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

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Date:   Tue, 30 Jan 2018 14:13:50 +0800
From:   Dou Liyang <douly.fnst@...fujitsu.com>
To:     "Misono, Tomohiro" <misono.tomohiro@...fujitsu.com>,
        <linux-kernel@...r.kernel.org>
CC:     <dwmw@...zon.co.uk>, <tglx@...utronix.de>, <arjan@...ux.intel.com>,
        <mingo@...nel.org>, <ak@...ux.intel.com>, <peterz@...radead.org>,
        <bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>,
        <dave.hansen@...el.com>
Subject: Re: Question about dmesg/sysfs output when retpoline config is
 disabled

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
> 
> 
> 
>