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Message-Id: <1438626217-23970-3-git-send-email-w@1wt.eu>
Date: Mon, 3 Aug 2015 20:23:37 +0200
From: Willy Tarreau <w@....eu>
To: Andy Lutomirski <luto@...nel.org>,
Kees Cook <keescook@...omium.org>
Cc: Willy Tarreau <w@....eu>, Steven Rostedt <rostedt@...dmis.org>,
"security@...nel.org" <security@...nel.org>,
X86 ML <x86@...nel.org>, Borislav Petkov <bp@...en8.de>,
Sasha Levin <sasha.levin@...cle.com>,
LKML <linux-kernel@...r.kernel.org>,
Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
Boris Ostrovsky <boris.ostrovsky@...cle.com>,
Andrew Cooper <andrew.cooper3@...rix.com>,
Jan Beulich <jbeulich@...e.com>,
xen-devel <xen-devel@...ts.xen.org>
Subject: [PATCH 2/2] x86/ldt: allow to disable modify_ldt at runtime
For distros who prefer not to take the risk of completely disabling the
modify_ldt syscall using CONFIG_MODIFY_LDT_SYSCALL, this patch adds a
sysctl to enable, temporarily disable, or permanently disable it at
runtime, and proposes to temporarily disable it by default. This can be
a safe alternative. A message is logged if an attempt was stopped so that
it's easy to spot if/when it is needed.
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Kees Cook <keescook@...omium.org>
Signed-off-by: Willy Tarreau <w@....eu>
---
Documentation/sysctl/kernel.txt | 16 ++++++++++++++++
arch/x86/Kconfig | 17 +++++++++++++++++
arch/x86/kernel/ldt.c | 15 +++++++++++++++
kernel/sysctl.c | 14 ++++++++++++++
4 files changed, 62 insertions(+)
diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
index 6fccb69..55648b9 100644
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -41,6 +41,7 @@ show up in /proc/sys/kernel:
- kptr_restrict
- kstack_depth_to_print [ X86 only ]
- l2cr [ PPC only ]
+- modify_ldt [ X86 only ]
- modprobe ==> Documentation/debugging-modules.txt
- modules_disabled
- msg_next_id [ sysv ipc ]
@@ -391,6 +392,21 @@ This flag controls the L2 cache of G3 processor boards. If
==============================================================
+modify_ldt: (X86 only)
+
+Enables (1), disables (0) or permanently disables (-1) the modify_ldt syscall.
+Modifying the LDT (Local Descriptor Table) may be needed to run a 16-bit or
+segmented code such as Dosemu or Wine. This is done via a system call which is
+not needed to run portable applications, and which can sometimes be abused to
+exploit some weaknesses of the architecture, opening new vulnerabilities.
+
+This sysctl allows one to increase the system's security by disabling the
+system call, or to restore compatibility with specific applications when it
+was already disabled. When permanently disabled, it is not possible to change
+the value anymore until the next system reboot.
+
+==============================================================
+
modules_disabled:
A toggle value indicating if modules are allowed to be loaded
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index beabf30..88d10a0 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2069,6 +2069,23 @@ config MODIFY_LDT_SYSCALL
surface. Disabling it removes the modify_ldt(2) system call.
Saying 'N' here may make sense for embedded or server kernels.
+ If really unsure, say 'Y', you'll be able to disable it at runtime.
+
+config DEFAULT_MODIFY_LDT_SYSCALL
+ bool "Allow userspace to modify the LDT by default"
+ depends on MODIFY_LDT_SYSCALL
+ default y
+ ---help---
+ Modifying the LDT (Local Descriptor Table) may be needed to run a
+ 16-bit or segmented code such as Dosemu or Wine. This is done via
+ a system call which is not needed to run portable applications,
+ and which can sometimes be abused to exploit some weaknesses of
+ the architecture, opening new vulnerabilities.
+
+ For this reason this option allows one to enable or disable the
+ feature at runtime. It is recommended to say 'N' here to leave
+ the system protected, and to enable it at runtime only if needed
+ by setting the sys.kernel.modify_ldt sysctl.
source "kernel/livepatch/Kconfig"
diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c
index 2bcc052..420fc8f 100644
--- a/arch/x86/kernel/ldt.c
+++ b/arch/x86/kernel/ldt.c
@@ -11,6 +11,7 @@
#include <linux/sched.h>
#include <linux/string.h>
#include <linux/mm.h>
+#include <linux/ratelimit.h>
#include <linux/smp.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>
@@ -21,6 +22,11 @@
#include <asm/mmu_context.h>
#include <asm/syscalls.h>
+#ifdef CONFIG_MODIFY_LDT_SYSCALL
+int sysctl_modify_ldt __read_mostly =
+ IS_ENABLED(CONFIG_DEFAULT_MODIFY_LDT_SYSCALL);
+#endif
+
/* context.lock is held for us, so we don't need any locking. */
static void flush_ldt(void *current_mm)
{
@@ -276,6 +282,15 @@ asmlinkage int sys_modify_ldt(int func, void __user *ptr,
{
int ret = -ENOSYS;
+ if (sysctl_modify_ldt <= 0) {
+ printk_ratelimited(KERN_INFO
+ "Denied a call to modify_ldt() from %s[%d] (uid: %d)."
+ " Adjust sysctl if this was not an exploit attempt.\n",
+ current->comm, task_pid_nr(current),
+ from_kuid_munged(current_user_ns(), current_uid()));
+ return ret;
+ }
+
switch (func) {
case 0:
ret = read_ldt(ptr, bytecount);
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 86c95a8..ec1170d 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -111,6 +111,9 @@ extern int sysctl_nr_open_min, sysctl_nr_open_max;
#ifndef CONFIG_MMU
extern int sysctl_nr_trim_pages;
#endif
+#ifdef CONFIG_MODIFY_LDT_SYSCALL
+extern int sysctl_modify_ldt;
+#endif
/* Constants used for minimum and maximum */
#ifdef CONFIG_LOCKUP_DETECTOR
@@ -963,6 +966,17 @@ static struct ctl_table kern_table[] = {
.mode = 0644,
.proc_handler = proc_dointvec,
},
+#ifdef CONFIG_MODIFY_LDT_SYSCALL
+ {
+ .procname = "modify_ldt",
+ .data = &sysctl_modify_ldt,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = proc_dointvec_minmax_negperm,
+ .extra1 = &neg_one,
+ .extra2 = &one,
+ },
+#endif
#endif
#if defined(CONFIG_MMU)
{
--
1.7.12.1
--
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