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

[Nitesh Narayan Lal]

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@xxxxxxxxxx>
---
 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