Re: [PATCH V34 06/29] kexec_load: Disable at runtime if the kernel is locked down

Kees Cook
2019-06-22T23:53:03Z

On Fri, Jun 21, 2019 at 05:03:35PM -0700, Matthew Garrett wrote:
> From: Matthew Garrett <mjg59@srcf.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.
>
> Signed-off-by: David Howells <dhowells@redhat.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> Signed-off-by: Matthew Garrett <mjg59@google.com>
> Acked-by: Dave Young <dyoung@redhat.com>
> cc: kexec@lists.infradead.org
> ---
> include/linux/security.h | 1 +
> kernel/kexec.c | 8 ++++++++
> security/lockdown/lockdown.c | 1 +
> 3 files changed, 10 insertions(+)
>
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 200175c8605a..00a31ab2e5ba 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -84,6 +84,7 @@ enum lockdown_reason {
> LOCKDOWN_NONE,
> LOCKDOWN_MODULE_SIGNATURE,
> LOCKDOWN_DEV_MEM,
> + LOCKDOWN_KEXEC,
> LOCKDOWN_INTEGRITY_MAX,
> LOCKDOWN_CONFIDENTIALITY_MAX,
> };
> diff --git a/kernel/kexec.c b/kernel/kexec.c
> index 68559808fdfa..ec3f07a4b1c0 100644
> --- a/kernel/kexec.c
> +++ b/kernel/kexec.c
> @@ -207,6 +207,14 @@ static inline int kexec_load_check(unsigned long nr_segments,
> if (result < 0)
> return result;
>
> + /*
> + * kexec can be used to circumvent module loading restrictions, so
> + * prevent loading in that case
> + */
> + result = security_locked_down(LOCKDOWN_KEXEC);
> + if (result)
> + return result;
> +
> /*
> * Verify we have a legal set of flags
> * This leaves us room for future extensions.
> diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
> index 565c87451f0f..08fcd8116db3 100644
> --- a/security/lockdown/lockdown.c
> +++ b/security/lockdown/lockdown.c
> @@ -20,6 +20,7 @@ static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
> [LOCKDOWN_NONE] = "none",
> [LOCKDOWN_MODULE_SIGNATURE] = "unsigned module loading",
> [LOCKDOWN_DEV_MEM] = "/dev/mem,kmem,port",
> + [LOCKDOWN_KEXEC] = "kexec of unsigned images",
> [LOCKDOWN_INTEGRITY_MAX] = "integrity",
> [LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
> };
> --
> 2.22.0.410.gd8fdbe21b5-goog
>

--
Kees Cook