linux-kernel - [tip:x86/urgent] x86/entry/32: Clear the CS high bits

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Message-ID: <tip-8cad6c58c9effb59b830bcf0103d8267ad2e312d@git.kernel.org>
Date:   Mon, 15 Oct 2018 08:09:53 -0700
From:   tip-bot for Jan Kiszka <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     jpoimboe@...hat.com, dave.hansen@...el.com,
        torvalds@...ux-foundation.org, jan.kiszka@...mens.com,
        dvlasenk@...hat.com, bp@...e.de, linux-mm@...ck.org,
        David.Laight@...lab.com, linux-kernel@...r.kernel.org,
        luto@...nel.org, jgross@...e.com, hpa@...or.com, brgerst@...il.com,
        eduval@...zon.com, gregkh@...uxfoundation.org, jroedel@...e.de,
        tglx@...utronix.de, peterz@...radead.org, mingo@...nel.org,
        will.deacon@....com, x86@...nel.org, jkosina@...e.cz,
        boris.ostrovsky@...cle.com, aarcange@...hat.com
Subject: [tip:x86/urgent] x86/entry/32: Clear the CS high bits

Commit-ID:  8cad6c58c9effb59b830bcf0103d8267ad2e312d
Gitweb:     https://git.kernel.org/tip/8cad6c58c9effb59b830bcf0103d8267ad2e312d
Author:     Jan Kiszka <jan.kiszka@...mens.com>
AuthorDate: Mon, 15 Oct 2018 16:09:29 +0200
Committer:  Borislav Petkov <bp@...e.de>
CommitDate: Mon, 15 Oct 2018 16:54:28 +0200

x86/entry/32: Clear the CS high bits

Even if not on an entry stack, the CS's high bits must be
initialized because they are unconditionally evaluated in
PARANOID_EXIT_TO_KERNEL_MODE.

Failing to do so broke the boot on Galileo Gen2 and IOT2000 boards.

 [ bp: Make the commit message tone passive and impartial. ]

Fixes: b92a165df17e ("x86/entry/32: Handle Entry from Kernel-Mode on Entry-Stack")
Signed-off-by: Jan Kiszka <jan.kiszka@...mens.com>
Signed-off-by: Borislav Petkov <bp@...e.de>
Reviewed-by: Joerg Roedel <jroedel@...e.de>
Acked-by: Joerg Roedel <jroedel@...e.de>
CC: "H. Peter Anvin" <hpa@...or.com>
CC: Andrea Arcangeli <aarcange@...hat.com>
CC: Andy Lutomirski <luto@...nel.org>
CC: Boris Ostrovsky <boris.ostrovsky@...cle.com>
CC: Brian Gerst <brgerst@...il.com>
CC: Dave Hansen <dave.hansen@...el.com>
CC: David Laight <David.Laight@...lab.com>
CC: Denys Vlasenko <dvlasenk@...hat.com>
CC: Eduardo Valentin <eduval@...zon.com>
CC: Greg KH <gregkh@...uxfoundation.org>
CC: Ingo Molnar <mingo@...nel.org>
CC: Jiri Kosina <jkosina@...e.cz>
CC: Josh Poimboeuf <jpoimboe@...hat.com>
CC: Juergen Gross <jgross@...e.com>
CC: Linus Torvalds <torvalds@...ux-foundation.org>
CC: Peter Zijlstra <peterz@...radead.org>
CC: Thomas Gleixner <tglx@...utronix.de>
CC: Will Deacon <will.deacon@....com>
CC: aliguori@...zon.com
CC: daniel.gruss@...k.tugraz.at
CC: hughd@...gle.com
CC: keescook@...gle.com
CC: linux-mm <linux-mm@...ck.org>
CC: x86-ml <x86@...nel.org>
Link: http://lkml.kernel.org/r/f271c747-1714-5a5b-a71f-ae189a093b8d@siemens.com
---
 arch/x86/entry/entry_32.S | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 2767c625a52c..fbbf1ba57ec6 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -389,6 +389,13 @@
 	 * that register for the time this macro runs
 	 */
 
+	/*
+	 * The high bits of the CS dword (__csh) are used for
+	 * CS_FROM_ENTRY_STACK and CS_FROM_USER_CR3. Clear them in case
+	 * hardware didn't do this for us.
+	 */
+	andl	$(0x0000ffff), PT_CS(%esp)
+
 	/* Are we on the entry stack? Bail out if not! */
 	movl	PER_CPU_VAR(cpu_entry_area), %ecx
 	addl	$CPU_ENTRY_AREA_entry_stack + SIZEOF_entry_stack, %ecx
@@ -407,12 +414,6 @@
 	/* Load top of task-stack into %edi */
 	movl	TSS_entry2task_stack(%edi), %edi
 
-	/*
-	 * Clear unused upper bits of the dword containing the word-sized CS
-	 * slot in pt_regs in case hardware didn't clear it for us.
-	 */
-	andl	$(0x0000ffff), PT_CS(%esp)
-
 	/* Special case - entry from kernel mode via entry stack */
 #ifdef CONFIG_VM86
 	movl	PT_EFLAGS(%esp), %ecx		# mix EFLAGS and CS