Re: Question about dmesg/sysfs output when retpoline config is disabled

[Dou Liyang]

Hi Misono-san,

At 01/30/2018 12:52 PM, Misono, Tomohiro wrote:

Hello,

I think dmesg/sysfs output messages are not suitable if retpoline config is off:

I intentionally compiled the kernel 4.15.0 with CONFIG_RETPOLINE=n for test and
boot it with the following kernel command line option to check dmesg/sysfs:

(a) no command line option or "spectre_v2=on" or "spectre_v2=auto"
$ dmesg | grep -i spectre
[    0.017714] Spectre V2 mitigation: Vulnerable: Minimal generic ASM retpoline
$ cat /sys/devices/system/cpu/vulnerabilities/spectre_v2
Minimal generic ASM retpoline

(b) "spectre_v2=off"
$ dmesg | grep -i spectre
[    0.017002] Spectre V2 mitigation: disabled on command line.
$ cat /sys/devices/system/cpu/vulnerabilities/spectre_v2
Vulnerable

(c) "spectre_v2=retpoline"
$ dmesg | grep -i spectre
[    0.018002] Spectre V2 mitigation: kernel not compiled with retpoline; no 
mitigation available!
$ cat /sys/devices/system/cpu/vulnerabilities/spectre_v2
Vulnerable

I think the output of (c) is correct for this case, or are these outputs 
actually right?

Also, the output of (a) is the same with following condition:
  (1) CONFIG_RETPOLINE=n, and
  (2) CONFIG_RETPOLINE=y but the compiler did not support retpoline
These cannot be distinguished unless option of (c) is explicitly used.

+Cc maintainers and x86 mail list... first.

IMO, Selecting 'on' or 'auto' to "spectre_v2=" should also consider the
setting of the CONFIG_RETPOLINE configuration option.

So, check if CONFIG_RETPOLINE is y before setup CPU capability.

Thanks,
        dou.

--------------8<-----------------------------
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 390b3dc3d438..10188f856099 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -193,8 +193,9 @@ static void __init spectre_v2_select_mitigation(void)
        case SPECTRE_V2_CMD_FORCE:
                /* FALLTRHU */
        case SPECTRE_V2_CMD_AUTO:
-               goto retpoline_auto;
-
+               if (IS_ENABLED(CONFIG_RETPOLINE))
+                       goto retpoline_auto;
+               break;
        case SPECTRE_V2_CMD_RETPOLINE_AMD:
                if (IS_ENABLED(CONFIG_RETPOLINE))
                        goto retpoline_amd;

Regards,
Tomohiro Misono