[tip: x86/shstk] x86: Introduce userspace API for shadow stack

From: tip-bot2 for Rick Edgecombe
Date: Fri Jun 16 2023 - 15:17:57 EST

The following commit has been merged into the x86/shstk branch of tip:

Commit-ID: c4610d713f17879a2f3d70a72f793557489fa2d4
Gitweb: https://git.kernel.org/tip/c4610d713f17879a2f3d70a72f793557489fa2d4
Author: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
AuthorDate: Mon, 12 Jun 2023 17:10:52 -07:00
Committer: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
CommitterDate: Thu, 15 Jun 2023 16:31:34 -07:00

x86: Introduce userspace API for shadow stack

Add three new arch_prctl() handles:

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

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

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

Co-developed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Reviewed-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Acked-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
Tested-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
Tested-by: John Allen <john.allen@xxxxxxx>
Tested-by: Kees Cook <keescook@xxxxxxxxxxxx>
Link: https://lore.kernel.org/all/20230613001108.3040476-27-rick.p.edgecombe%40intel.com
---
arch/x86/include/asm/processor.h | 6 ++++-
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, 85 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 a1e4fa5..407d555 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -28,6 +28,7 @@ struct vm86;
#include <asm/unwind_hints.h>
#include <asm/vmxfeatures.h>
#include <asm/vdso/processor.h>
+#include <asm/shstk.h>

#include <linux/personality.h>
#include <linux/cache.h>
@@ -475,6 +476,11 @@ 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 0000000..ec75380
--- /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 arg2) { 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 e8d7ebb..1cd44ec 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -23,9 +23,15 @@
#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 abee056..6b6bf47 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -147,6 +147,8 @@ obj-$(CONFIG_CALL_THUNKS) += callthunks.o

obj-$(CONFIG_X86_CET) += cet.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 3d181c1..0f89aa0 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -515,6 +515,8 @@ 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);
@@ -894,6 +896,10 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
else
return put_user(LAM_U57_BITS, (unsigned long __user *)arg2);
#endif
+ 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 0000000..41ed655
--- /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@xxxxxxxxx>
+ */
+
+#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;
+}