[PATCH v2 23/39] x86: Introduce userspace API for CET enabling

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

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

Add three new arch_prctl() handles:

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

 - ARCH_CET_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 impelement any features.

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>

---

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/cet.h        | 20 ++++++++++++++++
 arch/x86/include/asm/processor.h  |  3 +++
 arch/x86/include/uapi/asm/prctl.h |  6 +++++
 arch/x86/kernel/process.c         |  4 ++++
 arch/x86/kernel/process_64.c      |  5 +++-
 arch/x86/kernel/shstk.c           | 38 +++++++++++++++++++++++++++++++
 6 files changed, 75 insertions(+), 1 deletion(-)
 create mode 100644 arch/x86/include/asm/cet.h
 create mode 100644 arch/x86/kernel/shstk.c

diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
new file mode 100644
index 000000000000..0fa4dbc98c49
--- /dev/null
+++ b/arch/x86/include/asm/cet.h
@@ -0,0 +1,20 @@
+/* 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;
+
+#ifdef CONFIG_X86_SHADOW_STACK
+long cet_prctl(struct task_struct *task, int option,
+		      unsigned long features);
+#else
+static inline long cet_prctl(struct task_struct *task, int option,
+		      unsigned long features) { return -EINVAL; }
+#endif /* CONFIG_X86_SHADOW_STACK */
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* _ASM_X86_CET_H */
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 356308c73951..a92bf76edafe 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -530,6 +530,9 @@ struct thread_struct {
 	 */
 	u32			pkru;
 
+	unsigned long		features;
+	unsigned long		features_locked;
+
 	/* Floating point and extended processor state */
 	struct fpu		fpu;
 	/*
diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index 500b96e71f18..028158e35269 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -20,4 +20,10 @@
 #define ARCH_MAP_VDSO_32		0x2002
 #define ARCH_MAP_VDSO_64		0x2003
 
+/* Don't use 0x3001-0x3004 because of old glibcs */
+
+#define ARCH_CET_ENABLE			0x4001
+#define ARCH_CET_DISABLE		0x4002
+#define ARCH_CET_LOCK			0x4003
+
 #endif /* _ASM_X86_PRCTL_H */
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 58a6ea472db9..034880311e6b 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -367,6 +367,10 @@ void arch_setup_new_exec(void)
 		task_clear_spec_ssb_noexec(current);
 		speculation_ctrl_update(read_thread_flags());
 	}
+
+	/* Reset thread features on exec */
+	current->thread.features = 0;
+	current->thread.features_locked = 0;
 }
 
 #ifdef CONFIG_X86_IOPL_IOPERM
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 1962008fe743..8fa2c2b7de65 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -829,7 +829,10 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
 	case ARCH_MAP_VDSO_64:
 		return prctl_map_vdso(&vdso_image_64, arg2);
 #endif
-
+	case ARCH_CET_ENABLE:
+	case ARCH_CET_DISABLE:
+	case ARCH_CET_LOCK:
+		return cet_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..e3276ac9e9b9
--- /dev/null
+++ b/arch/x86/kernel/shstk.c
@@ -0,0 +1,38 @@
+// 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>
+
+long cet_prctl(struct task_struct *task, int option, unsigned long features)
+{
+	if (option == ARCH_CET_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_CET_DISABLE) {
+		return -EINVAL;
+	}
+
+	/* Handle ARCH_CET_ENABLE */
+	return -EINVAL;
+}
-- 
2.17.1