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Subject[PATCH v5 20/27] x86/cet/shstk: User-mode shadow stack support
Date
This patch adds basic shadow stack enabling/disabling routines.
A task's shadow stack is allocated from memory with VM_SHSTK
flag set and read-only protection. The shadow stack is
allocated to a fixed size of RLIMIT_STACK.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
---
arch/x86/include/asm/cet.h | 34 +++++++
arch/x86/include/asm/disabled-features.h | 8 +-
arch/x86/include/asm/msr-index.h | 14 +++
arch/x86/include/asm/processor.h | 5 ++
arch/x86/kernel/Makefile | 2 +
arch/x86/kernel/cet.c | 109 +++++++++++++++++++++++
arch/x86/kernel/cpu/common.c | 25 ++++++
arch/x86/kernel/process.c | 2 +
8 files changed, 198 insertions(+), 1 deletion(-)
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..c952a2ec65fe
--- /dev/null
+++ b/arch/x86/include/asm/cet.h
@@ -0,0 +1,34 @@
+/* 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_shstk(void);
+void cet_disable_free_shstk(struct task_struct *p);
+#else
+static inline int cet_setup_shstk(void) { return -EINVAL; }
+static inline void cet_disable_shstk(void) {}
+static inline void cet_disable_free_shstk(struct task_struct *p) {}
+#endif
+
+#define cpu_x86_cet_enabled() \
+ (cpu_feature_enabled(X86_FEATURE_SHSTK) || \
+ cpu_feature_enabled(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 33833d1909af..3624a11e5ba6 100644
--- a/arch/x86/include/asm/disabled-features.h
+++ b/arch/x86/include/asm/disabled-features.h
@@ -56,6 +56,12 @@
# 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
*/
@@ -75,7 +81,7 @@
#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/msr-index.h b/arch/x86/include/asm/msr-index.h
index 4731f0cf97c5..e073801a44e0 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -777,4 +777,18 @@
#define MSR_VM_IGNNE 0xc0010115
#define MSR_VM_HSAVE_PA 0xc0010117

+/* Control-flow Enforcement Technology MSRs */
+#define MSR_IA32_U_CET 0x6a0 /* user mode cet setting */
+#define MSR_IA32_S_CET 0x6a2 /* kernel mode cet setting */
+#define MSR_IA32_PL0_SSP 0x6a4 /* kernel shstk pointer */
+#define MSR_IA32_PL3_SSP 0x6a7 /* user shstk pointer */
+#define MSR_IA32_INT_SSP_TAB 0x6a8 /* exception shstk table */
+
+/* MSR_IA32_U_CET and MSR_IA32_S_CET bits */
+#define MSR_IA32_CET_SHSTK_EN 0x0000000000000001ULL
+#define MSR_IA32_CET_WRSS_EN 0x0000000000000002ULL
+#define MSR_IA32_CET_ENDBR_EN 0x0000000000000004ULL
+#define MSR_IA32_CET_LEG_IW_EN 0x0000000000000008ULL
+#define MSR_IA32_CET_NO_TRACK_EN 0x0000000000000010ULL
+
#endif /* _ASM_X86_MSR_INDEX_H */
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index d53c54b842da..63918cecf367 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -24,6 +24,7 @@ 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>
@@ -505,6 +506,10 @@ 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 8824d01c0c35..fbb2d91fb756 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -139,6 +139,8 @@ 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..ec256ae27a31
--- /dev/null
+++ b/arch/x86/kernel/cet.c
@@ -0,0 +1,109 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * cet.c - Control Flow Enforcement (CET)
+ *
+ * Copyright (c) 2018, 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 <asm/msr.h>
+#include <asm/user.h>
+#include <asm/fpu/xstate.h>
+#include <asm/fpu/types.h>
+#include <asm/compat.h>
+#include <asm/cet.h>
+
+static int set_shstk_ptr(unsigned long addr)
+{
+ u64 r;
+
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+ return -1;
+
+ if ((addr >= TASK_SIZE_MAX) || (!IS_ALIGNED(addr, 4)))
+ return -1;
+
+ rdmsrl(MSR_IA32_U_CET, r);
+ wrmsrl(MSR_IA32_PL3_SSP, addr);
+ wrmsrl(MSR_IA32_U_CET, r | MSR_IA32_CET_SHSTK_EN);
+ return 0;
+}
+
+static unsigned long get_shstk_addr(void)
+{
+ unsigned long ptr;
+
+ if (!current->thread.cet.shstk_enabled)
+ return 0;
+
+ rdmsrl(MSR_IA32_PL3_SSP, ptr);
+ return ptr;
+}
+
+int cet_setup_shstk(void)
+{
+ unsigned long addr, size;
+
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+ return -EOPNOTSUPP;
+
+ size = rlimit(RLIMIT_STACK);
+ addr = do_mmap_locked(0, size, PROT_READ,
+ MAP_ANONYMOUS | MAP_PRIVATE, VM_SHSTK);
+
+ /*
+ * Return actual error from do_mmap().
+ */
+ if (addr >= TASK_SIZE_MAX)
+ return addr;
+
+ set_shstk_ptr(addr + size - sizeof(u64));
+ current->thread.cet.shstk_base = addr;
+ current->thread.cet.shstk_size = size;
+ current->thread.cet.shstk_enabled = 1;
+ return 0;
+}
+
+void cet_disable_shstk(void)
+{
+ u64 r;
+
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+ return;
+
+ rdmsrl(MSR_IA32_U_CET, r);
+ r &= ~(MSR_IA32_CET_SHSTK_EN);
+ wrmsrl(MSR_IA32_U_CET, r);
+ wrmsrl(MSR_IA32_PL3_SSP, 0);
+ current->thread.cet.shstk_enabled = 0;
+}
+
+void cet_disable_free_shstk(struct task_struct *tsk)
+{
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK) ||
+ !tsk->thread.cet.shstk_enabled)
+ return;
+
+ if (tsk == current)
+ cet_disable_shstk();
+
+ /*
+ * Free only when tsk is current or shares mm
+ * with current but has its own shstk.
+ */
+ if (tsk->mm && (tsk->mm == current->mm) &&
+ (tsk->thread.cet.shstk_base)) {
+ vm_munmap(tsk->thread.cet.shstk_base,
+ tsk->thread.cet.shstk_size);
+ tsk->thread.cet.shstk_base = 0;
+ tsk->thread.cet.shstk_size = 0;
+ }
+
+ tsk->thread.cet.shstk_enabled = 0;
+}
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 44c4ef3d989b..c3960326b67f 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -52,6 +52,7 @@
#include <asm/microcode_intel.h>
#include <asm/intel-family.h>
#include <asm/cpu_device_id.h>
+#include <asm/cet.h>

#ifdef CONFIG_X86_LOCAL_APIC
#include <asm/uv/uv.h>
@@ -411,6 +412,29 @@ 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
+
/*
* Some CPU features depend on higher CPUID levels, which may not always
* be available due to CPUID level capping or broken virtualization
@@ -1376,6 +1400,7 @@ 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 c93fcfdf1673..4a776da4c28c 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -39,6 +39,7 @@
#include <asm/desc.h>
#include <asm/prctl.h>
#include <asm/spec-ctrl.h>
+#include <asm/cet.h>

/*
* per-CPU TSS segments. Threads are completely 'soft' on Linux,
@@ -134,6 +135,7 @@ void flush_thread(void)
flush_ptrace_hw_breakpoint(tsk);
memset(tsk->thread.tls_array, 0, sizeof(tsk->thread.tls_array));

+ cet_disable_shstk();
fpu__clear(&tsk->thread.fpu);
}

--
2.17.1
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