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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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