Re: [RFC][Patch v8 4/7] KVM: Disabling page poisoning to prevent corruption

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Feb 04, 2019 at 03:18:51PM -0500, Nitesh Narayan Lal wrote:
> This patch disables page poisoning if guest page hinting is enabled.
> It is required to avoid possible guest memory corruption errors.
> Page Poisoning is a feature in which the page is filled with a specific
> pattern of (0x00 or 0xaa) after arch_free_page and the same is verified
> before arch_alloc_page to prevent following issues:
>     *information leak from the freed data
>     *use after free bugs
>     *memory corruption
> Selection of the pattern depends on the CONFIG_PAGE_POISONING_ZERO
> Once the guest pages which are supposed to be freed are sent to the
> hypervisor it frees them. After freeing the pages in the global list
> following things may happen:
>     *Hypervisor reallocates the freed memory back to the guest
>     *Hypervisor frees the memory and maps a different physical memory
> In order to prevent any information leak hypervisor before allocating
> memory to the guest fills it with zeroes.
> The issue arises when the pattern used for Page Poisoning is 0xaa while
> the newly allocated page received from the hypervisor by the guest is
> filled with the pattern 0x00. This will result in memory corruption errors.
> 
> Signed-off-by: Nitesh Narayan Lal <nitesh@redhat.com>

IMHO it's better to take the approach of the existing balloon code
and just send the poison value to host. Host can then avoid filling
memory with zeroes.


> ---
>  include/linux/page_hinting.h | 8 ++++++++
>  mm/page_poison.c             | 2 +-
>  virt/kvm/page_hinting.c      | 1 +
>  3 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/page_hinting.h b/include/linux/page_hinting.h
> index 2d7ff59f3f6a..e800c6b07561 100644
> --- a/include/linux/page_hinting.h
> +++ b/include/linux/page_hinting.h
> @@ -19,7 +19,15 @@ struct hypervisor_pages {
>  extern int guest_page_hinting_flag;
>  extern struct static_key_false guest_page_hinting_key;
>  extern struct smp_hotplug_thread hinting_threads;
> +extern bool want_page_poisoning;
>  
>  int guest_page_hinting_sysctl(struct ctl_table *table, int write,
>  			      void __user *buffer, size_t *lenp, loff_t *ppos);
>  void guest_free_page(struct page *page, int order);
> +
> +static inline void disable_page_poisoning(void)
> +{
> +#ifdef CONFIG_PAGE_POISONING
> +	want_page_poisoning = 0;
> +#endif
> +}
> diff --git a/mm/page_poison.c b/mm/page_poison.c
> index f0c15e9017c0..9af96021133b 100644
> --- a/mm/page_poison.c
> +++ b/mm/page_poison.c
> @@ -7,7 +7,7 @@
>  #include <linux/poison.h>
>  #include <linux/ratelimit.h>
>  
> -static bool want_page_poisoning __read_mostly;
> +bool want_page_poisoning __read_mostly;
>  
>  static int __init early_page_poison_param(char *buf)
>  {
> diff --git a/virt/kvm/page_hinting.c b/virt/kvm/page_hinting.c
> index 636990e7fbb3..be529f6f2bc0 100644
> --- a/virt/kvm/page_hinting.c
> +++ b/virt/kvm/page_hinting.c
> @@ -103,6 +103,7 @@ void guest_free_page(struct page *page, int order)
>  
>  	local_irq_save(flags);
>  	if (page_hinting_obj->kvm_pt_idx != MAX_FGPT_ENTRIES) {
> +		disable_page_poisoning();
>  		page_hinting_obj->kvm_pt[page_hinting_obj->kvm_pt_idx].pfn =
>  							page_to_pfn(page);
>  		page_hinting_obj->kvm_pt[page_hinting_obj->kvm_pt_idx].zonenum =
> -- 
> 2.17.2