Re: [PATCH V34 05/29] Restrict /dev/{mem,kmem,port} when the kernel is locked down

Kees Cook
2019-06-22T23:52:44Z

On Fri, Jun 21, 2019 at 05:03:34PM -0700, Matthew Garrett wrote:
> From: Matthew Garrett <mjg59@srcf.ucam.org>
>
> Allowing users to read and write to core kernel memory makes it possible
> for the kernel to be subverted, avoiding module loading restrictions, and
> also to steal cryptographic information.
>
> Disallow /dev/mem and /dev/kmem from being opened this when the kernel has
> been locked down to prevent this.
>
> Also disallow /dev/port from being opened to prevent raw ioport access and
> thus DMA from being used to accomplish the same thing.
>
> Signed-off-by: David Howells <dhowells@redhat.com>
> Signed-off-by: Matthew Garrett <mjg59@google.com>
> Cc: x86@kernel.org
> ---
> drivers/char/mem.c | 6 +++++-
> include/linux/security.h | 1 +
> security/lockdown/lockdown.c | 1 +
> 3 files changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/char/mem.c b/drivers/char/mem.c
> index b08dc50f9f26..93c02493f0fa 100644
> --- a/drivers/char/mem.c
> +++ b/drivers/char/mem.c
> @@ -29,8 +29,8 @@
> #include <linux/export.h>
> #include <linux/io.h>
> #include <linux/uio.h>
> -
> #include <linux/uaccess.h>
> +#include <linux/security.h>
>
> #ifdef CONFIG_IA64
> # include <linux/efi.h>
> @@ -786,6 +786,10 @@ static loff_t memory_lseek(struct file *file, loff_t offset, int orig)
>
> static int open_port(struct inode *inode, struct file *filp)
> {
> + int ret = security_locked_down(LOCKDOWN_DEV_MEM);
> +
> + if (ret)
> + return ret;
> return capable(CAP_SYS_RAWIO) ? 0 : -EPERM;

Usually the ordering for LSM tests tends to follow capable checks, which
allows for things like audit to generate logs for capability rejections,
etc. I'd expect this to be:

if (!capable(CAP_SYS_RAWIO))
return -EPERM;

return security_locked_down(LOCKDOWN_DEV_MEM)

With that fixed:

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

-Kees

> }
>
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 46d85cd63b06..200175c8605a 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -83,6 +83,7 @@ enum lsm_event {
> enum lockdown_reason {
> LOCKDOWN_NONE,
> LOCKDOWN_MODULE_SIGNATURE,
> + LOCKDOWN_DEV_MEM,
> LOCKDOWN_INTEGRITY_MAX,
> LOCKDOWN_CONFIDENTIALITY_MAX,
> };
> diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
> index 25a3a5b0aa9c..565c87451f0f 100644
> --- a/security/lockdown/lockdown.c
> +++ b/security/lockdown/lockdown.c
> @@ -19,6 +19,7 @@ static enum lockdown_reason kernel_locked_down;
> 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_INTEGRITY_MAX] = "integrity",
> [LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
> };
> --
> 2.22.0.410.gd8fdbe21b5-goog
>

--
Kees Cook