[PATCH v4 25/39] x86: Introduce userspace API for shadow stack

[Rick Edgecombe <rick.p.edgecombe@intel.com>]

From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

Add three new arch_prctl() handles:

 - ARCH_SHSTK_ENABLE/DISABLE enables or disables the specified
   feature. Returns 0 on success or an error.

 - ARCH_SHSTK_LOCK prevents future disabling or enabling of the
   specified feature. Returns 0 on success or an error

The features are handled per-thread and inherited over fork(2)/clone(2),
but reset on exec().

This is preparation patch. It does not implement any features.

Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
[tweaked with feedback from tglx]
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
---

v4:
 - Remove references to CET and replace with shadow stack (Peterz)

v3:
 - Move shstk.c Makefile changes earlier (Kees)
 - Add #ifdef around features_locked and features (Kees)
 - Encapsulate features reset earlier in reset_thread_features() so
   features and features_locked are not referenced in code that would be
   compiled !CONFIG_X86_USER_SHADOW_STACK. (Kees)
 - Fix typo in commit log (Kees)
 - Switch arch_prctl() numbers to avoid conflict with LAM

v2:
 - Only allow one enable/disable per call (tglx)
 - Return error code like a normal arch_prctl() (Alexander Potapenko)
 - Make CET only (tglx)

 arch/x86/include/asm/processor.h  |  5 ++++
 arch/x86/include/asm/shstk.h      | 21 +++++++++++++++
 arch/x86/include/uapi/asm/prctl.h |  6 +++++
 arch/x86/kernel/Makefile          |  2 ++
 arch/x86/kernel/process_64.c      |  6 +++++
 arch/x86/kernel/shstk.c           | 44 +++++++++++++++++++++++++++++++
 6 files changed, 84 insertions(+)
 create mode 100644 arch/x86/include/asm/shstk.h
 create mode 100644 arch/x86/kernel/shstk.c

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 4e35c66edeb7..ff1c0b1aca8c 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -475,6 +475,11 @@ struct thread_struct {
 	 */
 	u32			pkru;
 
+#ifdef CONFIG_X86_USER_SHADOW_STACK
+	unsigned long		features;
+	unsigned long		features_locked;
+#endif
+
 	/* Floating point and extended processor state */
 	struct fpu		fpu;
 	/*
diff --git a/arch/x86/include/asm/shstk.h b/arch/x86/include/asm/shstk.h
new file mode 100644
index 000000000000..58f9ee675be0
--- /dev/null
+++ b/arch/x86/include/asm/shstk.h
@@ -0,0 +1,21 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_SHSTK_H
+#define _ASM_X86_SHSTK_H
+
+#ifndef __ASSEMBLY__
+#include <linux/types.h>
+
+struct task_struct;
+
+#ifdef CONFIG_X86_USER_SHADOW_STACK
+long shstk_prctl(struct task_struct *task, int option, unsigned long features);
+void reset_thread_features(void);
+#else
+static inline long shstk_prctl(struct task_struct *task, int option,
+			     unsigned long features) { return -EINVAL; }
+static inline void reset_thread_features(void) {}
+#endif /* CONFIG_X86_USER_SHADOW_STACK */
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* _ASM_X86_SHSTK_H */
diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index eb290d89cb32..8b427aea2345 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -20,9 +20,15 @@
 #define ARCH_MAP_VDSO_32		0x2002
 #define ARCH_MAP_VDSO_64		0x2003
 
+/* Don't use 0x3001-0x3004 because of old glibcs */
+
 #define ARCH_GET_UNTAG_MASK		0x4001
 #define ARCH_ENABLE_TAGGED_ADDR		0x4002
 #define ARCH_GET_MAX_TAG_BITS		0x4003
 #define ARCH_FORCE_TAGGED_SVA		0x4004
 
+#define ARCH_SHSTK_ENABLE		0x5001
+#define ARCH_SHSTK_DISABLE		0x5002
+#define ARCH_SHSTK_LOCK			0x5003
+
 #endif /* _ASM_X86_PRCTL_H */
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 96d51bbc2bd4..2260891609b4 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -145,6 +145,8 @@ obj-$(CONFIG_CFI_CLANG)			+= cfi.o
 
 obj-$(CONFIG_CALL_THUNKS)		+= callthunks.o
 
+obj-$(CONFIG_X86_USER_SHADOW_STACK)	+= shstk.o
+
 ###
 # 64 bit specific files
 ifeq ($(CONFIG_X86_64),y)
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 084ec467dbb1..4ddd7d9209e1 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -514,6 +514,8 @@ start_thread_common(struct pt_regs *regs, unsigned long new_ip,
 		load_gs_index(__USER_DS);
 	}
 
+	reset_thread_features();
+
 	loadsegment(fs, 0);
 	loadsegment(es, _ds);
 	loadsegment(ds, _ds);
@@ -916,6 +918,10 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
 			return put_user(0, (unsigned long __user *)arg2);
 		else
 			return put_user(LAM_U57_BITS, (unsigned long __user *)arg2);
+	case ARCH_SHSTK_ENABLE:
+	case ARCH_SHSTK_DISABLE:
+	case ARCH_SHSTK_LOCK:
+		return shstk_prctl(task, option, arg2);
 	default:
 		ret = -EINVAL;
 		break;
diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
new file mode 100644
index 000000000000..41ed6552e0a5
--- /dev/null
+++ b/arch/x86/kernel/shstk.c
@@ -0,0 +1,44 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * shstk.c - Intel shadow stack support
+ *
+ * Copyright (c) 2021, Intel Corporation.
+ * Yu-cheng Yu <yu-cheng.yu@intel.com>
+ */
+
+#include <linux/sched.h>
+#include <linux/bitops.h>
+#include <asm/prctl.h>
+
+void reset_thread_features(void)
+{
+	current->thread.features = 0;
+	current->thread.features_locked = 0;
+}
+
+long shstk_prctl(struct task_struct *task, int option, unsigned long features)
+{
+	if (option == ARCH_SHSTK_LOCK) {
+		task->thread.features_locked |= features;
+		return 0;
+	}
+
+	/* Don't allow via ptrace */
+	if (task != current)
+		return -EINVAL;
+
+	/* Do not allow to change locked features */
+	if (features & task->thread.features_locked)
+		return -EPERM;
+
+	/* Only support enabling/disabling one feature at a time. */
+	if (hweight_long(features) > 1)
+		return -EINVAL;
+
+	if (option == ARCH_SHSTK_DISABLE) {
+		return -EINVAL;
+	}
+
+	/* Handle ARCH_SHSTK_ENABLE */
+	return -EINVAL;
+}
-- 
2.17.1