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Message-ID: <CAGXu5jJ+GHJSgoHk3Vmf=JueVgwkP6ZSVm5kkMbCGBySp2VqmA@mail.gmail.com>
Date: Tue, 27 Jun 2017 16:13:08 -0700
From: Kees Cook <keescook@...omium.org>
To: Salvatore Mesoraca <s.mesoraca16@...il.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
linux-security-module <linux-security-module@...r.kernel.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
Brad Spengler <spender@...ecurity.net>, PaX Team <pageexec@...email.hu>,
Casey Schaufler <casey@...aufler-ca.com>, James Morris <james.l.morris@...cle.com>,
"Serge E. Hallyn" <serge@...lyn.com>, Linux-MM <linux-mm@...ck.org>, "x86@...nel.org" <x86@...nel.org>,
Jann Horn <jannh@...gle.com>, Christoph Hellwig <hch@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC v2 7/9] Trampoline emulation
On Thu, Jun 15, 2017 at 9:42 AM, Salvatore Mesoraca
<s.mesoraca16@...il.com> wrote:
> Some programs need to generate part of their code at runtime. Luckily
> enough, in some cases they only generate well-known code sequences (the
> "trampolines") that can be easily recognized and emulated by the kernel.
> This way WX Protection can still be active, so a potential attacker won't
> be able to generate arbitrary sequences of code, but just those that are
> explicitly allowed. This is not ideal, but it's still better than having WX
> Protection completely disabled.
> In particular S.A.R.A. is able to recognize trampolines used by GCC for
> nested C functions and libffi's trampolines.
> This feature is implemented only on x86_32 and x86_64.
> The assembly sequences used here were originally obtained from PaX source
> code.
See below about the language grsecurity has asked people to use in commit logs.
>
> Signed-off-by: Salvatore Mesoraca <s.mesoraca16@...il.com>
> ---
> security/sara/Kconfig | 17 ++++
> security/sara/include/trampolines.h | 171 ++++++++++++++++++++++++++++++++++++
> security/sara/wxprot.c | 140 +++++++++++++++++++++++++++++
> 3 files changed, 328 insertions(+)
> create mode 100644 security/sara/include/trampolines.h
>
> diff --git a/security/sara/Kconfig b/security/sara/Kconfig
> index 6c74069..f406805 100644
> --- a/security/sara/Kconfig
> +++ b/security/sara/Kconfig
> @@ -96,6 +96,23 @@ choice
> Documentation/security/SARA.rst.
> endchoice
>
> +config SECURITY_SARA_WXPROT_EMUTRAMP
> + bool "Enable emulation for some types of trampolines"
> + depends on SECURITY_SARA_WXPROT
> + depends on X86
> + default y
> + help
> + Some programs and libraries need to execute special small code
> + snippets from non-executable memory pages.
> + Most notable examples are the GCC and libffi trampolines.
> + This features make it possible to execute those trampolines even
> + if they reside in non-executable memory pages.
> + This features need to be enabled on a per-executable basis
> + via user-space utilities.
> + See Documentation/security/SARA.rst. for further information.
> +
> + If unsure, answer y.
> +
> config SECURITY_SARA_WXPROT_DISABLED
> bool "WX protection will be disabled at boot."
> depends on SECURITY_SARA_WXPROT
> diff --git a/security/sara/include/trampolines.h b/security/sara/include/trampolines.h
> new file mode 100644
> index 0000000..eab0a85
> --- /dev/null
> +++ b/security/sara/include/trampolines.h
> @@ -0,0 +1,171 @@
> +/*
> + * S.A.R.A. Linux Security Module
> + *
> + * Copyright (C) 2017 Salvatore Mesoraca <s.mesoraca16@...il.com>
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2, as
> + * published by the Free Software Foundation.
> + *
> + * Assembly sequences used here were copied from
> + * PaX patch by PaX Team <pageexec@...email.hu>
Given this copying, please include the grsecurity/PaX copyright notice
too. Please see the recommendations here:
http://kernsec.org/wiki/index.php/Kernel_Self_Protection_Project/Get_Involved
> + *
> + */
> +
> +#ifndef __SARA_TRAMPOLINES_H
> +#define __SARA_TRAMPOLINES_H
> +#ifdef CONFIG_SECURITY_SARA_WXPROT_EMUTRAMP
> +
> +
> +/* x86_32 */
> +
> +
> +struct libffi_trampoline_x86_32 {
> + unsigned char mov;
> + unsigned int addr1;
> + unsigned char jmp;
> + unsigned int addr2;
> +} __packed;
> +
> +struct gcc_trampoline_x86_32_type1 {
> + unsigned char mov1;
> + unsigned int addr1;
> + unsigned char mov2;
> + unsigned int addr2;
> + unsigned short jmp;
> +} __packed;
> +
> +struct gcc_trampoline_x86_32_type2 {
> + unsigned char mov;
> + unsigned int addr1;
> + unsigned char jmp;
> + unsigned int addr2;
> +} __packed;
> +
> +union trampolines_x86_32 {
> + struct libffi_trampoline_x86_32 lf;
> + struct gcc_trampoline_x86_32_type1 g1;
> + struct gcc_trampoline_x86_32_type2 g2;
> +};
> +
> +#define is_valid_libffi_trampoline_x86_32(UNION) \
> + (UNION.lf.mov == 0xB8 && \
> + UNION.lf.jmp == 0xE9)
> +
> +#define emulate_libffi_trampoline_x86_32(UNION, REGS) do { \
> + (REGS)->ax = UNION.lf.addr1; \
> + (REGS)->ip = (unsigned int) ((REGS)->ip + \
> + UNION.lf.addr2 + \
> + sizeof(UNION.lf)); \
> +} while (0)
> +
> +#define is_valid_gcc_trampoline_x86_32_type1(UNION, REGS) \
> + (UNION.g1.mov1 == 0xB9 && \
> + UNION.g1.mov2 == 0xB8 && \
> + UNION.g1.jmp == 0xE0FF && \
> + REGS->ip > REGS->sp)
> +
> +#define emulate_gcc_trampoline_x86_32_type1(UNION, REGS) do { \
> + (REGS)->cx = UNION.g1.addr1; \
> + (REGS)->ax = UNION.g1.addr2; \
> + (REGS)->ip = UNION.g1.addr2; \
> +} while (0)
> +
> +#define is_valid_gcc_trampoline_x86_32_type2(UNION, REGS) \
> + (UNION.g2.mov == 0xB9 && \
> + UNION.g2.jmp == 0xE9 && \
> + REGS->ip > REGS->sp)
> +
> +#define emulate_gcc_trampoline_x86_32_type2(UNION, REGS) do { \
> + (REGS)->cx = UNION.g2.addr1; \
> + (REGS)->ip = (unsigned int) ((REGS)->ip + \
> + UNION.g2.addr2 + \
> + sizeof(UNION.g2)); \
> +} while (0)
These all seem like they need to live in arch/x86/... somewhere rather
than in the LSM, but maybe this isn't needed on other architectures?
This seems to be very arch and compiler specific...
-Kees
--
Kees Cook
Pixel Security
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