On 2019-04-23, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > On Mon, Mar 25, 2019 at 6:05 AM Aleksa Sarai <cyphar@xxxxxxxxxx> wrote: > > On 2019-03-21, Andy Lutomirski <luto@xxxxxxxxxx> wrote: > > > On Wed, Mar 20, 2019 at 7:38 AM Aleksa Sarai <cyphar@xxxxxxxxxx> 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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