x86_64: Ignore compat mode SYSCALL when IA32_EMULATION is not defined

Previously a program could switch to a compat mode segment and then
execute SYSCALL and it would jump to an uninitialized MSR and crash
the kernel.

Instead supply a dummy target for this case.

Pointed out by Jan Beulich

Cc: [e-mail]
Signed-off-by: Andi Kleen <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/x86_64/kernel/entry.S

···
 	ret
 	CFI_ENDPROC
 ENDPROC(call_softirq)
+
+KPROBE_ENTRY(ignore_sysret)
+	CFI_STARTPROC
+	mov $-ENOSYS,%eax
+	sysret
+	CFI_ENDPROC
+ENDPROC(ignore_sysret)

arch/x86_64/kernel/setup64.c

···
 char boot_exception_stacks[(N_EXCEPTION_STACKS - 1) * EXCEPTION_STKSZ + DEBUG_STKSZ]
 __attribute__((section(".bss.page_aligned")));
 
+extern asmlinkage void ignore_sysret(void);
+
 /* May not be marked __init: used by software suspend */
 void syscall_init(void)
 {
···
 	 */ 
 	wrmsrl(MSR_STAR,  ((u64)__USER32_CS)<<48  | ((u64)__KERNEL_CS)<<32); 
 	wrmsrl(MSR_LSTAR, system_call); 
+	wrmsrl(MSR_CSTAR, ignore_sysret);
 
 #ifdef CONFIG_IA32_EMULATION   		
 	syscall32_cpu_init ();