linux-kernel - Re: [PATCH v2 1/2] x86/traps: Initialize DR6 by writing its architectural reset value

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <3e4c2e32-3c12-47a6-bf84-79690f78e1bf@zytor.com>
Date: Tue, 17 Jun 2025 15:49:00 -0700
From: Xin Li <xin@...or.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, seanjc@...gle.com, pbonzini@...hat.com,
        sohil.mehta@...el.com, brgerst@...il.com, tony.luck@...el.com,
        fenghuay@...dia.com
Subject: Re: [PATCH v2 1/2] x86/traps: Initialize DR6 by writing its
 architectural reset value

On 6/17/2025 2:03 AM, Peter Zijlstra wrote:
> On Tue, Jun 17, 2025 at 12:32:33AM -0700, Xin Li (Intel) wrote:
>> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
>> index 8feb8fd2957a..3bd7c9ac7576 100644
>> --- a/arch/x86/kernel/cpu/common.c
>> +++ b/arch/x86/kernel/cpu/common.c
>> @@ -2243,20 +2243,17 @@ EXPORT_PER_CPU_SYMBOL(__stack_chk_guard);
>>   #endif
>>   #endif
>>   
>> -/*
>> - * Clear all 6 debug registers:
>> - */
>> -static void clear_all_debug_regs(void)
>> +static void initialize_debug_regs(void)
>>   {
>>   	int i;
>>   
>> -	for (i = 0; i < 8; i++) {
>> -		/* Ignore db4, db5 */
>> -		if ((i == 4) || (i == 5))
>> -			continue;
>> +	/* Control register first */
>> +	set_debugreg(0, 7);
>> +	set_debugreg(DR6_RESERVED, 6);
>>   
>> +	/* Ignore db4, db5 */
>> +	for (i = 0; i < 4; i++)
>>   		set_debugreg(0, i);
>> -	}
>>   }
>>   
>>   #ifdef CONFIG_KGDB
> 
> Maybe like so?
> 
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -2206,15 +2206,14 @@ EXPORT_PER_CPU_SYMBOL(__stack_chk_guard)
>   
>   static void initialize_debug_regs(void)
>   {
> -	int i;
> -
> -	/* Control register first */
> +	/* Control register first -- to make sure everything is disabled. */
>   	set_debugreg(0, 7);
>   	set_debugreg(DR6_RESERVED, 6);
> -
> -	/* Ignore db4, db5 */
> -	for (i = 0; i < 4; i++)
> -		set_debugreg(0, i);
> +	/* dr5 and dr4 don't exist */
> +	set_debugreg(0, 3);
> +	set_debugreg(0, 2);
> +	set_debugreg(0, 1);
> +	set_debugreg(0, 0);
>   }
>   
>   #ifdef CONFIG_KGDB

Yeah, I think it makes sense to make the change.

Thanks!
     Xin