linux-kernel - Re: [tip: x86/urgent] x86/bugs: Use code 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] [thread-next>] [day] [month] [year] [list]

Message-ID: <20241009093257.GDZwZNyfIjw0lTZJqL@fat_crate.local>
Date: Wed, 9 Oct 2024 11:32:57 +0200
From: Borislav Petkov <bp@...en8.de>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, 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 Wed, Oct 09, 2024 at 09:34:37AM +0200, Peter Zijlstra wrote:
> You need ifdeffery either way around, either directly like this or for
> that macro. This is simple and straight forward.

Nothing in this file full of macros is simple. In any case, I would've done
this as the ifdeffery is shorter and the macro is simpler. We have this coding
pattern in a lot of headers, abstracting 32-bit vs 64-bit machine details, and
it is a very common and familiar one:

/*
 * 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).
 */
#ifdef CONFIG_X86_64
#define VERW_ARG "verw mds_verw_sel(%rip)"
#else /* CONFIG_X86_32 */
#define VERW_ARG "verw %cs:mds_verw_sel"
#endif

/*
 * Macro to execute VERW instruction that mitigate transient data sampling
 * attacks such as MDS. On affected systems a microcode update overloaded VERW
 * instruction to also clear the CPU buffers. VERW clobbers CFLAGS.ZF.
 *
 * Note: Only the memory operand variant of VERW clears the CPU buffers.
 */
.macro CLEAR_CPU_BUFFERS
        ALTERNATIVE "", VERW_ARG, X86_FEATURE_CLEAR_CPU_BUF
.endm

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette