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Message-ID: <CAGXu5j+yEWLuxJ9JUGyN7BxME+iOVXfpFgrZ-_2WXUn6A=0dVg@mail.gmail.com>
Date: Thu, 23 Jul 2015 16:36:37 -0700
From: Kees Cook <keescook@...omium.org>
To: Willy Tarreau <w@....eu>
Cc: Andy Lutomirski <luto@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
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: Re: [PATCH v3 2/3] x86/ldt: Make modify_ldt optional
On Thu, Jul 23, 2015 at 3:24 AM, Willy Tarreau <w@....eu> wrote:
> 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)
> {
I've been pondering something like this that is even MORE generic, for
any syscall. Something like a "syscalls" directory under
/proc/sys/kernel, with 1 entry per syscall. "0" is "available", "1" is
disabled, and "-1" disabled until next boot.
-Kees
--
Kees Cook
Chrome OS Security
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