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

[Nitesh Narayan Lal <nitesh@redhat.com>]

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On 2/7/19 12:23 PM, Alexander Duyck wrote:
> On Mon, Feb 4, 2019 at 2:11 PM Nitesh Narayan Lal <nitesh@redhat.com> 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>
> This seems kind of backwards to me. Why disable page poisoning instead
> of just not hinting about the free pages? There shouldn't be that many
> instances when page poisoning is enabled, and when it is it would make
> more sense to leave it enabled rather than silently disable it.
As I have mentioned in the cover email, I intend to reuse Wei's already
merged work.

This will enable the guest to communicate the poison value which is in
use to the host.

>
>> ---
>>  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 =
> At a minimum it seems like you should have some sort of warning
> message that you are disabling page poisoning rather than just
> silently turning it off.
-- 
Regards
Nitesh


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