Fix hardened usercopy under CONFIG_DEBUG_VIRTUAL

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Merge tag 'usercopy-v5.4-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux

Pull usercopy fix from Kees Cook:
 "Fix hardened usercopy under CONFIG_DEBUG_VIRTUAL"

* tag 'usercopy-v5.4-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux:
  usercopy: Avoid HIGHMEM pfn warning

mm/usercopy.c

@@ -11,6 +11,7 @@
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
 #include <linux/mm.h>
+#include <linux/highmem.h>
 #include <linux/slab.h>
 #include <linux/sched.h>
 #include <linux/sched/task.h>
@@ -227,7 +228,12 @@ static inline void check_heap_object(const void *ptr, unsigned long n,
 	if (!virt_addr_valid(ptr))
 		return;
 
-	page = virt_to_head_page(ptr);
+	/*
+	 * When CONFIG_HIGHMEM=y, kmap_to_page() will give either the
+	 * highmem page or fallback to virt_to_page(). The following
+	 * is effectively a highmem-aware virt_to_head_page().
+	 */
+	page = compound_head(kmap_to_page((void *)ptr));
 
 	if (PageSlab(page)) {
 		/* Check slab allocator for flags and size. */