linux-kernel - Re: [RFC PATCH v2 2/6] x86/arch_prctl: add ARCH_GET_NOPTI and ARCH_SET

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Date:   Tue, 9 Jan 2018 15:17:13 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Willy Tarreau <w@....eu>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Kees Cook <keescook@...omium.org>
Subject: Re: [RFC PATCH v2 2/6] x86/arch_prctl: add ARCH_GET_NOPTI and
 ARCH_SET_NOPTI to enable/disable PTI

On Tue, Jan 09, 2018 at 01:56:16PM +0100, Willy Tarreau wrote:
> This allows to report the current state of the PTI protection and to
> enable or disable it for the current process. The state change is only
> allowed if the mm is not shared (no threads have been created yet).
> 
> Setting the flag to disable the protection is subject to CAP_SYS_RAWIO.
> However it is possible to re-enable the protection without this privilege.
> 
> Signed-off-by: Willy Tarreau <w@....eu>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Brian Gerst <brgerst@...il.com>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Josh Poimboeuf <jpoimboe@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Kees Cook <keescook@...omium.org>
> 
> v2:
>   - use {set,clear}_thread_flag() as recommended by Peter
>   - use task->mm->context.pti_disable instead of task flag
>   - check for mm_users == 1
>   - check for CAP_SYS_RAWIO only when setting, not clearing
>   - make the code depend on CONFIG_PAGE_TABLE_ISOLATION
> ---
>  arch/x86/include/uapi/asm/prctl.h |  3 +++
>  arch/x86/kernel/process_64.c      | 15 +++++++++++++++
>  2 files changed, 18 insertions(+)
> 
> diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
> index 5a6aac9..1f1b5bc 100644
> --- a/arch/x86/include/uapi/asm/prctl.h
> +++ b/arch/x86/include/uapi/asm/prctl.h
> @@ -10,6 +10,9 @@
>  #define ARCH_GET_CPUID		0x1011
>  #define ARCH_SET_CPUID		0x1012
>  
> +#define ARCH_GET_NOPTI		0x1021
> +#define ARCH_SET_NOPTI		0x1022
> +
>  #define ARCH_MAP_VDSO_X32	0x2001
>  #define ARCH_MAP_VDSO_32	0x2002
>  #define ARCH_MAP_VDSO_64	0x2003
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index c754662..9516310 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -654,7 +654,22 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
>  		ret = put_user(base, (unsigned long __user *)arg2);
>  		break;
>  	}
> +#ifdef CONFIG_PAGE_TABLE_ISOLATION

Actually, I meant to add a new CONFIG item only for this feature which
depends on CONFIG_PAGE_TABLE_ISOLATION. So that people can disable the
per-mm thing when they don't want it.

Thx.

-- 
Regards/Gruss,
    Boris.

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