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 <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: balbir-aws

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"

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.