Re: [PATCH v2 1/3] cifs: clear tcon after cifsFileInfo_put() in cifs_file_set_size()

[Paulo Alcantara <pc@manguebit.org>]

Frank Sorenson <sorenson@redhat.com> writes:

> When the else branch of cifs_file_set_size() finds a writable file handle
> via find_writable_file(), it borrows tcon and server from the handle's
> tlink, attempts the handle-based set_file_size() RPC, and then releases
> the handle with cifsFileInfo_put().
>
> If set_file_size() fails, execution falls through to the path-based
> fallback, which reuses the borrowed tcon and server under the
> "if (tcon == NULL)" guard.  Since tcon is not NULL at that point, the
> guard is skipped.  If cifsFileInfo_put() dropped the last reference on a
> tlink that was already removed from the tlink tree (TCON_LINK_IN_TREE
> cleared, as happens during reconnection or session teardown),
> cifs_put_tlink() will have freed tcon; the subsequent set_path_size()
> call is then a use-after-free.
>
> Setting tcon = NULL after cifsFileInfo_put() causes the existing guard
> to take the cifs_sb_tlink() path, which acquires a fresh reference for
> the path-based operation or fails cleanly if the session is gone.
> ...

Applied.