[RFCv2 03/10] x86: Introduce userspace API to handle per-thread features

[Kirill A. Shutemov]

Add three new arch_prctl() handles:

 - ARCH_THREAD_FEATURE_ENABLE/DISABLE enables or disables the specified
   features. Returns what features are enabled after the operation.

 - ARCH_THREAD_FEATURE_LOCK prevents future disabling or enabling of the
   specified features. Returns the new set of locked features.

The features 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@xxxxxxxxxxxxxxx>
---
 arch/x86/include/asm/processor.h  |  3 +++
 arch/x86/include/uapi/asm/prctl.h |  5 +++++
 arch/x86/kernel/process.c         | 37 +++++++++++++++++++++++++++++++
 3 files changed, 45 insertions(+)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 91d0f93a00c7..ff0c34e18cc6 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..67fc30d36c73 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -20,4 +20,9 @@
 #define ARCH_MAP_VDSO_32		0x2002
 #define ARCH_MAP_VDSO_64		0x2003
 
+/* Never implement 0x3001, it will confuse old glibc's */
+#define ARCH_THREAD_FEATURE_ENABLE	0x3002
+#define ARCH_THREAD_FEATURE_DISABLE	0x3003
+#define ARCH_THREAD_FEATURE_LOCK	0x3004
+
 #endif /* _ASM_X86_PRCTL_H */
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index b370767f5b19..cb8fc28f2eae 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
@@ -985,6 +989,35 @@ unsigned long __get_wchan(struct task_struct *p)
 	return addr;
 }
 
+static long thread_feature_prctl(struct task_struct *task, int option,
+				 unsigned long features)
+{
+	const unsigned long known_features = 0;
+
+	if (features & ~known_features)
+		return -EINVAL;
+
+	if (option == ARCH_THREAD_FEATURE_LOCK) {
+		task->thread.features_locked |= features;
+		return task->thread.features_locked;
+	}
+
+	/* Do not allow to change locked features */
+	if (features & task->thread.features_locked)
+		return -EPERM;
+
+	if (option == ARCH_THREAD_FEATURE_DISABLE) {
+		task->thread.features &= ~features;
+		goto out;
+	}
+
+	/* Handle ARCH_THREAD_FEATURE_ENABLE */
+
+	task->thread.features |= features;
+out:
+	return task->thread.features;
+}
+
 long do_arch_prctl_common(struct task_struct *task, int option,
 			  unsigned long arg2)
 {
@@ -999,6 +1032,10 @@ long do_arch_prctl_common(struct task_struct *task, int option,
 	case ARCH_GET_XCOMP_GUEST_PERM:
 	case ARCH_REQ_XCOMP_GUEST_PERM:
 		return fpu_xstate_prctl(task, option, arg2);
+	case ARCH_THREAD_FEATURE_ENABLE:
+	case ARCH_THREAD_FEATURE_DISABLE:
+	case ARCH_THREAD_FEATURE_LOCK:
+		return thread_feature_prctl(task, option, arg2);
 	}
 
 	return -EINVAL;
-- 
2.35.1