linux-kernel - Re: [patch V163 51/51] x86/ldt: Make the LDT mapping RO

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Date:   Mon, 18 Dec 2017 14:48:03 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirsky <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Borislav Petkov <bpetkov@...e.de>,
        Greg KH <gregkh@...uxfoundation.org>, keescook@...gle.com,
        hughd@...gle.com, Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        David Laight <David.Laight@...lab.com>,
        Eduardo Valentin <eduval@...zon.com>, aliguori@...zon.com,
        Will Deacon <will.deacon@....com>, daniel.gruss@...k.tugraz.at
Subject: Re: [patch V163 51/51] x86/ldt: Make the LDT mapping RO

On Mon, Dec 18, 2017 at 12:43:06PM +0100, Thomas Gleixner wrote:

>  arch/x86/include/asm/desc.h           |    2 ++
>  arch/x86/kernel/ldt.c                 |    7 ++++++-
>  tools/testing/selftests/x86/ldt_gdt.c |    3 +--
>  3 files changed, 9 insertions(+), 3 deletions(-)
> 
> --- a/arch/x86/include/asm/desc.h
> +++ b/arch/x86/include/asm/desc.h
> @@ -20,6 +20,8 @@ static inline void fill_ldt(struct desc_
>  
>  	desc->type		= (info->read_exec_only ^ 1) << 1;
>  	desc->type	       |= info->contents << 2;
> +	/* Set the ACCESS bit so it can be mapped RO */
> +	desc->type	       |= 1;
>  
>  	desc->s			= 1;
>  	desc->dpl		= 0x3;


--- a/arch/x86/kernel/tls.c
+++ b/arch/x86/kernel/tls.c
@@ -93,17 +93,10 @@ static void set_tls_desc(struct task_str
 	cpu = get_cpu();
 
 	while (n-- > 0) {
-		if (LDT_empty(info) || LDT_zero(info)) {
+		if (LDT_empty(info) || LDT_zero(info))
 			memset(desc, 0, sizeof(*desc));
-		} else {
+		else
 			fill_ldt(desc, info);
-
-			/*
-			 * Always set the accessed bit so that the CPU
-			 * doesn't try to write to the (read-only) GDT.
-			 */
-			desc->type |= 1;
-		}
 		++info;
 		++desc;
 	}