[PATCH 02/12] Unionfs: prevent races in unionfs_fault

[Erez Zadok <ezk@cs.sunysb.edu>]

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>
---
 fs/unionfs/mmap.c |   21 +++++++++++++--------
 1 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/fs/unionfs/mmap.c b/fs/unionfs/mmap.c
index 07db5b0..febde7c 100644
--- 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;
 }
 
-- 
1.5.2.2