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From: Peter Zijlstra <peterz@infradead.org>
To: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	linux-kernel@vger.kernel.org, x86@kernel.org,
	Robert Gill <rtgill82@gmail.com>,
	Jari Ruusu <jariruusu@protonmail.com>,
	Brian Gerst <brgerst@gmail.com>,
	"Linux regression tracking (Thorsten Leemhuis)"
	<regressions@leemhuis.info>,
	antonio.gomez.iglesias@linux.intel.com,
	daniel.sneddon@linux.intel.com, stable@vger.kernel.org
Subject: Re: [PATCH v4] x86/entry_32: Use stack segment selector for VERW operand
Date: Thu, 11 Jul 2024 11:03:29 +0200	[thread overview]
Message-ID: <20240711090329.GI4587@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <20240710-fix-dosemu-vm86-v4-1-aa6464e1de6f@linux.intel.com>

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)

and call it a day?

  parent reply	other threads:[~2024-07-11  9:03 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-07-10 19:06 [PATCH v4] x86/entry_32: Use stack segment selector for VERW operand Pawan Gupta
2024-07-10 21:50 ` Uros Bizjak
2024-07-10 23:16   ` Pawan Gupta
2024-07-11  5:49     ` Jiri Slaby
2024-07-11 18:34       ` Pawan Gupta
2024-07-11  8:30     ` Uros Bizjak
2024-07-11  8:44       ` Jiri Slaby
2024-07-11  6:52 ` Jari Ruusu
2024-07-11  9:03 ` Peter Zijlstra [this message]
2024-07-11 18:36   ` Pawan Gupta

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