* Suren Baghdasaryan surenb@google.com [230707 00:32]:
mmap_region adds a newly created VMA into VMA tree and might modify it afterwards before dropping the mmap_lock. This poses a problem for page faults handled under per-VMA locks because they don't take the mmap_lock and can stumble on this VMA while it's still being modified. Currently this does not pose a problem since post-addition modifications are done only for file-backed VMAs, which are not handled under per-VMA lock. However, once support for handling file-backed page faults with per-VMA locks is added, this will become a race. Fix this by write-locking the VMA before inserting it into the VMA tree. Other places where a new VMA is added into VMA tree do not modify it after the insertion, so do not need the same locking.
Signed-off-by: Suren Baghdasaryan surenb@google.com
mm/mmap.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/mm/mmap.c b/mm/mmap.c index c66e4622a557..84c71431a527 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -2812,6 +2812,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr, if (vma->vm_file) i_mmap_lock_write(vma->vm_file->f_mapping);
- /* Lock the VMA since it is modified after insertion into VMA tree */
So it is modified, but that i_mmap_lock_write() directly above this comment is potentially moving below the insert and that is why this lock is needed.
- vma_start_write(vma); vma_iter_store(&vmi, vma); mm->map_count++; if (vma->vm_file) {
-- 2.41.0.255.g8b1d071c50-goog