lockin/x86: Implement sync_core_before_usermode()

Ensure that a core serializing instruction is issued before returning to
user-mode. x86 implements return to user-space through sysexit, sysrel,
and sysretq, which are not core serializing.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrea Parri <parri.andrea@gmail.com>
Cc: Andrew Hunter <ahh@google.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Avi Kivity <avi@scylladb.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Dave Watson <davejwatson@fb.com>
Cc: David Sehr <sehr@google.com>
Cc: Greg Hackmann <ghackmann@google.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Maged Michael <maged.michael@gmail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-api@vger.kernel.org
Cc: linux-arch@vger.kernel.org
Link: http://lkml.kernel.org/r/20180129202020.8515-8-mathieu.desnoyers@efficios.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 29 insertions, 0 deletions

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 423e4b64e683..31030ade4690 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -61,6 +61,7 @@ config X86
         select ARCH_HAS_SG_CHAIN
         select ARCH_HAS_STRICT_KERNEL_RWX
         select ARCH_HAS_STRICT_MODULE_RWX
+        select ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
         select ARCH_HAS_UBSAN_SANITIZE_ALL
         select ARCH_HAS_ZONE_DEVICE             if X86_64
         select ARCH_HAVE_NMI_SAFE_CMPXCHG
diff --git a/arch/x86/include/asm/sync_core.h b/arch/x86/include/asm/sync_core.h
new file mode 100644
index 000000000000..c67caafd3381
--- /dev/null
+++ b/arch/x86/include/asm/sync_core.h
@@ -0,0 +1,28 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_SYNC_CORE_H
+#define _ASM_X86_SYNC_CORE_H
+
+#include <linux/preempt.h>
+#include <asm/processor.h>
+#include <asm/cpufeature.h>
+
+/*
+ * Ensure that a core serializing instruction is issued before returning
+ * to user-mode. x86 implements return to user-space through sysexit,
+ * sysrel, and sysretq, which are not core serializing.
+ */
+static inline void sync_core_before_usermode(void)
+{
+        /* With PTI, we unconditionally serialize before running user code. */
+        if (static_cpu_has(X86_FEATURE_PTI))
+                return;
+        /*
+         * Return from interrupt and NMI is done through iret, which is core
+         * serializing.
+         */
+        if (in_irq() || in_nmi())
+                return;
+        sync_core();
+}
+
+#endif /* _ASM_X86_SYNC_CORE_H */