mm: move_ptes -- Set soft dirty bit depending on pte type

Dave reported corrupted swap entries

 | [ 4588.541886] swap_free: Unused swap offset entry 00002d15
 | [ 4588.541952] BUG: Bad page map in process trinity-kid12  pte:005a2a80 pmd:22c01f067

and Hugh pointed that in move_ptes _PAGE_SOFT_DIRTY bit set regardless
the type of entry pte consists of.  The trick here is that when we carry
soft dirty status in swap entries we are to use _PAGE_SWP_SOFT_DIRTY
instead, because this is the only place in pte which can be used for own
needs without intersecting with bits owned by swap entry type/offset.

Reported-and-tested-by: Dave Jones <[email protected]>
Signed-off-by: Cyrill Gorcunov <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Analyzed-by: Hugh Dickins <[email protected]>
Cc: Hillf Danton <[email protected]>
Cc: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/mremap.c

@@ -15,6 +15,7 @@
 #include <linux/swap.h>
 #include <linux/capability.h>
 #include <linux/fs.h>
+#include <linux/swapops.h>
 #include <linux/highmem.h>
 #include <linux/security.h>
 #include <linux/syscalls.h>
@@ -69,6 +70,23 @@ static pmd_t *alloc_new_pmd(struct mm_struct *mm, struct vm_area_struct *vma,
 	return pmd;
 }
 
+static pte_t move_soft_dirty_pte(pte_t pte)
+{
+	/*
+	 * Set soft dirty bit so we can notice
+	 * in userspace the ptes were moved.
+	 */
+#ifdef CONFIG_MEM_SOFT_DIRTY
+	if (pte_present(pte))
+		pte = pte_mksoft_dirty(pte);
+	else if (is_swap_pte(pte))
+		pte = pte_swp_mksoft_dirty(pte);
+	else if (pte_file(pte))
+		pte = pte_file_mksoft_dirty(pte);
+#endif
+	return pte;
+}
+
 static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
 		unsigned long old_addr, unsigned long old_end,
 		struct vm_area_struct *new_vma, pmd_t *new_pmd,
@@ -126,7 +144,8 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
 			continue;
 		pte = ptep_get_and_clear(mm, old_addr, old_pte);
 		pte = move_pte(pte, new_vma->vm_page_prot, old_addr, new_addr);
-		set_pte_at(mm, new_addr, new_pte, pte_mksoft_dirty(pte));
+		pte = move_soft_dirty_pte(pte);
+		set_pte_at(mm, new_addr, new_pte, pte);
 	}
 
 	arch_leave_lazy_mmu_mode();