[PATCH v7 2/3] prctl: Hook L1D flushing in via prctl

[Balbir Singh <sblbir@amazon.com>]

Use the existing PR_GET/SET_SPECULATION_CTRL API to expose the L1D
flush capability. For L1D flushing PR_SPEC_FORCE_DISABLE and
PR_SPEC_DISABLE_NOEXEC are not supported.

There is also no seccomp integration for the feature.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Balbir Singh <sblbir@amazon.com>
---
 arch/x86/kernel/cpu/bugs.c | 28 ++++++++++++++++++++++++++++
 include/uapi/linux/prctl.h |  1 +
 2 files changed, 29 insertions(+)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index ed54b3b21c39..3eb9139fcf50 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1121,6 +1121,19 @@ static void task_update_spec_tif(struct task_struct *tsk)
 		speculation_ctrl_update_current();
 }
 
+static int l1d_flush_out_prctl_set(struct task_struct *task, unsigned long ctrl)
+{
+	switch (ctrl) {
+	case PR_SPEC_ENABLE:
+		return enable_l1d_flush_for_task(task);
+	case PR_SPEC_DISABLE:
+		return disable_l1d_flush_for_task(task);
+	default:
+		return -ERANGE;
+	}
+	return 0;
+}
+
 static int ssb_prctl_set(struct task_struct *task, unsigned long ctrl)
 {
 	if (ssb_mode != SPEC_STORE_BYPASS_PRCTL &&
@@ -1206,6 +1219,8 @@ int arch_prctl_spec_ctrl_set(struct task_struct *task, unsigned long which,
 		return ssb_prctl_set(task, ctrl);
 	case PR_SPEC_INDIRECT_BRANCH:
 		return ib_prctl_set(task, ctrl);
+	case PR_SPEC_L1D_FLUSH_OUT:
+		return l1d_flush_out_prctl_set(task, ctrl);
 	default:
 		return -ENODEV;
 	}
@@ -1221,6 +1236,17 @@ void arch_seccomp_spec_mitigate(struct task_struct *task)
 }
 #endif
 
+static int l1d_flush_out_prctl_get(struct task_struct *task)
+{
+	int ret;
+
+	ret = test_ti_thread_flag(&task->thread_info, TIF_SPEC_L1D_FLUSH);
+	if (ret)
+		return PR_SPEC_PRCTL | PR_SPEC_ENABLE;
+	else
+		return PR_SPEC_PRCTL | PR_SPEC_DISABLE;
+}
+
 static int ssb_prctl_get(struct task_struct *task)
 {
 	switch (ssb_mode) {
@@ -1272,6 +1298,8 @@ int arch_prctl_spec_ctrl_get(struct task_struct *task, unsigned long which)
 		return ssb_prctl_get(task);
 	case PR_SPEC_INDIRECT_BRANCH:
 		return ib_prctl_get(task);
+	case PR_SPEC_L1D_FLUSH_OUT:
+		return l1d_flush_out_prctl_get(task);
 	default:
 		return -ENODEV;
 	}
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 07b4f8131e36..1e864867a367 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -213,6 +213,7 @@ struct prctl_mm_map {
 /* Speculation control variants */
 # define PR_SPEC_STORE_BYPASS		0
 # define PR_SPEC_INDIRECT_BRANCH	1
+# define PR_SPEC_L1D_FLUSH_OUT		2
 /* Return and control values for PR_SET/GET_SPECULATION_CTRL */
 # define PR_SPEC_NOT_AFFECTED		0
 # define PR_SPEC_PRCTL			(1UL << 0)
-- 
2.17.1