linux-kernel - Re: [PATCH] x86/bhi: BHI mitigation can trigger warning in #DB handler

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Date: Thu, 23 May 2024 15:59:59 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Alexandre Chartre <alexandre.chartre@...cle.com>, x86@...nel.org,
 kvm@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, daniel.sneddon@...ux.intel.com,
 pawan.kumar.gupta@...ux.intel.com, tglx@...utronix.de,
 konrad.wilk@...cle.com, peterz@...radead.org, gregkh@...uxfoundation.org,
 seanjc@...gle.com, dave.hansen@...ux.intel.com, nik.borisov@...e.com,
 kpsingh@...nel.org, longman@...hat.com, bp@...en8.de, pbonzini@...hat.com,
 "Kaplan, David" <david.kaplan@....com>
Subject: Re: [PATCH] x86/bhi: BHI mitigation can trigger warning in #DB
 handler

On 23/05/2024 1:59 pm, Alexandre Chartre wrote:
>
>
> On 5/23/24 14:42, Andrew Cooper wrote:
>> On 23/05/2024 1:33 pm, Alexandre Chartre wrote:
>>> diff --git a/arch/x86/entry/entry_64_compat.S
>>> b/arch/x86/entry/entry_64_compat.S
>>> index 11c9b8efdc4c..7fa04edc87e9 100644
>>> --- a/arch/x86/entry/entry_64_compat.S
>>> +++ b/arch/x86/entry/entry_64_compat.S
>>> @@ -91,7 +91,6 @@
>>> SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
>>>         IBRS_ENTER
>>>       UNTRAIN_RET
>>> -    CLEAR_BRANCH_HISTORY
>>>         /*
>>>        * SYSENTER doesn't filter flags, so we need to clear NT and AC
>>> @@ -116,6 +115,12 @@
>>> SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
>>>       jnz    .Lsysenter_fix_flags
>>>   .Lsysenter_flags_fixed:
>>>   +    /*
>>> +     * CLEAR_BRANCH_HISTORY can call other functions. It should be
>>> invoked
>>> +     * after making sure TF is cleared because single-step is
>>> ignored only
>>> +     * for instructions inside the entry_SYSENTER_compat function.
>>> +     */
>>> +    CLEAR_BRANCH_HISTORY
>>
>> Exactly the same is true of UNTRAIN_RET, although it will only manifest
>> in i386 builds running on AMD hardware (SYSENTER is #UD on AMD hardware
>> in Long mode.)
>>
>> #DB is IST so does handle it's own speculation safety.  It should be
>> safe to move all the speculation safety logic in the sysenter handler to
>> after .Lsysenter_flags_fixed:, I think?
>>
>
> Right, so something like this:
>
> --- a/arch/x86/entry/entry_64_compat.S
> +++ b/arch/x86/entry/entry_64_compat.S
> @@ -89,10 +89,6 @@
> SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
>  
>         cld
>  
> -       IBRS_ENTER
> -       UNTRAIN_RET
> -       CLEAR_BRANCH_HISTORY
> -
>         /*
>          * SYSENTER doesn't filter flags, so we need to clear NT and AC
>          * ourselves.  To save a few cycles, we can check whether
> @@ -116,6 +112,15 @@
> SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
>         jnz     .Lsysenter_fix_flags
>  .Lsysenter_flags_fixed:
>  
> +       /*
> +        * CPU bugs mitigations mechanisms can call other functions. They
> +        * should be invoked after making sure TF is cleared because
> +        * single-step is ignored only for instructions inside the
> +        * entry_SYSENTER_compat function.
> +        */
> +       IBRS_ENTER
> +       UNTRAIN_RET
> +       CLEAR_BRANCH_HISTORY

Yeah - this looks rather better.

Although I'd suggest a blank line here if you're going to formalise the
patch.

~Andrew