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Message-ID: <CAGXu5jKOQPNuXR5At7984FSMHHrT7G6z7Tx5SdfP+62WON7JNA@mail.gmail.com>
Date: Wed, 7 Mar 2018 11:21:46 -0800
From: Kees Cook <keescook@...omium.org>
To: Andy Lutomirski <luto@...nel.org>
Cc: X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>, Borislav Petkov <bp@...en8.de>,
Dominik Brodowski <linux@...inikbrodowski.net>
Subject: Re: [PATCH] x86/vsyscall/64: Drop "native" vsyscalls
On Wed, Mar 7, 2018 at 11:12 AM, Andy Lutomirski <luto@...nel.org> wrote:
> Since Linux 3.2, vsyscalls have been deprecated and slow. From 3.2
> on, Linux had three vsyscall modes: "native", "emulate", and "none".
>
> "emulate" is the default. All known user programs work correctly in
> emulate mode, but vsyscalls turn into page faults and are emulated.
> This is very slow. In "native" mode, the vsyscall page is easily
> usable as an exploit gadget, but vsyscalls are a bit faster -- they
> turn into normal syscalls. (This is in contrast to vDSO functions,
> which can be much faster than syscalls.) In "none" mode, there are
> no vsyscalls.
>
> For all practical purposes, "native" was really just a chicken bit
> in case something went wrong with the emulation. It's been over six
> years, and nothing has gone wrong. Delete it.
>
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
Acked-by: Kees Cook <keescook@...omium.org>
I related news, I wonder how long before we can switch from EMULATE to
NONE as the default? glibc 2.15 (which stopped using vsyscall) is (not
coincidentally) 6 years old too...
-Kees
> ---
> arch/x86/Kconfig | 11 +----------
> arch/x86/entry/vsyscall/vsyscall_64.c | 16 +++-------------
> arch/x86/include/asm/pgtable_types.h | 2 --
> tools/testing/selftests/x86/test_vsyscall.c | 11 ++++++-----
> 4 files changed, 10 insertions(+), 30 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index eb7f43f23521..0fa71a78ec99 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2307,7 +2307,7 @@ choice
> it can be used to assist security vulnerability exploitation.
>
> This setting can be changed at boot time via the kernel command
> - line parameter vsyscall=[native|emulate|none].
> + line parameter vsyscall=[emulate|none].
>
> On a system with recent enough glibc (2.14 or newer) and no
> static binaries, you can say None without a performance penalty
> @@ -2315,15 +2315,6 @@ choice
>
> If unsure, select "Emulate".
>
> - config LEGACY_VSYSCALL_NATIVE
> - bool "Native"
> - help
> - Actual executable code is located in the fixed vsyscall
> - address mapping, implementing time() efficiently. Since
> - this makes the mapping executable, it can be used during
> - security vulnerability exploitation (traditionally as
> - ROP gadgets). This configuration is not recommended.
> -
> config LEGACY_VSYSCALL_EMULATE
> bool "Emulate"
> help
> diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
> index 577fa8adb785..8560ef68a9d6 100644
> --- a/arch/x86/entry/vsyscall/vsyscall_64.c
> +++ b/arch/x86/entry/vsyscall/vsyscall_64.c
> @@ -42,10 +42,8 @@
> #define CREATE_TRACE_POINTS
> #include "vsyscall_trace.h"
>
> -static enum { EMULATE, NATIVE, NONE } vsyscall_mode =
> -#if defined(CONFIG_LEGACY_VSYSCALL_NATIVE)
> - NATIVE;
> -#elif defined(CONFIG_LEGACY_VSYSCALL_NONE)
> +static enum { EMULATE, NONE } vsyscall_mode =
> +#ifdef CONFIG_LEGACY_VSYSCALL_NONE
> NONE;
> #else
> EMULATE;
> @@ -56,8 +54,6 @@ static int __init vsyscall_setup(char *str)
> if (str) {
> if (!strcmp("emulate", str))
> vsyscall_mode = EMULATE;
> - else if (!strcmp("native", str))
> - vsyscall_mode = NATIVE;
> else if (!strcmp("none", str))
> vsyscall_mode = NONE;
> else
> @@ -139,10 +135,6 @@ bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
>
> WARN_ON_ONCE(address != regs->ip);
>
> - /* This should be unreachable in NATIVE mode. */
> - if (WARN_ON(vsyscall_mode == NATIVE))
> - return false;
> -
> if (vsyscall_mode == NONE) {
> warn_bad_vsyscall(KERN_INFO, regs,
> "vsyscall attempted with vsyscall=none");
> @@ -370,9 +362,7 @@ void __init map_vsyscall(void)
>
> if (vsyscall_mode != NONE) {
> __set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
> - vsyscall_mode == NATIVE
> - ? PAGE_KERNEL_VSYSCALL
> - : PAGE_KERNEL_VVAR);
> + PAGE_KERNEL_VVAR);
> set_vsyscall_pgtable_user_bits(swapper_pg_dir);
> }
>
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index 246f15b4e64c..acfe755562a6 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -174,7 +174,6 @@ enum page_cache_mode {
> #define __PAGE_KERNEL_RO (__PAGE_KERNEL & ~_PAGE_RW)
> #define __PAGE_KERNEL_RX (__PAGE_KERNEL_EXEC & ~_PAGE_RW)
> #define __PAGE_KERNEL_NOCACHE (__PAGE_KERNEL | _PAGE_NOCACHE)
> -#define __PAGE_KERNEL_VSYSCALL (__PAGE_KERNEL_RX | _PAGE_USER)
> #define __PAGE_KERNEL_VVAR (__PAGE_KERNEL_RO | _PAGE_USER)
> #define __PAGE_KERNEL_LARGE (__PAGE_KERNEL | _PAGE_PSE)
> #define __PAGE_KERNEL_LARGE_EXEC (__PAGE_KERNEL_EXEC | _PAGE_PSE)
> @@ -206,7 +205,6 @@ enum page_cache_mode {
> #define PAGE_KERNEL_NOCACHE __pgprot(__PAGE_KERNEL_NOCACHE | _PAGE_ENC)
> #define PAGE_KERNEL_LARGE __pgprot(__PAGE_KERNEL_LARGE | _PAGE_ENC)
> #define PAGE_KERNEL_LARGE_EXEC __pgprot(__PAGE_KERNEL_LARGE_EXEC | _PAGE_ENC)
> -#define PAGE_KERNEL_VSYSCALL __pgprot(__PAGE_KERNEL_VSYSCALL | _PAGE_ENC)
> #define PAGE_KERNEL_VVAR __pgprot(__PAGE_KERNEL_VVAR | _PAGE_ENC)
>
> #define PAGE_KERNEL_IO __pgprot(__PAGE_KERNEL_IO)
> diff --git a/tools/testing/selftests/x86/test_vsyscall.c b/tools/testing/selftests/x86/test_vsyscall.c
> index be81621446f0..0b4f1cc2291c 100644
> --- a/tools/testing/selftests/x86/test_vsyscall.c
> +++ b/tools/testing/selftests/x86/test_vsyscall.c
> @@ -450,7 +450,7 @@ static void sigtrap(int sig, siginfo_t *info, void *ctx_void)
> num_vsyscall_traps++;
> }
>
> -static int test_native_vsyscall(void)
> +static int test_emulation(void)
> {
> time_t tmp;
> bool is_native;
> @@ -458,7 +458,7 @@ static int test_native_vsyscall(void)
> if (!vtime)
> return 0;
>
> - printf("[RUN]\tchecking for native vsyscall\n");
> + printf("[RUN]\tchecking that vsyscalls are emulated\n");
> sethandler(SIGTRAP, sigtrap, 0);
> set_eflags(get_eflags() | X86_EFLAGS_TF);
> vtime(&tmp);
> @@ -474,11 +474,12 @@ static int test_native_vsyscall(void)
> */
> is_native = (num_vsyscall_traps > 1);
>
> - printf("\tvsyscalls are %s (%d instructions in vsyscall page)\n",
> + printf("[%s]\tvsyscalls are %s (%d instructions in vsyscall page)\n",
> + (is_native ? "FAIL" : "OK"),
> (is_native ? "native" : "emulated"),
> (int)num_vsyscall_traps);
>
> - return 0;
> + return is_native;
> }
> #endif
>
> @@ -498,7 +499,7 @@ int main(int argc, char **argv)
> nerrs += test_vsys_r();
>
> #ifdef __x86_64__
> - nerrs += test_native_vsyscall();
> + nerrs += test_emulation();
> #endif
>
> return nerrs ? 1 : 0;
> --
> 2.14.3
>
--
Kees Cook
Pixel Security
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