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

Aleksa Sarai
2020-04-14T10:44:20Z

[-- Attachment #1: Type: text/plain, Size: 7571 bytes --]

On 2020-04-13, Michael Kerrisk (man-pages) <mtk.manpages@gmail.com> wrote:
> >>>> .\" FIXME I find the "previously-functional systems" in the previous
> >>>> .\" sentence a little odd (since openat2() ia new sysycall), so I would
> >>>> .\" like to clarify a little...
> >>>> .\" Are you referring to the scenario where someone might take an
> >>>> .\" existing application that uses openat() and replaces the uses
> >>>> .\" of openat() with openat2()? In which case, is it correct to
> >>>> .\" understand that you mean that one should not just indiscriminately
> >>>> .\" add the RESOLVE_NO_XDEV flag to all of the openat2() calls?
> >>>> .\" If I'm not on the right track, could you point me in the right
> >>>> .\" direction please.
> >>>
> >>> This is mostly meant as a warning to hopefully avoid applications
> >>> because the developer didn't realise that system paths may contain
> >>> symlinks or bind-mounts. For an application which has switched to
> >>> openat2() and then uses RESOLVE_NO_SYMLINKS for a non-security reason,
> >>> it's possible that on some distributions (or future versions of a
> >>> distribution) that their application will stop working because a system
> >>> path suddenly contains a symlink or is a bind-mount.
> >>>
> >>> This was a concern which was brought up on LWN some time ago. If you can
> >>> think of a phrasing that makes this more clear, I'd appreciate it.
> >>
> >> Thanks. I've made the text:
> >>
> >> Applications that employ the RESOLVE_NO_XDEV flag
> >> are encouraged to make its use configurable (unless
> >> it is used for a specific security purpose), as bind
> >> mounts are widely used by end-users. Setting this
> >> flag indiscriminately—i.e., for purposes not specif‐
> >> ically related to security—for all uses of openat2()
> >> may result in spurious errors on previously-func‐
> >> tional systems. This may occur if, for example, a
> >> system pathname that is used by an application is
> >> modified (e.g., in a new distribution release) so
> >> that a pathname component (now) contains a bind
> >> mount.
> >>
> >> Okay?
> >
> > Yup,
>
> Thanks.
>
> > and the same text should be used for the same warning I gave for
> > RESOLVE_NO_SYMLINKS (for the same reason, because system paths may
> > switch to symlinks -- the prime example being what Arch Linux did
> > several years ago).
>
> Okay -- I added similar text to RESOLVE_NO_SYMLINKS.

Much appreciated.

> >>>> .\" FIXME: what specific details in symlink(7) are being referred
> >>>> .\" by the following sentence? It's not clear.
> >>>
> >>> The section on magic-links, but you're right that the sentence ordering
> >>> is a bit odd. It should probably go after the first sentence.
> >>
> >> 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.

> >> And, while I think of it, the text just preceding that FIXME says:
> >>
> >> Due to the potential danger of unknowingly opening
> >> these magic links, it may be preferable for users to
> >> disable their resolution entirely.
> >>
> >> This sentence reads a little strangely. Could you please give me some
> >> concrete examples, and I will try rewording that sentence a bit.
> >
> > The primary example is that certain files (such as tty devices) are
> > best not opened by an unsuspecting program (if you do not have a
> > controlling TTY, and you open such a file that console becomes your
> > controlling TTY unless you use O_NOCTTY).
> >
> > But more generally, magic-links allow programs to be "beamed" all over
> > the system (bypassing ordinary mount namespace restrictions). Since they
> > are fairly rarely used intentionally by most programs, this is more of a
> > tip to programmers that maybe they should play it safe and disallow
> > magic-links unless they are expecting to have to use them.
>
>
> 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.

> 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 -- 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).

Thanks!

--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]