Question about dmesg/sysfs output when retpoline config is disabled

Question about dmesg/sysfs output when retpoline config is disabled

[Misono, Tomohiro]

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.

Regards,
Tomohiro Misono

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
> 
> 
> 
> 

[tip:x86/pti] x86/spectre: Check CONFIG_RETPOLINE in command line parser

[tip-bot for Dou Liyang]

Commit-ID:  9471eee9186a46893726e22ebb54cade3f9bc043
Gitweb:     https://git.kernel.org/tip/9471eee9186a46893726e22ebb54cade3f9bc043
Author:     Dou Liyang <douly.fnst@cn.fujitsu.com>
AuthorDate: Tue, 30 Jan 2018 14:13:50 +0800
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Tue, 30 Jan 2018 15:30:35 +0100

x86/spectre: Check CONFIG_RETPOLINE in command line parser

The spectre_v2 option 'auto' does not check whether CONFIG_RETPOLINE is
enabled. As a consequence it fails to emit the appropriate warning and sets
feature flags which have no effect at all.

Add the missing IS_ENABLED() check.

Fixes: da285121560e ("x86/spectre: Add boot time option to select Spectre v2 mitigation")
Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: ak@linux.intel.com
Cc: peterz@infradead.org
Cc: Tomohiro" <misono.tomohiro@jp.fujitsu.com>
Cc: dave.hansen@intel.com
Cc: bp@alien8.de
Cc: arjan@linux.intel.com
Cc: dwmw@amazon.co.uk
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/f5892721-7528-3647-08fb-f8d10e65ad87@cn.fujitsu.com

---
 arch/x86/kernel/cpu/bugs.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 3bfb2b2..400c34e 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -213,10 +213,10 @@ static void __init spectre_v2_select_mitigation(void)
 		return;
 
 	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;