[PATCH v2 1/5] kexec: handover: add helper to check preserved page ranges

[Cris Jacob Maamor <crisjacobmaamor@gmail.com>]

Restore code should not use phys_to_virt() on physical addresses from
restored metadata until it verifies that the range was preserved by KHO.

Add kho_is_preserved() so callers can check a preserved page range without
restoring, freeing, or taking ownership of the pages.

Signed-off-by: Cris Jacob Maamor <crisjacobmaamor@gmail.com>
---
 include/linux/kexec_handover.h     |  6 +++++
 kernel/liveupdate/kexec_handover.c | 35 ++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/include/linux/kexec_handover.h b/include/linux/kexec_handover.h
index 8968c56d2d73..fb09943ab232 100644
--- a/include/linux/kexec_handover.h
+++ b/include/linux/kexec_handover.h
@@ -19,6 +19,7 @@ struct page;
 #ifdef CONFIG_KEXEC_HANDOVER
 bool kho_is_enabled(void);
 bool is_kho_boot(void);
+bool kho_is_preserved(phys_addr_t phys, unsigned long nr_pages);
 
 int kho_preserve_folio(struct folio *folio);
 void kho_unpreserve_folio(struct folio *folio);
@@ -51,6 +52,11 @@ static inline bool is_kho_boot(void)
 	return false;
 }
 
+static inline bool kho_is_preserved(phys_addr_t phys, unsigned long nr_pages)
+{
+	return false;
+}
+
 static inline int kho_preserve_folio(struct folio *folio)
 {
 	return -EOPNOTSUPP;
diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
index 94762de1fe5f..fe9f11190705 100644
--- a/kernel/liveupdate/kexec_handover.c
+++ b/kernel/liveupdate/kexec_handover.c
@@ -10,6 +10,7 @@
 
 #define pr_fmt(fmt) "KHO: " fmt
 
+#include <linux/bits.h>
 #include <linux/cleanup.h>
 #include <linux/cma.h>
 #include <linux/kmemleak.h>
@@ -429,6 +430,40 @@ static struct page *kho_restore_page(phys_addr_t phys, bool is_folio)
 	return page;
 }
 
+/**
+ * kho_is_preserved - Verify that a physical page range belongs to KHO.
+ * @phys: physical address of the first page in the range.
+ * @nr_pages: number of pages that the caller expects to access.
+ *
+ * Use this before phys_to_virt() when a physical address comes from restored
+ * metadata. It checks that @phys starts a KHO-preserved allocation large
+ * enough to cover @nr_pages.
+ *
+ * This only checks the KHO marker. It does not restore, free, or take
+ * ownership of the pages.
+ *
+ * Return: true if @phys starts a preserved KHO allocation large enough to cover
+ * @nr_pages, false otherwise.
+ */
+bool kho_is_preserved(phys_addr_t phys, unsigned long nr_pages)
+{
+	struct page *page;
+	union kho_page_info info;
+
+	if (!nr_pages || !IS_ALIGNED(phys, PAGE_SIZE))
+		return false;
+
+	page = pfn_to_online_page(PHYS_PFN(phys));
+	if (!page)
+		return false;
+
+	info.page_private = page->private;
+	if (info.magic != KHO_PAGE_MAGIC || info.order >= BITS_PER_LONG)
+		return false;
+
+	return nr_pages <= BIT(info.order);
+}
+
 /**
  * kho_restore_folio - recreates the folio from the preserved memory.
  * @phys: physical address of the folio.
-- 
2.53.0