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    SubjectRe: [PATCH RESEND v5 0/5] namei: vfs flags to restrict path resolution
    On 2019-04-23, Kees Cook <keescook@chromium.org> wrote:
    > On Mon, Mar 25, 2019 at 6:05 AM Aleksa Sarai <cyphar@cyphar.com> wrote:
    > > On 2019-03-21, Andy Lutomirski <luto@kernel.org> wrote:
    > > > On Wed, Mar 20, 2019 at 7:38 AM Aleksa Sarai <cyphar@cyphar.com> wrote:
    > > > > Now that the holiday break is over, it's time to re-send this patch
    > > > > series (with a few additions, due to new information we got from
    > > > > CVE-2019-5736 -- which this patchset mostly protected against but had
    > > > > some holes with regards to #!-style scripts).
    > > >
    > > > I generally like this, but, as Linus pointed out, it will be
    > > > unfortunate if application authors see this as just another
    > > > non-portable weird Linux API and don't use it. Would it be worthwhile
    > > > to put some thought into making it an API that other OSes might be
    > > > willing to implement? As it stands, the openat(2) flags are getting
    > > > rather crazy in this patch set.
    >
    > I think many of the issues are specific to Linux (and Linux containers
    > especially), so I'm not sure this should get blocked because we want
    > something more portable.

    I agree these issues are quite Linux-specific (*especially* the ability
    to re-open fds through /proc and the existence of "magic links").

    However, I feel there are a few more good reasons for resolveat(2):

    * openat(2) ignores unknown flags, meaning that old kernels will ignore
    new programs trying to use O_THISROOT and might end up causing
    security issues. Yes, it'd be trivial to check whether the new O_*
    flags are supported at start-up, but I think a security feature
    shouldn't have a foot-gun associated with it. In fact, I didn't know
    openat(2) ignored unknown flags until I wrote this patchset -- I
    doubt many other userspace developers do either.

    * resolveat(2) allows for improvement to the O_PATH interface, which I
    think might be necessary (completely separately to this patchset).
    I've been working on a patchset which would make nd_jump_link()
    transitions in trailing_symlink() depend on the mode of the magic
    link being traversed through (this would allow us to block a
    read-only fd being re-opened as a read-write fd or similar such
    nonsense). One aspect of this could be to allow userspace to enable
    certain re-opening operations by passing a "link mode" to
    resolveat(2).

    * I would argue that O_PATH should've been a separate syscall from the
    beginning, given how different its semantics are to other openat(2)
    flags (not to mention how O_PATH is incompatible with and thus
    ignores so many other openat(2) flags).

    * If we end up needing a resolveat(2) for any of the above reasons,
    then we will have wasted quite a few openat(2) flag slots for naught.
    (Then again, there are plenty of flag slots still left.)

    All of that aside, what I'd really like to know is what I should do to
    get this patchset reviewed and merged. It's been largely radio-silence
    for the last few revisions.

    A simple resolveat(2) is fairly trivial (I have a version of it lying
    around somewhere), but it doesn't make sense to polish it if there's no
    chance Al is interested in it.

    > This series provides solutions to so many different race and confusion
    > issues, I'd really like to see it land. What's the next step here? Is
    > this planned to go directly to Linus for v5.2, or is it going to live
    > in -mm for a while? I'd really like to see this moving forward.

    Given some of the security requirements of this interface, I think
    getting it to live in -mm wouldn't be a bad idea so folks can shake the
    bugs out before it's depended on by container runtimes.

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