Re: [PATCH v2 1/2] powerpc/pseries: block untrusted device tree changes when locked down

[Andrew Donnellan]

On Mon, 2022-09-26 at 08:16 -0500, Nathan Lynch 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>

Seems sensible to me.

Reviewed-by: Andrew Donnellan <ajd@xxxxxxxxxxxxx>

> ---
>  arch/powerpc/platforms/pseries/reconfig.c | 5 +++++
>  include/linux/security.h                  | 1 +
>  security/security.c                       | 1 +
>  3 files changed, 7 insertions(+)
> 
> diff --git a/arch/powerpc/platforms/pseries/reconfig.c
> b/arch/powerpc/platforms/pseries/reconfig.c
> index cad7a0c93117..599bd2c78514 100644
> --- a/arch/powerpc/platforms/pseries/reconfig.c
> +++ b/arch/powerpc/platforms/pseries/reconfig.c
> @@ -10,6 +10,7 @@
>  #include <linux/kernel.h>
>  #include <linux/notifier.h>
>  #include <linux/proc_fs.h>
> +#include <linux/security.h>
>  #include <linux/slab.h>
>  #include <linux/of.h>
>  
> @@ -361,6 +362,10 @@ static ssize_t ofdt_write(struct file *file,
> const char __user *buf, size_t coun
>         char *kbuf;
>         char *tmp;
>  
> +       rv = security_locked_down(LOCKDOWN_DEVICE_TREE);
> +       if (rv)
> +               return rv;
> +
>         kbuf = memdup_user_nul(buf, count);
>         if (IS_ERR(kbuf))
>                 return PTR_ERR(kbuf);
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 7bd0c490703d..39e7c0e403d9 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -114,6 +114,7 @@ enum lockdown_reason {
>         LOCKDOWN_IOPORT,
>         LOCKDOWN_MSR,
>         LOCKDOWN_ACPI_TABLES,
> +       LOCKDOWN_DEVICE_TREE,
>         LOCKDOWN_PCMCIA_CIS,
>         LOCKDOWN_TIOCSSERIAL,
>         LOCKDOWN_MODULE_PARAMETERS,
> diff --git a/security/security.c b/security/security.c
> index 4b95de24bc8d..51bf66d4f472 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -52,6 +52,7 @@ const char *const
> lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
>         [LOCKDOWN_IOPORT] = "raw io port access",
>         [LOCKDOWN_MSR] = "raw MSR access",
>         [LOCKDOWN_ACPI_TABLES] = "modifying ACPI tables",
> +       [LOCKDOWN_DEVICE_TREE] = "modifying device tree contents",
>         [LOCKDOWN_PCMCIA_CIS] = "direct PCMCIA CIS storage",
>         [LOCKDOWN_TIOCSSERIAL] = "reconfiguration of serial port IO",
>         [LOCKDOWN_MODULE_PARAMETERS] = "unsafe module parameters",

-- 
Andrew Donnellan    OzLabs, ADL Canberra
ajd@xxxxxxxxxxxxx   IBM Australia Limited