[tip: x86/cpu] sched: Add task_work callback for paranoid L1D flush

["tip-bot2 for Balbir Singh" <tip-bot2@linutronix.de>]

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

Commit-ID:     58e106e725eed59896b9141a1c9a917d2f67962a
Gitweb:        https://git.kernel.org/tip/58e106e725eed59896b9141a1c9a917d2f67962a
Author:        Balbir Singh <sblbir@amazon.com>
AuthorDate:    Mon, 26 Apr 2021 21:59:11 +02:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Wed, 28 Jul 2021 11:42:24 +02:00

sched: Add task_work callback for paranoid L1D flush

The upcoming paranoid L1D flush infrastructure allows to conditionally
(opt-in) flush L1D in switch_mm() as a defense against potential new side
channels or for paranoia reasons. As the flush makes only sense when a task
runs on a non-SMT enabled core, because SMT siblings share L1, the
switch_mm() logic will kill a task which is flagged for L1D flush when it
is running on a SMT thread.

Add a taskwork callback so switch_mm() can queue a SIG_KILL command which
is invoked when the task tries to return to user space.

Signed-off-by: Balbir Singh <sblbir@amazon.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210108121056.21940-1-sblbir@amazon.com
---
 arch/Kconfig          |  3 +++
 include/linux/sched.h | 10 ++++++++++
 2 files changed, 13 insertions(+)

diff --git a/arch/Kconfig b/arch/Kconfig
index 129df49..98db634 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1282,6 +1282,9 @@ config ARCH_SPLIT_ARG64
 config ARCH_HAS_ELFCORE_COMPAT
 	bool
 
+config ARCH_HAS_PARANOID_L1D_FLUSH
+	bool
+
 source "kernel/gcov/Kconfig"
 
 source "scripts/gcc-plugins/Kconfig"
diff --git a/include/linux/sched.h b/include/linux/sched.h
index ec8d07d..c048e59 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1400,6 +1400,16 @@ struct task_struct {
 	struct llist_head               kretprobe_instances;
 #endif
 
+#ifdef CONFIG_ARCH_HAS_PARANOID_L1D_FLUSH
+	/*
+	 * If L1D flush is supported on mm context switch
+	 * then we use this callback head to queue kill work
+	 * to kill tasks that are not running on SMT disabled
+	 * cores
+	 */
+	struct callback_head		l1d_flush_kill;
+#endif
+
 	/*
 	 * New fields for task_struct should be added above here, so that
 	 * they are included in the randomized portion of task_struct.