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

Michael Kerrisk (man-pages)
2020-04-13T07:22:47Z

Hello Aleksa,

On 4/12/20 6:49 PM, Aleksa Sarai wrote:
> Sorry, I could've sworn I responded when you posted this -- comments
> below. And sorry for not getting back to you before the 5.06 release.

No worries and ahanks for your feedback below.

[...]

>>>> .\" 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.

>>>> .\" 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!

>> 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?

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.

?

[...]

>>>> .\" FIXME The next piece is unclear (to me). What kind of ".." escape
>>>> .\" attempts does chroot() not detect that RESOLVE_IN_ROOT does?
>>>
>>> If the root is moved, you can escape from a chroot(2). But this sentence
>>> might not really belong in a man-page since it's describing (important)
>>> aspects of the implementation and not the semantics.
>>
>> So, should I just remove the sentence?
>
> Yup, sounds reasonable.

Done.

Thanks,

Michael

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