x86/asm/entry/64: Always set up SYSENTER MSRs

On CONFIG_IA32_EMULATION=y kernels we set up
MSR_IA32_SYSENTER_CS/ESP/EIP, but on !CONFIG_IA32_EMULATION
kernels we leave them unchanged.

Clear them to make sure the instruction is disabled properly.

SYSCALL is set up properly in both cases.

Acked-by: Denys Vlasenko <dvlasenk@redhat.com>
Acked-by: Andy Lutomirski <luto@amacapital.net>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/cpu/common.c

@@ -1169,9 +1169,8 @@ void syscall_init(void)
 	 */
 	wrmsrl(MSR_STAR,  ((u64)__USER32_CS)<<48  | ((u64)__KERNEL_CS)<<32);
 	wrmsrl(MSR_LSTAR, system_call);
-#ifndef CONFIG_IA32_EMULATION
-	wrmsrl(MSR_CSTAR, ignore_sysret);
-#else
+
+#ifdef CONFIG_IA32_EMULATION
 	wrmsrl(MSR_CSTAR, ia32_cstar_target);
 	/*
 	 * Always load these, in case some future 64-bit CPU supports
@@ -1180,6 +1179,11 @@ void syscall_init(void)
 	wrmsrl_safe(MSR_IA32_SYSENTER_CS, (u64)__KERNEL_CS);
 	wrmsrl_safe(MSR_IA32_SYSENTER_ESP, 0ULL);
 	wrmsrl_safe(MSR_IA32_SYSENTER_EIP, (u64)ia32_sysenter_target);
+#else
+	wrmsrl(MSR_CSTAR, ignore_sysret);
+	wrmsrl_safe(MSR_IA32_SYSENTER_CS, 0);
+	wrmsrl_safe(MSR_IA32_SYSENTER_ESP, 0ULL);
+	wrmsrl_safe(MSR_IA32_SYSENTER_EIP, 0ULL);
 #endif
 
 	/* Flags to clear on syscall */