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Subject[PATCH 22/34] union-mount: Prevent improper union-related remounts
Date
A remount request must (a) not convert a union to a non-union (or vice
versa), or (b) make the topmost layer of a union read-only.

Note that we only have to worry about attempts to remount the vfsmount
of the topmost read-write of the union (the one with MNT_UNION set).
The vfsmounts of the read-only layers are hidden in a cloned tree
hanging of the superblock of the topmost layer and aren't visible to
userspace.

Signed-off-by: Valerie Aurora <vaurora@redhat.com>
---
fs/namespace.c | 12 ++++++++++++
1 files changed, 12 insertions(+), 0 deletions(-)
diff --git a/fs/namespace.c b/fs/namespace.c
index ff83cee..61256e6 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1824,6 +1824,18 @@ static int do_remount(struct path *path, int flags, int mnt_flags,
if (!check_mnt(path->mnt))
return -EINVAL;

+ if ((path->mnt->mnt_flags & MNT_UNION) &&
+ !(mnt_flags & MNT_UNION))
+ return -EINVAL;
+
+ if ((mnt_flags & MNT_UNION) &&
+ !(path->mnt->mnt_flags & MNT_UNION))
+ return -EINVAL;
+
+ if ((path->mnt->mnt_flags & MNT_UNION) &&
+ (mnt_flags & MNT_READONLY))
+ return -EINVAL;
+
if (path->dentry != path->mnt->mnt_root)
return -EINVAL;

--
1.6.3.3


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