,7 +87,7 @@
> #define DISABLED_MASK13 0
> #define DISABLED_MASK14 0
> #define DISABLED_MASK15 0
> -#define DISABLED_MASK16 (DISABLE_PKU|DISABLE_OSPKE|DISABLE_LA57|DISABLE_UMIP)
> +#define DISABLED_MASK16 (DISABLE_PKU|DISABLE_OSPKE|DISABLE_LA57|DISABLE_UMIP|DISABLE_SHSTK)
> #define DISABLED_MASK17 0
> #define DISABLED_MASK18 0
> #define DISABLED_MASK_CHECK BUILD_BUG_ON_ZERO(NCAPINTS != 19)
> --
> 2.21.0
>
>

Otherwise, looks good to me. :)

--
Kees Cook