linux-kernel - [tip:x86/asm] x86/segments/64: When load_gs

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Date:	Fri, 29 Apr 2016 03:49:53 -0700
From:	tip-bot for Andy Lutomirski <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	bp@...en8.de, tglx@...utronix.de, dvlasenk@...hat.com,
	mingo@...nel.org, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org, peterz@...radead.org,
	luto@...nel.org, luto@...capital.net, brgerst@...il.com,
	hpa@...or.com
Subject: [tip:x86/asm] x86/segments/64: When load_gs_index fails, clear the
 base

Commit-ID:  b038c842b385f1470f991078e71b7c5b084a7341
Gitweb:     http://git.kernel.org/tip/b038c842b385f1470f991078e71b7c5b084a7341
Author:     Andy Lutomirski <luto@...nel.org>
AuthorDate: Tue, 26 Apr 2016 12:23:27 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 29 Apr 2016 11:56:41 +0200

x86/segments/64: When load_gs_index fails, clear the base

On AMD CPUs, a failed load_gs_base currently may not clear the FS
base.  Fix it.

Signed-off-by: Andy Lutomirski <luto@...nel.org>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/1a6c4d3a8a4e7be79ba448b42685e0321d50c14c.1461698311.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/entry/entry_64.S | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 1693c17..6344629 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -794,6 +794,12 @@ END(native_load_gs_index)
 	/* running with kernelgs */
 bad_gs:
 	SWAPGS					/* switch back to user gs */
+.macro ZAP_GS
+	/* This can't be a string because the preprocessor needs to see it. */
+	movl $__USER_DS, %eax
+	movl %eax, %gs
+.endm
+	ALTERNATIVE "", "ZAP_GS", X86_BUG_NULL_SEG
 	xorl	%eax, %eax
 	movl	%eax, %gs
 	jmp	2b