When restoring a page, no sanity checks are done to make sure the page
actually came from a kexec handover. The caller is trusted to pass in
the right address. If the caller has a bug and passes in a wrong
address, an in-use page might be "restored" and returned, causing all
sorts of memory corruption.

Harden the page restore logic by stashing in a magic number in
page->private along with the order. If the magic number does not match,
the page won't be touched. page->private is an unsigned long. The union
kho_page_info splits it into two parts, with one holding the order and
the other holding the magic number.

Signed-off-by: Pratyush Yadav <pratyush@kernel.org>
---

Notes:
    Changes in v2:
    
    - Add a WARN_ON_ONCE() if order or magic is invalid.
    - Add a comment explaining why the magic check also implicitly makes
      sure phys is order-aligned.
    - Clear page private to make sure later restores of the same page error
      out.
    - Move the checks to kho_restore_page() since patch 1 now moves sanity
      checking to it.

 kernel/kexec_handover.c | 41 ++++++++++++++++++++++++++++++++++-------
 1 file changed, 34 insertions(+), 7 deletions(-)

diff --git a/kernel/kexec_handover.c b/kernel/kexec_handover.c
index 69cab82abaaef..911fda8532b2e 100644
--- a/kernel/kexec_handover.c
+++ b/kernel/kexec_handover.c
@@ -32,6 +32,22 @@
 #define PROP_PRESERVED_MEMORY_MAP "preserved-memory-map"
 #define PROP_SUB_FDT "fdt"
 
+#define KHO_PAGE_MAGIC 0x4b484f50U /* ASCII for 'KHOP' */
+
+/*
+ * KHO uses page->private, which is an unsigned long, to store page metadata.
+ * Use it to store both the magic and the order.
+ */
+union kho_page_info {
+	unsigned long page_private;
+	struct {
+		unsigned int order;
+		unsigned int magic;
+	};
+};
+
+static_assert(sizeof(union kho_page_info) == sizeof(((struct page *)0)->private));
+
 static bool kho_enable __ro_after_init;
 
 bool kho_is_enabled(void)
@@ -186,16 +202,24 @@ static int __kho_preserve_order(struct kho_mem_track *track, unsigned long pfn,
 static struct page *kho_restore_page(phys_addr_t phys)
 {
 	struct page *page = pfn_to_online_page(PHYS_PFN(phys));
-	unsigned int nr_pages, order;
+	union kho_page_info info;
+	unsigned int nr_pages;
 
 	if (!page)
 		return NULL;
 
-	order = page->private;
-	if (order > MAX_PAGE_ORDER)
+	info.page_private = page->private;
+	/*
+	 * deserialize_bitmap() only sets the magic on the head page. This magic
+	 * check also implicitly makes sure phys is order-aligned since for
+	 * non-order-aligned phys addresses, magic will never be set.
+	 */
+	if (WARN_ON_ONCE(info.magic != KHO_PAGE_MAGIC || info.order > MAX_PAGE_ORDER))
 		return NULL;
-	nr_pages = (1 << order);
+	nr_pages = (1 << info.order);
 
+	/* Clear private to make sure later restores on this page error out. */
+	page->private = 0;
 	/* Head page gets refcount of 1. */
 	set_page_count(page, 1);
 
@@ -203,8 +227,8 @@ static struct page *kho_restore_page(phys_addr_t phys)
 	for (unsigned int i = 1; i < nr_pages; i++)
 		set_page_count(page + i, 0);
 
-	if (order > 0)
-		prep_compound_page(page, order);
+	if (info.order > 0)
+		prep_compound_page(page, info.order);
 
 	adjust_managed_page_count(page, nr_pages);
 	return page;
@@ -341,10 +365,13 @@ static void __init deserialize_bitmap(unsigned int order,
 		phys_addr_t phys =
 			elm->phys_start + (bit << (order + PAGE_SHIFT));
 		struct page *page = phys_to_page(phys);
+		union kho_page_info info;
 
 		memblock_reserve(phys, sz);
 		memblock_reserved_mark_noinit(phys, sz);
-		page->private = order;
+		info.magic = KHO_PAGE_MAGIC;
+		info.order = order;
+		page->private = info.page_private;
 	}
 }
 
-- 
2.47.3