Re: [PATCH man-pages v2 2/2] openat2.2: document new openat2(2) syscall

Michael Kerrisk (man-pages)
2020-04-15T20:25:16Z

Hello Aleksa,

On Tue, 14 Apr 2020 at 12:35, Aleksa Sarai <asarai@suse.de> wrote:

[...]

> > >> I must admit that I'm still confused. There's only the briefest of
> > >> mentions of magic links in symlink(7). Perhaps that needs to be fixed?
> > >
> > > It wouldn't hurt to add a longer description of magic-links in
> > > symlink(7). I'll send you a small patch to beef up the description (I
> > > had planned to include a longer rewrite with the O_EMPTYPATH patches but
> > > those require quite a bit more work to land).
> >
> > That would be great. Thank you!
>
> I'll cook something up later this week.

Thank you!

[...]

> > I've reworked the text on RESOLVE_NO_MAGICLINKS substantially:
> >
> > RESOLVE_NO_MAGICLINKS
> > Disallow all magic-link resolution during path reso‐
> > lution.
> >
> > Magic links are symbolic link-like objects that are
> > most notably found in proc(5); examples include
> > /proc/[pid]/exe and /proc/[pid]/fd/*. (See sym‐
> > link(7) for more details.)
> >
> > Unknowingly opening magic links can be risky for
> > some applications. Examples of such risks include
> > the following:
> >
> > · If the process opening a pathname is a controlling
> > process that currently has no controlling terminal
> > (see credentials(7)), then opening a magic link
> > inside /proc/[pid]/fd that happens to refer to a
> > terminal would cause the process to acquire a con‐
> > trolling terminal.
> >
> > · In a containerized environment, a magic link
> > inside /proc may refer to an object outside the
> > container, and thus may provide a means to escape
> > from the container.
> >
> > [The above example derives from https://lwn.net/Articles/796868/]
> >
> > Because of such risks, an application may prefer to
> > disable magic link resolution using the
> > RESOLVE_NO_MAGICLINKS flag.
> >
> > If the trailing component (i.e., basename) of path‐
> > name is a magic link, and how.flags contains both
> > O_PATH and O_NOFOLLOW, then an O_PATH file descrip‐
> > tor referencing the magic link will be returned.
> >
> > How does the above look?
>
> The changes look correct, though you could end up going through procfs
> even if you weren't resolving a path inside proc directly (since you can
> bind-mount symlinks or have a symlink to procfs). But I'm not sure if
> it's necessary to outline all the ways a program could be tricked into
> doing something unintended.

Yes, indeed. These paragraphs are merely intended to give the reader
some ideas about what the issues are.

> > Also, regarding the last paragraph, I have a question. The
> > text doesn't seem quite to relate to the rest of the discussion.
> > Should it be saying something like:
> >
> > If the trailing component (i.e., basename) of pathname is a magic link,
> > **how.resolve contains RESOLVE_NO_MAGICLINKS,**
> > and how.flags contains both O_PATH and O_NOFOLLOW, then an O_PATH
> > file descriptor referencing the magic link will be returned.
> >
> > ?
>
> Yes, that is what I meant to write --

Good. Fixed.

> and I believe that the
> RESOLVE_NO_SYMLINKS section is missing similar text in the second
> paragraph (except it should refer to RESOLVE_NO_SYMLINKS, obviously).

Also fixed.

Thanks,

Michael

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/ …