linux-kernel - Re: [patch V3 17/20] x86/iopl: Restrict iopl() permission scope

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Date:   Thu, 14 Nov 2019 19:13:24 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Willy Tarreau <w@....eu>, Juergen Gross <jgross@...e.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch V3 17/20] x86/iopl: Restrict iopl() permission scope

On Wed, Nov 13, 2019 at 09:42:57PM +0100, Thomas Gleixner wrote:
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> The access to the full I/O port range can be also provided by the TSS I/O
> bitmap, but that would require to copy 8k of data on scheduling in the
> task. As shown with the sched out optimization TSS.io_bitmap_base can be
> used to switch the incoming task to a preallocated I/O bitmap which has all
> bits zero, i.e. allows access to all I/O ports.
> 
> Implementing this allows to provide an iopl() emulation mode which restricts
> the IOPL level 3 permissions to I/O port access but removes the STI/CLI
> permission which is coming with the hardware IOPL mechansim.
> 
> Provide a config option to switch IOPL to emulation mode, make it the
> default and while at it also provide an option to disable IOPL completely.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Acked-by: Andy Lutomirski <luto@...nel.org>
> ---
> V3:	Folded the missing NULL pointer check, reduced preempt disable
> 	scope (Ingo)
> 
> V2:	Fixed the 32bit build fail by increasing the cpu entry area size
> 	Move the TSS update out of the iopl() emulation code.
> ---
>  arch/x86/Kconfig                        |   32 +++++++++++
>  arch/x86/include/asm/pgtable_32_types.h |    2 
>  arch/x86/include/asm/processor.h        |   28 +++++++---
>  arch/x86/kernel/cpu/common.c            |    5 +
>  arch/x86/kernel/ioport.c                |   87 ++++++++++++++++++++++----------
>  arch/x86/kernel/process.c               |   28 ++++++----
>  6 files changed, 137 insertions(+), 45 deletions(-)
> 
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1254,6 +1254,38 @@ config X86_VSYSCALL_EMULATION
>  	 Disabling this option saves about 7K of kernel size and
>  	 possibly 4K of additional runtime pagetable memory.
>  
> +choice
> +	prompt "IOPL"
> +	default X86_IOPL_EMULATION
> +
> +config X86_IOPL_EMULATION
> +	bool "IOPL Emulation"
> +	---help---
> +	  Legacy IOPL support is an overbroad mechanism which allows user
> +	  space aside of accessing all 65536 I/O ports also to disable
> +	  interrupts. To gain this access the caller needs CAP_SYS_RAWIO
> +	  capabilities and permission from eventually active security

I think you mean here: s/eventually/potentially/ or so. "eventually" is
one of the false friends. :)

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette