Re: [Xen-devel] [PATCH] x86/mm: Make PV linear pagetables optional

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 10/17/2017 06:10 PM, George Dunlap wrote:
> Allowing pagetables to point to other pagetables of the same level
> (often called 'linear pagetables') has been included in Xen since its
> inception; but recently it has been the source of a number of subtle
> reference-counting bugs.
> 
> It is not used by Linux or MiniOS; but it used used by NetBSD and
> Novell Netware.  There are significant numbers of people who are never
> going to use the feature, along with significant numbers who need the
> feature.
> 
> Add a Kconfig option for the feature (default to 'y').  Also add a
> command-line option to control whether PV linear pagetables are
> allowed (default to 'true').
> 
> In order to make the code clean:
> - Introduce LPT_ASSERT(), which only exists if CONFIG_PV_LINEAR_PT is defined
> - Introduce zero_linear_entries() to set page->linear_pt_count to zero
>   (or do nothing, as appropriate)
> 
> Reported-by: Jann Horn <jannh@xxxxxxxxxx>
> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
> ---
> Changes since XSA
> - Add a Kconfig option
> - Default to 'on' (rather than 'off').
> 
> Release justification: This was originally part of a security fix
> embargoed until after the freeze date; it wasn't checked in with the
> other security patches in order to allow a discussion about the
> default.
> 
> CC: Ian Jackson <ian.jackson@xxxxxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> CC: Jan Beulich <jbeulich@xxxxxxxx>
> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> CC: Konrad Wilk <konrad.wilk@xxxxxxxxxx>
> CC: Julien Grall <julien.grall@xxxxxxx>
> ---
>  docs/misc/xen-command-line.markdown | 16 ++++++++++++++++
>  xen/arch/Kconfig                    |  1 +
>  xen/arch/arm/mm.c                   |  1 +
>  xen/arch/x86/Kconfig                | 21 ++++++++++++++++++++
>  xen/arch/x86/mm.c                   | 38 
> +++++++++++++++++++++++++++++++++----
>  xen/include/asm-x86/mm.h            |  5 +++++
>  6 files changed, 78 insertions(+), 4 deletions(-)
> 
> diff --git a/docs/misc/xen-command-line.markdown 
> b/docs/misc/xen-command-line.markdown
> index eb4995e68b..952368d3be 100644
> --- a/docs/misc/xen-command-line.markdown
> +++ b/docs/misc/xen-command-line.markdown
> @@ -1422,6 +1422,22 @@ The following resources are available:
>      CDP, one COS will corespond two CBMs other than one with CAT, due to the
>      sum of CBMs is fixed, that means actual `cos_max` in use will 
> automatically
>      reduce to half when CDP is enabled.
> +     
> +### pv-linear-pt
> +> `= <boolean>`
> +
> +> Default: `false`
> +
> +Allow PV guests to have pagetable entries pointing to other pagetables
> +of the same level (i.e., allowing L2 PTEs to point to other L2 pages).
> +This technique is often called "linear pagetables", and is sometimes
> +used to allow operating systems a simple way to consistently map the
> +current process's pagetables into its own virtual address space.
> +
> +Linux and MiniOS don't use this technique.  NetBSD and Novell Netware
> +do; there may be other custom operating systems which do.  If you're
> +certain you don't plan on having PV guests which use this feature,
> +turning it off can reduce the attack surface.
>  
>  ### rcu-idle-timer-period-ms
>  > `= <integer>`
> diff --git a/xen/arch/Kconfig b/xen/arch/Kconfig
> index cf0acb7e89..47287a4985 100644
> --- a/xen/arch/Kconfig
> +++ b/xen/arch/Kconfig
> @@ -6,3 +6,4 @@ config NR_CPUS
>       default "128" if ARM
>       ---help---
>         Specifies the maximum number of physical CPUs which Xen will support.
> +
> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> index 3c328e2df5..199155fcd8 100644
> --- a/xen/arch/arm/mm.c
> +++ b/xen/arch/arm/mm.c
> @@ -42,6 +42,7 @@
>  #include <xen/libfdt/libfdt.h>
>  #include <asm/setup.h>
>  
> +

Gah -- sorry about the blank lines.  Should have looked over the patch
better first.

I'll wait for feedback on the rest of the patch before I resend.

 -George

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