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    Subject[PATCH 4.9 41/62] debugfs: fix use-after-free on symlink traversal
    Date
    [ Upstream commit 93b919da64c15b90953f96a536e5e61df896ca57 ]

    symlink body shouldn't be freed without an RCU delay. Switch debugfs to
    ->destroy_inode() and use of call_rcu(); free both the inode and symlink
    body in the callback. Similar to solution for bpf, only here it's even
    more obvious that ->evict_inode() can be dropped.

    Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    fs/debugfs/inode.c | 13 +++++++++----
    1 file changed, 9 insertions(+), 4 deletions(-)

    diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
    index 77e9cd7a0137..20ee612017bf 100644
    --- a/fs/debugfs/inode.c
    +++ b/fs/debugfs/inode.c
    @@ -170,19 +170,24 @@ static int debugfs_show_options(struct seq_file *m, struct dentry *root)
    return 0;
    }

    -static void debugfs_evict_inode(struct inode *inode)
    +static void debugfs_i_callback(struct rcu_head *head)
    {
    - truncate_inode_pages_final(&inode->i_data);
    - clear_inode(inode);
    + struct inode *inode = container_of(head, struct inode, i_rcu);
    if (S_ISLNK(inode->i_mode))
    kfree(inode->i_link);
    + free_inode_nonrcu(inode);
    +}
    +
    +static void debugfs_destroy_inode(struct inode *inode)
    +{
    + call_rcu(&inode->i_rcu, debugfs_i_callback);
    }

    static const struct super_operations debugfs_super_operations = {
    .statfs = simple_statfs,
    .remount_fs = debugfs_remount,
    .show_options = debugfs_show_options,
    - .evict_inode = debugfs_evict_inode,
    + .destroy_inode = debugfs_destroy_inode,
    };

    static struct vfsmount *debugfs_automount(struct path *path)
    --
    2.20.1


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