[3.13.y-ckt stable] Patch "x86/tls: Disallow unusual TLS segments" has been added to staging queue

[Kamal Mostafa]

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

    x86/tls: Disallow unusual TLS segments

to the linux-3.13.y-queue branch of the 3.13.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11-ckt15.

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

For more information about the 3.13.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 4a4641343322c027248b431505eef207c04fba53 Mon Sep 17 00:00:00 2001
From: Andy Lutomirski <luto at amacapital.net>
Date: Thu, 4 Dec 2014 16:48:17 -0800
Subject: x86/tls: Disallow unusual TLS segments

commit 0e58af4e1d2166e9e33375a0f121e4867010d4f8 upstream.

Users have no business installing custom code segments into the
GDT, and segments that are not present but are otherwise valid
are a historical source of interesting attacks.

For completeness, block attempts to set the L bit.  (Prior to
this patch, the L bit would have been silently dropped.)

This is an ABI break.  I've checked glibc, musl, and Wine, and
none of them look like they'll have any trouble.

Note to stable maintainers: this is a hardening patch that fixes
no known bugs.  Given the possibility of ABI issues, this
probably shouldn't be backported quickly.

Signed-off-by: Andy Lutomirski <luto at amacapital.net>
Acked-by: H. Peter Anvin <hpa at zytor.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk at oracle.com>
Cc: Linus Torvalds <torvalds at linux-foundation.org>
Cc: security at kernel.org <security at kernel.org>
Cc: Willy Tarreau <w at 1wt.eu>
Signed-off-by: Ingo Molnar <mingo at kernel.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 arch/x86/kernel/tls.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
index e7650bd..3e551ee 100644
--- a/arch/x86/kernel/tls.c
+++ b/arch/x86/kernel/tls.c
@@ -39,6 +39,28 @@ static bool tls_desc_okay(const struct user_desc *info)
 	if (!info->seg_32bit)
 		return false;

+	/* Only allow data segments in the TLS array. */
+	if (info->contents > 1)
+		return false;
+
+	/*
+	 * Non-present segments with DPL 3 present an interesting attack
+	 * surface.  The kernel should handle such segments correctly,
+	 * but TLS is very difficult to protect in a sandbox, so prevent
+	 * such segments from being created.
+	 *
+	 * If userspace needs to remove a TLS entry, it can still delete
+	 * it outright.
+	 */
+	if (info->seg_not_present)
+		return false;
+
+#ifdef CONFIG_X86_64
+	/* The L bit makes no sense for data. */
+	if (info->lm)
+		return false;
+#endif
+
 	return true;
 }

--
1.9.1