linux-kernel - Re: [PATCH v5] x86/entry_32: Use stack segment selector for VERW operand

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Message-ID: <cdcc5b91-321b-4e33-9a86-829b8f632ae6@intel.com>
Date: Thu, 29 Aug 2024 15:28:13 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
 Robert Gill <rtgill82@...il.com>, Jari Ruusu <jariruusu@...tonmail.com>,
 Brian Gerst <brgerst@...il.com>,
 "Linux regression tracking (Thorsten Leemhuis)" <regressions@...mhuis.info>,
 antonio.gomez.iglesias@...ux.intel.com, daniel.sneddon@...ux.intel.com,
 stable@...r.kernel.org
Subject: Re: [PATCH v5] x86/entry_32: Use stack segment selector for VERW
 operand

On 7/11/24 15:03, Pawan Gupta wrote:
> +/*
> + * Safer version of CLEAR_CPU_BUFFERS that uses %ss to reference VERW operand
> + * mds_verw_sel. This ensures VERW will not #GP for an arbitrary user %ds.
> + */
> +.macro CLEAR_CPU_BUFFERS_SAFE
> +	ALTERNATIVE "", __stringify(verw %ss:_ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
> +.endm

One other thing...

Instead of making a "_SAFE" variant, let's just make the 32-bit version
always safe.

Also, is there any downside to using %ss: on 64-bit?  If not, let's just
update the one and only CLEAR_CPU_BUFFERS use %ss:.