linux-kernel - Re: [tip: x86/urgent] x86/bugs: Use code segment selector for VERW operand

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Message-ID: <b4cae913-282a-4772-a2b0-d2638adfe486@intel.com>
Date: Wed, 9 Oct 2024 08:00:56 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org
Cc: linux-tip-commits@...r.kernel.org, Robert Gill <rtgill82@...il.com>,
 Andrew Cooper <andrew.cooper3@...rix.com>, stable@...r.kernel.org,
 #@...-bot2.tec.linutronix.de, 5.10+@...-bot2.tec.linutronix.de,
 Dave Hansen <dave.hansen@...ux.intel.com>, Brian Gerst <brgerst@...il.com>,
 Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, x86@...nel.org
Subject: Re: [tip: x86/urgent] x86/bugs: Use code segment selector for VERW
 operand

On 10/8/24 23:11, Borislav Petkov wrote:
>>  .macro CLEAR_CPU_BUFFERS
>> -	ALTERNATIVE "", __stringify(verw _ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
>> +#ifdef CONFIG_X86_64
>> +	ALTERNATIVE "", "verw mds_verw_sel(%rip)", X86_FEATURE_CLEAR_CPU_BUF
>> +#else
>> +	/*
>> +	 * In 32bit mode, the memory operand must be a %cs reference. The data
>> +	 * segments may not be usable (vm86 mode), and the stack segment may not
>> +	 * be flat (ESPFIX32).
>> +	 */
>> +	ALTERNATIVE "", "verw %cs:mds_verw_sel", X86_FEATURE_CLEAR_CPU_BUF
>> +#endif
> So why didn't we ifdef the "verw mds_verw_sel(%rip)" and "verw
> %cs:mds_verw_sel" macro argument instead of adding more bigger ugly ifdeffery?

I'm not jumping for joy about how it looks, but I applied it because
it's good enough and the regression was about to get its driver's
license. ;)

I did spend some time noodling to come up with _some_ common 32/64-bit
implementation, but 32-bit is just too special of a snowflake.