linux-kernel - Re: [PATCH 04/27] Restrict /dev/mem and /dev/kmem when the kernel is locked down

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Message-ID: <CABawtvMWhHJHtog9BWNVJ3Xk-ipOhPp+TGSVNjjZD27QAfB+Nw@mail.gmail.com>
Date:   Tue, 24 Oct 2017 18:48:44 +0800
From:   Ethan Zhao <ethan.kernel@...il.com>
To:     David Howells <dhowells@...hat.com>
Cc:     joeyli <jlee@...e.com>, linux-security-module@...r.kernel.org,
        gnomes@...rguk.ukuu.org.uk, linux-efi@...r.kernel.org,
        Greg KH <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>, jforbes@...hat.com
Subject: Re: [PATCH 04/27] Restrict /dev/mem and /dev/kmem when the kernel is
 locked down

David，

    May I ask a question here -- Is it intentionally enabling the
read-only mode, so userspace
tools like dmidecode could work with kernel_is_locked_down ?  while it
was impossible to work
with the attached patch applied. Is it a security policy change with
secure boot ?

Thanks,
Ethan

On Mon, Oct 23, 2017 at 10:34 PM, David Howells <dhowells@...hat.com> wrote:
> I think I should replace this patch with the attached.  This will prevent
> /dev/mem, /dev/kmem and /dev/port from being *opened*, and thereby preventing
> read, write and ioctl.
>
> David
> ---
> commit e68daa2256986932b9a7d6709cf9e24b30d93583
> Author: Matthew Garrett <matthew.garrett@...ula.com>
> Date:   Wed May 24 14:56:02 2017 +0100
>
>     Restrict /dev/{mem,kmem,port} when the kernel is locked down
>
>     Allowing users to read and write to core kernel memory makes it possible
>     for the kernel to be subverted, avoiding module loading restrictions, and
>     also to steal cryptographic information.
>
>     Disallow /dev/mem and /dev/kmem from being opened this when the kernel has
>     been locked down to prevent this.
>
>     Also disallow /dev/port from being opened to prevent raw ioport access and
>     thus DMA from being used to accomplish the same thing.
>
>     Signed-off-by: Matthew Garrett <matthew.garrett@...ula.com>
>     Signed-off-by: David Howells <dhowells@...hat.com>
>     Reviewed-by: "Lee, Chun-Yi" <jlee@...e.com>
>
> diff --git a/drivers/char/mem.c b/drivers/char/mem.c
> index 593a8818aca9..0ce5ac0a5c6b 100644
> --- a/drivers/char/mem.c
> +++ b/drivers/char/mem.c
> @@ -762,6 +762,8 @@ static loff_t memory_lseek(struct file *file, loff_t offset, int orig)
>
>  static int open_port(struct inode *inode, struct file *filp)
>  {
> +       if (kernel_is_locked_down("/dev/mem,kmem,port"))
> +               return -EPERM;
>         return capable(CAP_SYS_RAWIO) ? 0 : -EPERM;
>  }
>