Re: [PATCH 1/2] powerpc/pseries: block untrusted device tree changes when locked down

[Paul Moore]

On Thu, Sep 22, 2022 at 3:38 PM Nathan Lynch <nathanl@xxxxxxxxxxxxx> wrote:
>
> The /proc/powerpc/ofdt interface allows the root user to freely alter
> the in-kernel device tree, enabling arbitrary physical address writes
> via drivers that could bind to malicious device nodes, thus making it
> possible to disable lockdown.
>
> Historically this interface has been used on the pseries platform to
> facilitate the runtime addition and removal of processor, memory, and
> device resources (aka Dynamic Logical Partitioning or DLPAR). Years
> ago, the processor and memory use cases were migrated to designs that
> happen to be lockdown-friendly: device tree updates are communicated
> directly to the kernel from firmware without passing through untrusted
> user space. I/O device DLPAR via the "drmgr" command in powerpc-utils
> remains the sole legitimate user of /proc/powerpc/ofdt, but it is
> already broken in lockdown since it uses /dev/mem to allocate argument
> buffers for the rtas syscall. So only illegitimate uses of the
> interface should see a behavior change when running on a locked down
> kernel.
>
> Signed-off-by: Nathan Lynch <nathanl@xxxxxxxxxxxxx>
> ---
>  arch/powerpc/platforms/pseries/reconfig.c | 5 +++++
>  include/linux/security.h                  | 1 +
>  security/security.c                       | 1 +
>  3 files changed, 7 insertions(+)

A couple of small nits below, but in general this seems reasonable.
However, as we are currently at -rc6 I would like us to wait to merge
this until after the upcoming merge window closes (I don't like
merging new functionality into -next at -rc6).

https://git.kernel.org/pub/scm/linux/kernel/git/pcmoore/lsm.git/tree/README.md

> diff --git a/include/linux/security.h b/include/linux/security.h
> index 7bd0c490703d..1ca8dbacd3cc 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -122,6 +122,7 @@ enum lockdown_reason {
>         LOCKDOWN_XMON_WR,
>         LOCKDOWN_BPF_WRITE_USER,
>         LOCKDOWN_DBG_WRITE_KERNEL,
> +       LOCKDOWN_DEVICE_TREE,

I would suggest moving LOCKDOWN_DEVICE_TREE to be next to
LOCKDOWN_ACPI_TABLES.  It's not a hard requirement, but it seems like
a nice idea to group similar things when we can.

>         LOCKDOWN_INTEGRITY_MAX,
>         LOCKDOWN_KCORE,
>         LOCKDOWN_KPROBES,
> diff --git a/security/security.c b/security/security.c
> index 4b95de24bc8d..2863fc31eec6 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -60,6 +60,7 @@ const char *const lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
>         [LOCKDOWN_XMON_WR] = "xmon write access",
>         [LOCKDOWN_BPF_WRITE_USER] = "use of bpf to write user RAM",
>         [LOCKDOWN_DBG_WRITE_KERNEL] = "use of kgdb/kdb to write kernel RAM",
> +       [LOCKDOWN_DEVICE_TREE] = "modifying device tree contents",

Might as well move this one too.

>         [LOCKDOWN_INTEGRITY_MAX] = "integrity",
>         [LOCKDOWN_KCORE] = "/proc/kcore access",
>         [LOCKDOWN_KPROBES] = "use of kprobes",
> --
> 2.37.3

-- 
paul-moore.com