[3.11.y.z extended stable] Patch "x86-64, modify_ldt: Ban 16-bit segments on 64-bit kernels" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    x86-64, modify_ldt: Ban 16-bit segments on 64-bit kernels

to the linux-3.11.y-queue branch of the 3.11.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.11.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.11.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From e4c4bd6e2e548f3accc5dc713a372b11e024e520 Mon Sep 17 00:00:00 2001
From: "H. Peter Anvin" <hpa at linux.intel.com>
Date: Sun, 16 Mar 2014 15:31:54 -0700
Subject: x86-64, modify_ldt: Ban 16-bit segments on 64-bit kernels

commit b3b42ac2cbae1f3cecbb6229964a4d48af31d382 upstream.

The IRET instruction, when returning to a 16-bit segment, only
restores the bottom 16 bits of the user space stack pointer.  We have
a software workaround for that ("espfix") for the 32-bit kernel, but
it relies on a nonzero stack segment base which is not available in
32-bit mode.

Since 16-bit support is somewhat crippled anyway on a 64-bit kernel
(no V86 mode), and most (if not quite all) 64-bit processors support
virtualization for the users who really need it, simply reject
attempts at creating a 16-bit segment when running on top of a 64-bit
kernel.

Cc: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: H. Peter Anvin <hpa at linux.intel.com>
Link: http://lkml.kernel.org/n/tip-kicdm89kzw9lldryb1br9od0@git.kernel.org
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 arch/x86/kernel/ldt.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c
index ebc9873..af1d14a 100644
--- a/arch/x86/kernel/ldt.c
+++ b/arch/x86/kernel/ldt.c
@@ -229,6 +229,17 @@ static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
 		}
 	}

+	/*
+	 * On x86-64 we do not support 16-bit segments due to
+	 * IRET leaking the high bits of the kernel stack address.
+	 */
+#ifdef CONFIG_X86_64
+	if (!ldt_info.seg_32bit) {
+		error = -EINVAL;
+		goto out_unlock;
+	}
+#endif
+
 	fill_ldt(&ldt, &ldt_info);
 	if (oldmode)
 		ldt.avl = 0;
--
1.9.1