kaiser: KAISER depends on SMP

It is absurd that KAISER should depend on SMP, but apparently nobody
has tried a UP build before: which breaks on implicit declaration of
function 'per_cpu_offset' in arch/x86/mm/kaiser.c.

Now, you would expect that to be trivially fixed up; but looking at
the System.map when that block is #ifdef'ed out of kaiser_init(),
I see that in a UP build __per_cpu_user_mapped_end is precisely at
__per_cpu_user_mapped_start, and the items carefully gathered into
that section for user-mapping on SMP, dispersed elsewhere on UP.

So, some other kind of section assignment will be needed on UP,
but implementing that is not a priority: just make KAISER depend
on SMP for now.

Also inserted a blank line before the option, tidied up the
brief Kconfig help message, and added an "If unsure, Y".
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

CVE-2017-5754
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

security/Kconfig

@@ -30,14 +30,16 @@ config SECURITY
 	  model will be used.
 
 	  If you are unsure how to answer this question, answer N.
+
 config KAISER
 	bool "Remove the kernel mapping in user mode"
 	default y
-	depends on X86_64
-	depends on !PARAVIRT
+	depends on X86_64 && SMP && !PARAVIRT
 	help
-	  This enforces a strict kernel and user space isolation in order to close
-	  hardware side channels on kernel address information.
+	  This enforces a strict kernel and user space isolation, in order
+	  to close hardware side channels on kernel address information.
+
+	  If you are unsure how to answer this question, answer Y.
 
 config KAISER_REAL_SWITCH
 	bool "KAISER: actually switch page tables"