On 2020-04-13, Michael Kerrisk (man-pages) <mtk.manpages@xxxxxxxxx> 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/>
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