[PATCH] mm/mmap: Fix race in mmap_region() with ftrucate()

[Liam R. Howlett]

From: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>

Avoiding the zeroing of the vma tree in mmap_region() introduced a race
with truncate in the page table walk.  To avoid any races, create a hole
in the rmap during the operation by clearing the pagetable entries
earlier under the mmap write lock and (critically) before the new vma is
installed into the vma tree.  The result is that the old vma is still in
the vma tree, but the page tables are cleared while holding the
i_mmap_rwsem.

This change extends the fix required for hugetblfs and the call_mmap()
function by moving the cleanup higher in the function and running it
unconditionally.

Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Fixes: f8d112a4e657 ("mm/mmap: avoid zeroing vma tree in mmap_region()")
Reported-by: Jann Horn <jannh@xxxxxxxxxx>
Closes: https://lore.kernel.org/all/CAG48ez0ZpGzxi=-5O_uGQ0xKXOmbjeQ0LjZsRJ1Qtf2X5eOr1w@xxxxxxxxxxxxxx/
Link: https://lore.kernel.org/all/CAG48ez0ZpGzxi=-5O_uGQ0xKXOmbjeQ0LjZsRJ1Qtf2X5eOr1w@xxxxxxxxxxxxxx/
Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
---
 mm/mmap.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index dd4b35a25aeb..a20998fb633c 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1413,6 +1413,13 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 		vmg.flags = vm_flags;
 	}
 
+	/*
+	 * clear PTEs while the vma is still in the tree so that rmap
+	 * cannot race with the freeing later in the truncate scenario.
+	 * This is also needed for call_mmap(), which is why vm_ops
+	 * close function is called.
+	 */
+	vms_clean_up_area(&vms, &mas_detach);
 	vma = vma_merge_new_range(&vmg);
 	if (vma)
 		goto expanded;
@@ -1432,11 +1439,6 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 
 	if (file) {
 		vma->vm_file = get_file(file);
-		/*
-		 * call_mmap() may map PTE, so ensure there are no existing PTEs
-		 * and call the vm_ops close function if one exists.
-		 */
-		vms_clean_up_area(&vms, &mas_detach);
 		error = call_mmap(file, vma);
 		if (error)
 			goto unmap_and_free_vma;
-- 
2.43.0