Unionfs: prevent races in unionfs_fault

vm_ops->fault may be called in parallel.  Because we have to resort to
temporarily changing the vma->vm_file to point to the lower file, a
concurrent invocation of unionfs_fault could see a different value.  In this
workaround, we keep a different copy of the vma structure in our stack, so
we never expose a different value of the vma->vm_file called to us, even
temporarily.  A better fix (already tested) would be to change the calling
semantics of ->fault to take an explicit file pointer.

Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>

diff --git a/fs/unionfs/mmap.c b/fs/unionfs/mmap.c
index 07db5b0f2bbd761cc37a2338f3ffb1be965de55b..febde7c1e76df5cd3a6428d5b2c05ddb26bb45f5 100644 (file)
--- a/fs/unionfs/mmap.c
+++ b/fs/unionfs/mmap.c
@@ -40,23 +40,28 @@ static int unionfs_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
        int err;
        struct file *file, *lower_file;
        struct vm_operations_struct *lower_vm_ops;
+       struct vm_area_struct lower_vma;
 
        BUG_ON(!vma);
-       file = vma->vm_file;
+       memcpy(&lower_vma, vma, sizeof(struct vm_area_struct));
+       file = lower_vma.vm_file;
        lower_vm_ops = UNIONFS_F(file)->lower_vm_ops;
        BUG_ON(!lower_vm_ops);
 
        lower_file = unionfs_lower_file(file);
        BUG_ON(!lower_file);
        /*
-        * XXX: we set the vm_file to the lower_file, before calling the
-        * lower ->fault op, then we restore the vm_file back to the upper
-        * file.  Need to change the ->fault prototype to take an explicit
-        * struct file, and fix all users accordingly.
+        * XXX: vm_ops->fault may be called in parallel.  Because we have to
+        * resort to temporarily changing the vma->vm_file to point to the
+        * lower file, a concurrent invocation of unionfs_fault could see a
+        * different value.  In this workaround, we keep a different copy of
+        * the vma structure in our stack, so we never expose a different
+        * value of the vma->vm_file called to us, even temporarily.  A
+        * better fix would be to change the calling semantics of ->fault to
+        * take an explicit file pointer.
         */
-       vma->vm_file = lower_file;
-       err = lower_vm_ops->fault(vma, vmf);
-       vma->vm_file = file;
+       lower_vma.vm_file = lower_file;
+       err = lower_vm_ops->fault(&lower_vma, vmf);
        return err;
 }