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

[Yu-cheng Yu]

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@xxxxxxxxx>
---
 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@xxxxxxxxx>
+ */
+
+#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
+
 /*
  * 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