Re: [PATCH] utimensat() non-conformances and fixes [v3]

["Michael Kerrisk" <mtk.manpages@googlemail.com>]

Hi Miklos,

> 2) I've found yet another divergence from the spec -- but this
> was in the original implementation, rather than being
> something that has been introduced.  In do_futimes() there is
>
>        if (!times && !(file->f_mode & FMODE_WRITE))
>                write_error = -EACCES;
>
> However, the check here should not be against the f_mode (file access
> mode), but the against actual permission of the file referred to by
> the underlying descriptor.  This means that for the do_futimes() +
> times==NULL case, a set-user-ID root program could open a file
> descriptor O_RDWR/O_WRONLY for which the real UID does not have write
> access, and then even after reverting the the effective UID, the real
> user could still update file.
>
> I'm not sure of the correct way to get the required nameidata (to do a
> vfs_permission() call) from the file descriptor.  Can you give me a
> tip there?

Could you point me at the right way of doing this?

Cheers,

Michael