linux-kernel - Re: [PATCH 06/24] kexec_load: Disable at runtime if the kernel is locked down

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Message-ID: <87po35k1q6.fsf@xmission.com>
Date:   Wed, 11 Apr 2018 14:00:17 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     David Howells <dhowells@...hat.com>
Cc:     torvalds@...ux-foundation.org, linux-man@...r.kernel.org,
        linux-api@...r.kernel.org, jmorris@...ei.org,
        linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: Re: [PATCH 06/24] kexec_load: Disable at runtime if the kernel is locked down

David Howells <dhowells@...hat.com> writes:

> From: Matthew Garrett <mjg59@...f.ucam.org>
>
> The kexec_load() syscall permits the loading and execution of arbitrary
> code in ring 0, which is something that lock-down is meant to prevent. It
> makes sense to disable kexec_load() in this situation.
>
> This does not affect kexec_file_load() syscall which can check for a
> signature on the image to be booted.

Maybing I am missing it but I am not seeing anything that would require
kexec_file_load be configured such that it checks the loaded kernel.

Without that I don't see the point of disabling kexec_load.

Nacked-by: "Eric W. Biederman" <ebiederm@...ssion.com>


Eric




> Signed-off-by: Matthew Garrett <mjg59@...f.ucam.org>
> Signed-off-by: David Howells <dhowells@...hat.com>
> Acked-by: Dave Young <dyoung@...hat.com>
> Reviewed-by: "Lee, Chun-Yi" <jlee@...e.com>
> Reviewed-by: James Morris <james.l.morris@...cle.com>
> cc: kexec@...ts.infradead.org
> ---
>
>  kernel/kexec.c |    7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/kernel/kexec.c b/kernel/kexec.c
> index aed8fb2564b3..1553ac765e73 100644
> --- a/kernel/kexec.c
> +++ b/kernel/kexec.c
> @@ -199,6 +199,13 @@ static inline int kexec_load_check(unsigned long nr_segments,
>  	if (!capable(CAP_SYS_BOOT) || kexec_load_disabled)
>  		return -EPERM;
>  
> +	/*
> +	 * kexec can be used to circumvent module loading restrictions, so
> +	 * prevent loading in that case
> +	 */
> +	if (kernel_is_locked_down("kexec of unsigned images"))
> +		return -EPERM;
> +
>  	/*
>  	 * Verify we have a legal set of flags
>  	 * This leaves us room for future extensions.