[MODERATED] [PATCH v6 11/43] MDSv6

Add basic infrastructure for code to request CPU buffer clearing
on the next kernel exit.

We have two functions lazy_clear_cpu to request clearing,
and lazy_clear_cpu_interrupt to request clearing if running
in an interrupt.

Non architecture specific code can include linux/clearcpu.h
and use lazy_clear_cpu / lazy_clear_interrupt. On x86
we provide low level implementations that sets a per cpu
flag to request cpu clearing on the next kernel exit.

Suggested-by: Linus Torvalds <[email protected]>
Signed-off-by: Andi Kleen <[email protected]>
---
 arch/Kconfig                    |  3 +++
 arch/x86/Kconfig                |  1 +
 arch/x86/include/asm/clearcpu.h | 12 +++++++++--
 arch/x86/kernel/cpu/bugs.c      |  3 +++
 include/linux/clearcpu.h        | 36 +++++++++++++++++++++++++++++++++
 5 files changed, 53 insertions(+), 2 deletions(-)
 create mode 100644 include/linux/clearcpu.h

diff --git a/arch/Kconfig b/arch/Kconfig
index 4cfb6de48f79..e6b7bf9174aa 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -808,6 +808,9 @@ config VMAP_STACK
 	  the stack to map directly to the KASAN shadow map using a formula
 	  that is incorrect if the stack is in vmalloc space.
 
+config ARCH_HAS_CLEAR_CPU
+	def_bool n
+
 config ARCH_OPTIONAL_KERNEL_RWX
 	def_bool n
 
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 68261430fe6e..1b837b61d07b 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -84,6 +84,7 @@ config X86
 	select ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
 	select ARCH_WANTS_DYNAMIC_TASK_STRUCT
 	select ARCH_WANTS_THP_SWAP		if X86_64
+	select ARCH_HAS_CLEAR_CPU
 	select BUILDTIME_EXTABLE_SORT
 	select CLKEVT_I8253
 	select CLOCKSOURCE_VALIDATE_LAST_CYCLE
diff --git a/arch/x86/include/asm/clearcpu.h b/arch/x86/include/asm/clearcpu.h
index 88ab27694aa6..15d25c2436da 100644
--- a/arch/x86/include/asm/clearcpu.h
+++ b/arch/x86/include/asm/clearcpu.h
@@ -5,7 +5,8 @@
 #include <linux/jump_label.h>
 #include <linux/sched/smt.h>
 #include <asm/alternative.h>
-#include <linux/thread_info.h>
+
+DECLARE_PER_CPU(bool, clear_cpu_flag);
 
 /*
  * Clear CPU buffers to avoid side channels.
@@ -34,8 +35,15 @@ static inline void clear_cpu(void)
 
 static inline void clear_cpu_idle(void)
 {
-	if (sched_smt_active())
+	if (sched_smt_active()) {
+		__this_cpu_write(clear_cpu_flag, false);
 		clear_cpu();
+	}
+}
+
+static inline void lazy_clear_cpu(void)
+{
+	__this_cpu_write(clear_cpu_flag, true);
 }
 
 #endif
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 26b890f18239..067309190a6a 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1061,6 +1061,9 @@ early_param("l1tf", l1tf_cmdline);
 
 #undef pr_fmt
 
+DEFINE_PER_CPU(bool, clear_cpu_flag);
+EXPORT_PER_CPU_SYMBOL(clear_cpu_flag);
+
 static void mds_select_mitigation(void)
 {
 	if (!boot_cpu_has(X86_BUG_MDS))
diff --git a/include/linux/clearcpu.h b/include/linux/clearcpu.h
new file mode 100644
index 000000000000..63a6952b46fa
--- /dev/null
+++ b/include/linux/clearcpu.h
@@ -0,0 +1,36 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_CLEARCPU_H
+#define _LINUX_CLEARCPU_H 1
+
+#include <linux/preempt.h>
+
+#ifdef CONFIG_ARCH_HAS_CLEAR_CPU
+#include <asm/clearcpu.h>
+#else
+static inline void lazy_clear_cpu(void)
+{
+}
+#endif
+
+/*
+ * Use this function when potentially touching (reading or writing)
+ * user data in an interrupt. In this case schedule to clear the
+ * CPU buffers on kernel exit to avoid any potential side channels.
+ *
+ * If not in an interrupt we assume the touched data belongs to the
+ * current process and doesn't need to be cleared.
+ *
+ * This version is for code who might be in an interrupt.
+ * If you know for sure you're in interrupt context call
+ * lazy_clear_cpu directly.
+ *
+ * lazy_clear_cpu is reasonably cheap (just sets a bit) and
+ * can be used in fast paths.
+ */
+static inline void lazy_clear_cpu_interrupt(void)
+{
+	if (in_interrupt())
+		lazy_clear_cpu();
+}
+
+#endif
-- 
2.17.2