RE: [PATCH v6 03/11] mm, x86: Add support for eXclusive Page Frame Ownership (XPFO)

[Ralph Campbell]

> -----Original Message-----
> From: owner-linux-mm@xxxxxxxxx [mailto:owner-linux-mm@xxxxxxxxx] On
> Behalf Of Tycho Andersen
> Sent: Thursday, September 7, 2017 10:36 AM
> To: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx; kernel-hardening@xxxxxxxxxxxxxxxxxx; Marco Benatto
> <marco.antonio.780@xxxxxxxxx>; Juerg Haefliger
> <juerg.haefliger@xxxxxxxxxxxxx>; x86@xxxxxxxxxx; Tycho Andersen
> <tycho@xxxxxxxxxx>
> Subject: [PATCH v6 03/11] mm, x86: Add support for eXclusive Page Frame
> Ownership (XPFO)
> 
> From: Juerg Haefliger <juerg.haefliger@xxxxxxxxxxxxx>
> 
> This patch adds support for XPFO which protects against 'ret2dir' kernel attacks.
> The basic idea is to enforce exclusive ownership of page frames by either the
> kernel or userspace, unless explicitly requested by the kernel. Whenever a page
> destined for userspace is allocated, it is unmapped from physmap (the kernel's
> page table). When such a page is reclaimed from userspace, it is mapped back to
> physmap.
> 
> Additional fields in the page_ext struct are used for XPFO housekeeping,
> specifically:
>   - two flags to distinguish user vs. kernel pages and to tag unmapped
>     pages.
>   - a reference counter to balance kmap/kunmap operations.
>   - a lock to serialize access to the XPFO fields.
> 
> This patch is based on the work of Vasileios P. Kemerlis et al. who published their
> work in this paper:
>   http://www.cs.columbia.edu/~vpk/papers/ret2dir.sec14.pdf
> 
> v6: * use flush_tlb_kernel_range() instead of __flush_tlb_one, so we flush
>       the tlb entry on all CPUs when unmapping it in kunmap
>     * handle lookup_page_ext()/lookup_xpfo() returning NULL
>     * drop lots of BUG()s in favor of WARN()
>     * don't disable irqs in xpfo_kmap/xpfo_kunmap, export
>       __split_large_page so we can do our own alloc_pages(GFP_ATOMIC) to
>       pass it
> 
> CC: x86@xxxxxxxxxx
> Suggested-by: Vasileios P. Kemerlis <vpk@xxxxxxxxxxxxxxx>
> Signed-off-by: Juerg Haefliger <juerg.haefliger@xxxxxxxxxxxxx>
> Signed-off-by: Tycho Andersen <tycho@xxxxxxxxxx>
> Signed-off-by: Marco Benatto <marco.antonio.780@xxxxxxxxx>
> ---
>  Documentation/admin-guide/kernel-parameters.txt |   2 +
>  arch/x86/Kconfig                                |   1 +
>  arch/x86/include/asm/pgtable.h                  |  25 +++
>  arch/x86/mm/Makefile                            |   1 +
>  arch/x86/mm/pageattr.c                          |  22 +--
>  arch/x86/mm/xpfo.c                              | 114 ++++++++++++
>  include/linux/highmem.h                         |  15 +-
>  include/linux/xpfo.h                            |  42 +++++
>  mm/Makefile                                     |   1 +
>  mm/page_alloc.c                                 |   2 +
>  mm/page_ext.c                                   |   4 +
>  mm/xpfo.c                                       | 222 ++++++++++++++++++++++++
>  security/Kconfig                                |  19 ++
>  13 files changed, 449 insertions(+), 21 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt
> b/Documentation/admin-guide/kernel-parameters.txt
> index d9c171ce4190..444d83183f75 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2736,6 +2736,8 @@
> 
>  	nox2apic	[X86-64,APIC] Do not enable x2APIC mode.
> 
> +	noxpfo		[X86-64] Disable XPFO when CONFIG_XPFO is on.
> +
>  	cpu0_hotplug	[X86] Turn on CPU0 hotplug feature when
>  			CONFIG_BOOTPARAM_HOTPLUG_CPU0 is off.
>  			Some features depend on CPU0. Known dependencies
<... snip>

A bit more description for system administrators would be very useful.
Perhaps something like:

noxpfo		[XPFO,X86-64] Disable eXclusive Page Frame Ownership (XPFO)
                             Physical pages mapped into user applications will also be mapped
                             in the kernel's address space as if CONFIG_XPFO was not enabled.

Patch 05 should also update kernel-parameters.txt and add "ARM64" to the config option list for noxpfo.