linux-kernel - Re: [PATCH v1 1/1] x86/fred: Clear the WFE bit in missing-ENDBRANCH #CP

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Message-ID: <e9376635-f578-41df-9e14-e4cc6def848a@zytor.com>
Date: Thu, 12 Sep 2024 01:25:55 -0700
From: Xin Li <xin@...or.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, andrew.cooper3@...rix.com
Subject: Re: [PATCH v1 1/1] x86/fred: Clear the WFE bit in missing-ENDBRANCH
 #CP

On 9/12/2024 12:57 AM, Peter Zijlstra wrote:
> On Wed, Sep 11, 2024 at 04:19:29PM -0700, Xin Li (Intel) wrote:
> 
>> +static void ibt_clear_fred_wfe(struct pt_regs *regs)
>> +{
>> +	if (cpu_feature_enabled(X86_FEATURE_FRED))
>> +		regs->fred_cs.wfe = 0;
>> +}
>> +
>>   static void do_kernel_cp_fault(struct pt_regs *regs, unsigned long error_code)
>>   {
>>   	if ((error_code & CP_EC) != CP_ENDBR) {
>> @@ -90,6 +107,7 @@ static void do_kernel_cp_fault(struct pt_regs *regs, unsigned long error_code)
>>   
>>   	if (unlikely(regs->ip == (unsigned long)&ibt_selftest_noendbr)) {
>>   		regs->ax = 0;
>> +		ibt_clear_fred_wfe(regs);
>>   		return;
>>   	}
>>   
>> @@ -97,6 +115,7 @@ static void do_kernel_cp_fault(struct pt_regs *regs, unsigned long error_code)
>>   	if (!ibt_fatal) {
>>   		printk(KERN_DEFAULT CUT_HERE);
>>   		__warn(__FILE__, __LINE__, (void *)regs->ip, TAINT_WARN, regs, NULL);
>> +		ibt_clear_fred_wfe(regs);
>>   		return;
>>   	}
>>   	BUG();
> 
> So, why not clear the bit the moment we know this is CP_ENDBR?
> 
> In the fatal case, we'll hit that BUG and die anyway, nobody cares about
> the tracker state in that case.

I did think about it, but preferred to leave it as in BUG().

But as you have asked, will change unless someone else disagrees with a
good reason :)

> 
> diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
> index d2c732a34e5d..fde4bdd25a73 100644
> --- a/arch/x86/kernel/cet.c
> +++ b/arch/x86/kernel/cet.c
> @@ -88,6 +88,8 @@ static void do_kernel_cp_fault(struct pt_regs *regs, unsigned long error_code)
>   		return;
>   	}
>   
> +	ibt_clear_fred_wfe(regs);
> +
>   	if (unlikely(regs->ip == (unsigned long)&ibt_selftest_noendbr)) {
>   		regs->ax = 0;
>   		return;
>