linux-kernel - [ 05/25] x86-32, espfix: Remove filter for espfix32 due to race

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Message-Id: <20141206174148.631937314@1wt.eu>
Date:	Sat, 06 Dec 2014 18:41:53 +0100
From:	Willy Tarreau <w@....eu>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Ben Hutchings <ben@...adent.org.uk>, Willy Tarreau <w@....eu>
Subject: [ 05/25] x86-32, espfix: Remove filter for espfix32 due to race

2.6.32-longterm review patch.  If anyone has any objections, please let me know.

------------------

From: "H. Peter Anvin" <hpa@...ux.intel.com>

commit 246f2d2ee1d715e1077fc47d61c394569c8ee692 upstream.

It is not safe to use LAR to filter when to go down the espfix path,
because the LDT is per-process (rather than per-thread) and another
thread might change the descriptors behind our back.  Fortunately it
is always *safe* (if a bit slow) to go down the espfix path, and a
32-bit LDT stack segment is extremely rare.

Signed-off-by: H. Peter Anvin <hpa@...ux.intel.com>
Link: http://lkml.kernel.org/r/1398816946-3351-1-git-send-email-hpa@linux.intel.com
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
(cherry picked from 3.2 commit 6806fa8b6795aba9be8742a8f598f60eed26f875)
Signed-off-by: Willy Tarreau <w@....eu>
---
 arch/x86/kernel/entry_32.S | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index 8b5370c..db7dbe7 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -571,11 +571,6 @@ ENTRY(iret_exc)
 
 	CFI_RESTORE_STATE
 ldt_ss:
-	larl PT_OLDSS(%esp), %eax
-	jnz restore_nocheck
-	testl $0x00400000, %eax		# returning to 32bit stack?
-	jnz restore_nocheck		# allright, normal return
-
 #ifdef CONFIG_PARAVIRT
 	/*
 	 * The kernel can't run on a non-flat stack if paravirt mode
-- 
1.7.12.2.21.g234cd45.dirty



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