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SubjectRe: [PATCH v3 2/3] x86/ldt: Make modify_ldt optional
Hi Andy,

On Wed, Jul 22, 2015 at 12:23:47PM -0700, Andy Lutomirski wrote:
> The modify_ldt syscall exposes a large attack surface and is
> unnecessary for modern userspace. Make it optional.

Wouldn't you prefer something like this which makes it possible to re-enable
it at runtime so that we can hope distros ship with it disabled by default ?

It's pretty efficient on your ldtgdt testcase :

# echo 1 > /proc/sys/kernel/modify_ldt
# ./a.out
[OK] LDT entry 0 has AR 0x0040FA00 and limit 0x0000000A
[OK] LDT entry 0 has AR 0x00C0FA00 and limit 0x0000AFFF
[OK] LDT entry 1 is invalid
[OK] LDT entry 2 has AR 0x00C0FA00 and limit 0x0000AFFF
[OK] LDT entry 1 is invalid
[OK] LDT entry 2 has AR 0x00C0FA00 and limit 0x0000AFFF
[OK] LDT entry 2 has AR 0x00D0FA00 and limit 0x0000AFFF
[OK] LDT entry 2 has AR 0x00D07A00 and limit 0x0000AFFF
[OK] LDT entry 2 has AR 0x00907A00 and limit 0x0000AFFF
[OK] LDT entry 2 has AR 0x00D07200 and limit 0x0000AFFF
[OK] LDT entry 2 has AR 0x00D07000 and limit 0x0000AFFF
[OK] LDT entry 2 has AR 0x00D07400 and limit 0x0000AFFF
[OK] LDT entry 2 has AR 0x00507600 and limit 0x0000000A
[OK] LDT entry 2 has AR 0x00507E00 and limit 0x0000000A
[OK] LDT entry 2 has AR 0x00507C00 and limit 0x0000000A
[OK] LDT entry 2 has AR 0x00507A00 and limit 0x0000000A
[OK] LDT entry 2 has AR 0x00507800 and limit 0x0000000A
[OK] LDT entry 2 has AR 0x00507800 and limit 0x0000000A
[RUN] Test fork
[OK] LDT entry 2 has AR 0x00507800 and limit 0x0000000A
[OK] LDT entry 1 is invalid
[OK] LDT entry 0 has AR 0x0040FA00 and limit 0x0000000A
[OK] LDT entry 0 has AR 0x00C0FA00 and limit 0x0000AFFF
[OK] LDT entry 1 is invalid
[OK] LDT entry 2 has AR 0x00C0FA00 and limit 0x0000AFFF
[OK] LDT entry 1 is invalid
[OK] LDT entry 2 has AR 0x00C0FA00 and limit 0x0000AFFF
[OK] LDT entry 2 has AR 0x00D0FA00 and limit 0x0000AFFF
[OK] LDT entry 2 has AR 0x00D07A00 and limit 0x0000AFFF
[OK] LDT entry 2 has AR 0x00907A00 and limit 0x0000AFFF
[OK] LDT entry 2 has AR 0x00D07200 and limit 0x0000AFFF
[OK] LDT entry 2 has AR 0x00D07000 and limit 0x0000AFFF
[OK] LDT entry 2 has AR 0x00D07400 and limit 0x0000AFFF
[OK] LDT entry 2 has AR 0x00507600 and limit 0x0000000A
[OK] LDT entry 2 has AR 0x00507E00 and limit 0x0000000A
[OK] LDT entry 2 has AR 0x00507C00 and limit 0x0000000A
[OK] LDT entry 2 has AR 0x00507A00 and limit 0x0000000A
[OK] LDT entry 2 has AR 0x00507800 and limit 0x0000000A
[OK] LDT entry 2 has AR 0x00507800 and limit 0x0000000A
[RUN] Test fork
[OK] Child succeeded
[OK] modify_ldt failure 22
[OK] LDT entry 0 has AR 0x0000F200 and limit 0x00000000
[OK] LDT entry 0 has AR 0x00007200 and limit 0x00000000
[OK] LDT entry 0 has AR 0x0000F000 and limit 0x00000000
[OK] LDT entry 0 has AR 0x00007200 and limit 0x00000000
[OK] LDT entry 0 has AR 0x00007000 and limit 0x00000001
[OK] LDT entry 0 has AR 0x00007000 and limit 0x00000000
[OK] LDT entry 0 is invalid
[OK] LDT entry 0 has AR 0x0040F200 and limit 0x00000000
[OK] LDT entry 0 is invalid
[SKIP] Cannot set affinity to CPU 1


# echo 0 > /proc/sys/kernel/modify_ldt
# ./a.out
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[OK] LDT entry 1 is invalid
[OK] modify_ldt is returned -ENOSYS
[OK] LDT entry 1 is invalid
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[SKIP] Skipping fork test because have no LDT
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[OK] modify_ldt is returned -ENOSYS
[SKIP] Cannot set affinity to CPU 1

The patch is quite small (I stole your comment for the config option).

Willy


diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 226d569..b926f65 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1012,6 +1012,23 @@ config X86_16BIT
this option saves about 300 bytes on i386, or around 6K text
plus 16K runtime memory on x86-64,

+config DEFAULT_MODIFY_LDT_SYSCALL
+ bool "Allow userspace to modify the LDT (local descriptor table)"
+ default y
+ ---help---
+ Linux can allow user programs to install a per-process x86
+ Local Descriptor Table (LDT) using the modify_ldt(2) system
+ call. This is required to run 16-bit or segmented code such as
+ DOSEMU or some Wine programs. It is also used by some very old
+ threading libraries.
+
+ Enabling this feature increases the low-level kernel attack
+ surface. Disabling it disables the modify_ldt(2) system call by
+ default. Note that even when disabled it remains possible to
+ enable it at runtime by setting the sys.kernel.modify_ldt sysctl.
+
+ Say 'N' here if you don't expect to use DOSEMU or Wine often.
+
config X86_ESPFIX32
def_bool y
depends on X86_16BIT && X86_32
diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c
index c37886d..2f10b6c 100644
--- a/arch/x86/kernel/ldt.c
+++ b/arch/x86/kernel/ldt.c
@@ -20,6 +20,12 @@
#include <asm/mmu_context.h>
#include <asm/syscalls.h>

+#ifdef CONFIG_DEFAULT_MODIFY_LDT_SYSCALL
+int sysctl_modify_ldt __read_mostly = 1;
+#else
+int sysctl_modify_ldt __read_mostly = 0;
+#endif
+
#ifdef CONFIG_SMP
static void flush_ldt(void *current_mm)
{
@@ -254,6 +260,9 @@ asmlinkage int sys_modify_ldt(int func, void __user *ptr,
{
int ret = -ENOSYS;

+ if (!sysctl_modify_ldt)
+ return ret;
+
switch (func) {
case 0:
ret = read_ldt(ptr, bytecount);
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 2082b1a..60270c6 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_X86
+extern int sysctl_modify_ldt;
+#endif

/* Constants used for minimum and maximum */
#ifdef CONFIG_LOCKUP_DETECTOR
@@ -962,6 +965,13 @@ static struct ctl_table kern_table[] = {
.mode = 0644,
.proc_handler = proc_dointvec,
},
+ {
+ .procname = "modify_ldt",
+ .data = &sysctl_modify_ldt,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = proc_dointvec,
+ },
#endif
#if defined(CONFIG_MMU)
{

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