linux-kernel - Re: [PATCH v4] x86/entry_32: Use stack segment selector for VERW operand

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] [day] [month] [year] [list]

Message-ID: <20240711183620.j5di5gnsn6bt2ppw@desk>
Date: Thu, 11 Jul 2024 11:36:20 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	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 v4] x86/entry_32: Use stack segment selector for VERW
 operand

On Thu, Jul 11, 2024 at 11:03:29AM +0200, Peter Zijlstra wrote:
> On Wed, Jul 10, 2024 at 12:06:47PM -0700, 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 "jmp .Lskip_verw\@", "", X86_FEATURE_CLEAR_CPU_BUF
> > +	verw	%ss:_ASM_RIP(mds_verw_sel)
> > +.Lskip_verw\@:
> > +.endm
> 
> I know this is somewhat of a common pattern, but I think it is silly in
> this case. Since we already have the ALTERNATIVE() why not NOP the one
> VERW instruction instead?
> 
> That is,
> 
> 	ALTERNATIVE("", "verw %ss:_ASM_RIP(mds_verw_sel)", X86_FEATURE_CLEAR_CPU_BUF)

Will do, thanks.