Re: [PATCH 2/2] restrict /dev/mem to idle io memory ranges

Kees Cook
2015-11-23T19:00:46Z

On Sat, Nov 21, 2015 at 7:57 PM, Dan Williams <dan.j.williams@intel.com> wrote:
> This effectively promotes IORESOURCE_BUSY to IORESOURCE_EXCLUSIVE
> semantics by default. If userspace really believes it is safe to access
> the memory region it can also perform the extra step of disabling an
> active driver. This protects device address ranges with read side
> effects and otherwise directs userspace to use the driver.
>
> Persistent memory presents a large "mistake surface" to /dev/mem as now
> accidental writes can corrupt a filesystem.
>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Russell King <linux@arm.linux.org.uk>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>

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

Thanks!

-Kees

> ---
> kernel/resource.c | 3 +++
> lib/Kconfig.debug | 23 ++++++++++++++++++++---
> 2 files changed, 23 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/resource.c b/kernel/resource.c
> index f150dbbe6f62..03a8b09f68a8 100644
> --- a/kernel/resource.c
> +++ b/kernel/resource.c
> @@ -1498,6 +1498,9 @@ int iomem_is_exclusive(u64 addr)
> break;
> if (p->end < addr)
> continue;
> + if (IS_ENABLED(CONFIG_IO_STRICT_DEVMEM)
> + && p->flags & IORESOURCE_BUSY)
> + break;
> if (p->flags & IORESOURCE_BUSY &&
> p->flags & IORESOURCE_EXCLUSIVE) {
> err = 1;
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index ad85145d0047..be47f99fb191 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -1866,9 +1866,26 @@ config STRICT_DEVMEM
> enabled, even in this case there are restrictions on /dev/mem
> use due to the cache aliasing requirements.
>
> + If this option is switched on, and IO_STRICT_DEVMEM=n, the /dev/mem
> + file only allows userspace access to PCI space and the BIOS code and
> + data regions. This is sufficient for dosemu and X and all common
> + users of /dev/mem.
> +
> + If in doubt, say Y.
> +
> +config IO_STRICT_DEVMEM
> + bool "Filter I/O access to /dev/mem"
> + depends on STRICT_DEVMEM
> + default STRICT_DEVMEM
> + ---help---
> + If this option is disabled, you allow userspace (root) access to all
> + io-memory regardless of whether a driver is actively using that
> + range. Accidental access to this is obviously disastrous, but
> + specific access can be used by people debugging kernel drivers.
> +
> If this option is switched on, the /dev/mem file only allows
> - userspace access to PCI space and the BIOS code and data regions.
> - This is sufficient for dosemu and X and all common users of
> - /dev/mem.
> + userspace access to *idle* io-memory ranges (see /proc/iomem) This
> + may break traditional users of /dev/mem (dosemu, legacy X, etc...)
> + if the driver using a given range cannot be disabled.
>
> If in doubt, say Y.
>

--
Kees Cook
Chrome OS Security