linux-kernel - Re: [PATCH 07/11] kexec: Disable in a secure boot environment

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Date:	Wed, 05 Sep 2012 17:13:49 -0400
From:	Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:	Matthew Garrett <mjg@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org, linux-efi@...r.kernel.org
Subject: Re: [PATCH 07/11] kexec: Disable in a secure boot environment

On Tue, 2012-09-04 at 11:55 -0400, Matthew Garrett wrote:
> kexec could be used as a vector for a malicious user to use a signed kernel
> to circumvent the secure boot trust model. In the long run we'll want to
> support signed kexec payloads, but for the moment we should just disable
> loading entirely in that situation.
> 
> Signed-off-by: Matthew Garrett <mjg@...hat.com>
> ---
>  kernel/kexec.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/kexec.c b/kernel/kexec.c
> index 0668d58..48852ec 100644
> --- a/kernel/kexec.c
> +++ b/kernel/kexec.c
> @@ -944,7 +944,7 @@ SYSCALL_DEFINE4(kexec_load, unsigned long, entry, unsigned long, nr_segments,
>  	int result;
> 
>  	/* We only trust the superuser with rebooting the system. */
> -	if (!capable(CAP_SYS_BOOT))
> +	if (!capable(CAP_SYS_BOOT) || !capable(CAP_SECURE_FIRMWARE))
>  		return -EPERM;
> 
>  	/*

Normally capabilities provide additional permissions. So if you don't
have the capability, an errno is returned.  CAP_SYS_BOOT is a good
example.  With CAP_SECURE_FIRMWARE, it reads backwards - if not
CAP_SECURE_FIRMWARE, return error.  I think you want to invert the name
to CAP_NOT_SECURE_FIRMWARE, CAP_NOT_SECURE_BOOT or perhaps
CAP_UNSECURED_BOOT.

Mimi

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