linux-kernel - [RFC 09/17] x86/asm/32: Pull MSR_IA32_SYSENTER_CS update code out of native_load

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Date:   Wed,  6 Sep 2017 14:36:54 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: [RFC 09/17] x86/asm/32: Pull MSR_IA32_SYSENTER_CS update code out of native_load_sp0()

This causees the MSR_IA32_SYSENTER_CS write to move out of the
paravirt hook.  This shouldn't affect Xen or lgeust: Xen already
ignores MSR_IA32_SYSENTER_ESP writes and lguest doesn't support
SYSENTER at all.  In any event, Xen doesn't support vm86()
in a useful way.

Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/include/asm/processor.h |  7 -------
 arch/x86/include/asm/switch_to.h | 11 +++++++++++
 arch/x86/kernel/process_32.c     |  1 +
 arch/x86/kernel/process_64.c     |  2 +-
 arch/x86/kernel/vm86_32.c        |  6 +++++-
 5 files changed, 18 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 028245e1c42b..ee37fb86900a 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -513,13 +513,6 @@ static inline void
 native_load_sp0(struct tss_struct *tss, struct thread_struct *thread)
 {
 	tss->x86_tss.sp0 = thread->sp0;
-#ifdef CONFIG_X86_32
-	/* Only happens when SEP is enabled, no need to test "SEP"arately: */
-	if (unlikely(tss->x86_tss.ss1 != thread->sysenter_cs)) {
-		tss->x86_tss.ss1 = thread->sysenter_cs;
-		wrmsr(MSR_IA32_SYSENTER_CS, thread->sysenter_cs, 0);
-	}
-#endif
 }
 
 static inline void native_swapgs(void)
diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
index fcc5cd387fd1..f3fa19925ae1 100644
--- a/arch/x86/include/asm/switch_to.h
+++ b/arch/x86/include/asm/switch_to.h
@@ -72,4 +72,15 @@ do {									\
 	((last) = __switch_to_asm((prev), (next)));			\
 } while (0)
 
+#ifdef CONFIG_X86_32
+static inline void refresh_sysenter_cs(struct thread_struct *thread)
+{
+	/* Only happens when SEP is enabled, no need to test "SEP"arately: */
+	if (unlikely(this_cpu_read(cpu_tss.x86_tss.ss1) == thread->sysenter_cs))
+		return;
+
+	this_cpu_write(cpu_tss.x86_tss.ss1, thread->sysenter_cs);
+}
+#endif
+
 #endif /* _ASM_X86_SWITCH_TO_H */
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index c6d6dc5f8bb2..9f217b5ef438 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -287,6 +287,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 	 * current_thread_info().
 	 */
 	load_sp0(tss, next);
+	refresh_sysenter_cs();  /* in case prev or next is vm86 */
 	this_cpu_write(cpu_current_top_of_stack,
 		       (unsigned long)task_stack_page(next_p) +
 		       THREAD_SIZE);
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index c3169be4c596..985e7569ab57 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -436,7 +436,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 	 */
 	this_cpu_write(current_task, next_p);
 
-	/* Reload esp0 and ss1.  This changes current_thread_info(). */
+	/* Reload sp0. */
 	load_sp0(tss, next);
 
 	/*
diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c
index 7924a5356c8a..9edda73dcc90 100644
--- a/arch/x86/kernel/vm86_32.c
+++ b/arch/x86/kernel/vm86_32.c
@@ -54,6 +54,7 @@
 #include <asm/irq.h>
 #include <asm/traps.h>
 #include <asm/vm86.h>
+#include <asm/switch_to.h>
 
 /*
  * Known problems:
@@ -149,6 +150,7 @@ void save_v86_state(struct kernel_vm86_regs *regs, int retval)
 	tsk->thread.sp0 = vm86->saved_sp0;
 	tsk->thread.sysenter_cs = __KERNEL_CS;
 	load_sp0(tss, &tsk->thread);
+	refresh_sysenter_cs();
 	vm86->saved_sp0 = 0;
 	put_cpu();
 
@@ -368,8 +370,10 @@ static long do_sys_vm86(struct vm86plus_struct __user *user_vm86, bool plus)
 	/* make room for real-mode segments */
 	tsk->thread.sp0 += 16;
 
-	if (static_cpu_has(X86_FEATURE_SEP))
+	if (static_cpu_has(X86_FEATURE_SEP)) {
 		tsk->thread.sysenter_cs = 0;
+		refresh_sysenter_cs();
+	}
 
 	load_sp0(tss, &tsk->thread);
 	put_cpu();
-- 
2.13.5