linux-kernel - RE: [PATCH] x86/speculation: Check whether speculation is force disabled

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Message-ID: <TYXPR01MB1503D6F73C6356DED5D2C849F5890@TYXPR01MB1503.jpnprd01.prod.outlook.com>
Date:   Thu, 4 Jun 2020 07:29:48 +0000
From:   "Tada, Kenta (Sony)" <Kenta.Tada@...y.com>
To:     Waiman Long <longman@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "hpa@...or.com" <hpa@...or.com>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "pawan.kumar.gupta@...ux.intel.com" 
        <pawan.kumar.gupta@...ux.intel.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] x86/speculation: Check whether speculation is force
 disabled

>It conflicts with your new code. We can have an argument on whether IB should follow how SSB is being handled. Before that is settled,

Thank you for the information.
It conflicts but I think users who read the below document get confused.
Documentation/userspace-api/spec_ctrl.rst.

Especially, seccomp users must know the difference of this implicit specification
because both IB and SSB are force disabled simultaneously when seccomp is enabled
without SECCOMP_FILTER_FLAG_SPEC_ALLOW on x86.

-----Original Message-----
From: Waiman Long <longman@...hat.com> 
Sent: Thursday, June 4, 2020 12:40 AM
To: Tada, Kenta (Sony) <Kenta.Tada@...y.com>; x86@...nel.org; tglx@...utronix.de; mingo@...hat.com; bp@...en8.de; hpa@...or.com; jpoimboe@...hat.com; peterz@...radead.org; tony.luck@...el.com; pawan.kumar.gupta@...ux.intel.com
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/speculation: Check whether speculation is force disabled

On 6/3/20 3:12 AM, Tada, Kenta (Sony) wrote:
> Once PR_SPEC_FORCE_DISABLE is set, users cannot set PR_SPEC_ENABLE.
> This commit checks whether PR_SPEC_FORCE_DISABLE was previously set.
>
> Signed-off-by: Kenta Tada <Kenta.Tada@...y.com>
> ---
>   arch/x86/kernel/cpu/bugs.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c 
> index ed54b3b21c39..678ace157035 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -1173,6 +1173,9 @@ static int ib_prctl_set(struct task_struct *task, unsigned long ctrl)
>   		if (spectre_v2_user == SPECTRE_V2_USER_STRICT ||
>   		    spectre_v2_user == SPECTRE_V2_USER_STRICT_PREFERRED)
>   			return -EPERM;
> +		/* If speculation is force disabled, enable is not allowed */
> +		if (task_spec_ib_force_disable(task))
> +			return -EPERM;
>   		task_clear_spec_ib_disable(task);
>   		task_update_spec_tif(task);
>   		break;

There is a comment up a few lines about this:

                 /*
                  * Indirect branch speculation is always allowed when
                  * mitigation is force disabled.
                  */
It conflicts with your new code. We can have an argument on whether IB should follow how SSB is being handled. Before that is settled,

Nacked-by: Waiman Long <longman@...hat.com>