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

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Message-ID: <CAKgNAkhCE0BHjHzc7My1shieDvohCRb-n3AL_E9P49EEsz5upA@mail.gmail.com>
Date:   Wed, 15 Apr 2020 22:24:00 +0200
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Aleksa Sarai <asarai@...e.de>
Cc:     Aleksa Sarai <cyphar@...har.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian@...uner.io>,
        linux-man <linux-man@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH man-pages v2 2/2] openat2.2: document new openat2(2) syscall

Hello Aleksa,

On Tue, 14 Apr 2020 at 12:35, Aleksa Sarai <asarai@...e.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/[...]