Re: [PATCH 2/3] namei: implement AT_THIS_ROOT chroot-like path resolution

[Christian Brauner]

On Mon, Oct 01, 2018 at 01:29:16PM +0200, Jann Horn wrote:
> On Mon, Oct 1, 2018 at 12:42 PM Christian Brauner <christian@xxxxxxxxxx> wrote:
> > On Mon, Oct 01, 2018 at 03:44:28PM +1000, Aleksa Sarai wrote:
> > > On 2018-09-29, Jann Horn <jannh@xxxxxxxxxx> wrote:
> > > > The problem is what happens if a folder you are walking through is
> > > > concurrently moved out of the chroot. Consider the following scenario:
> > > >
> > > > You attempt to open "C/../../etc/passwd" under the root "/A/B".
> > > > Something else concurrently moves /A/B/C to /A/C. This can result in
> > > > the following:
> > > >
> > > > 1. You start the path walk and reach /A/B/C.
> > > > 2. The other process moves /A/B/C to /A/C. Your path walk is now at /A/C.
> > > > 3. Your path walk follows the first ".." up into /A. This is outside
> > > > the process root, but you never actually encountered the process root,
> > > > so you don't notice.
> > > > 4. Your path walk follows the second ".." up to /. Again, this is
> > > > outside the process root, but you don't notice.
> > > > 5. Your path walk walks down to /etc/passwd, and the open completes
> > > > successfully. You now have an fd pointing outside your chroot.
> > > >
> > > > If the root of your walk is below an attacker-controlled directory,
> > > > this of course means that you lose instantly. If you point the root of
> > > > the walk at a directory out of which a process in the container
> > > > wouldn't be able to move the file, you're probably kinda mostly fine -
> > > > as long as you know, for certain, that nothing else on the system
> > > > would ever do that. But I still wouldn't feel good about that.
> > >
> > > Please correct me if I'm wrong here (this is the first patch I've
> > > written for VFS). Isn't the retry/LOOKUP_REVAL code meant to handle this
> > > -- or does that only handle if a particular path component changes
> > > *while* it's being walked through? Is it possible for a path walk to
> > > succeed after a path component was unmounted (obviously you can't delete
> > > a directory path component since you'd get -ENOTEMPTY)?
> > >
> > > If this is an issue for AT_THIS_ROOT, I believe this might also be an
> > > issue for AT_BENEATH since they are effectively both using the same
> > > nd->root trick (so you could similarly trick AT_BENEATH to not error
> > > out). So we'd need to figure out how to solve this problem in order for
> > > AT_BENEATH to be safe.
> > >
> > > Speaking naively, doesn't it make sense to invalidate the walk if a path
> > > component was modified? Or is this something that would be far too
> > > costly with little benefit? What if we do more aggressive nd->root
> > > checks when resolving with AT_BENEATH or AT_THIS_ROOT (or if nd->root !=
> > > current->mnt_ns->root)?
> > >
> > > Regarding chroot attacks, I was aware of the trivial
> > > chroot-open-chroot-fchdir attack but I was not aware that there was a
> > > rename attack for chroot. Thanks for bringing this up!
> > >
> > > > I believe that the only way to robustly use this would be to point the
> > > > dirfd at a mount point, such that you know that being moved out of the
> > > > chroot is impossible because the mount point limits movement of
> > > > directories under it. (Well, technically, it doesn't, but it ensures
> > > > that if a directory does dangerously move away, the syscall fails.) It
> > > > might make sense to hardcode this constraint in the implementation of
> > > > AT_THIS_ROOT, to keep people from shooting themselves in the foot.
> > >
> > > Unless I'm missing something, would this not also affect using a
> > > mountpoint as a dirfd-root (with MS_MOVE of an already-walked-through
> > > path component) -- or does MS_MOVE cause a rewalk in a way that rename
> > > does not?
> > >
> > > I wouldn't mind tying AT_THIS_ROOT to only work on mountpoints (I
> > > thought that bind-mounts would be an issue but you also get -EXDEV when
> > > trying to rename across bind-mounts even if they are on the same
> > > underlying filesystem). But AT_BENEATH might be a more bitter pill to
> > > swallow. I'm not sure.
> > >
> > > In the usecase of container runtimes, we wouldn't generally be doing
> > > resolution of attacker-controlled paths but it still definitely doesn't
> > > hurt to consider this part of the threat model -- to avoid foot-gunning
> > > as you've said. (There also might be some nested-container cases where
> > > you might want to do that.)
> > >
> > > > > Currently most container runtimes try to do this resolution in
> > > > > userspace[1], causing many potential race conditions. In addition, the
> > > > > "obvious" alternative (actually performing a {ch,pivot_}root(2))
> > > > > requires a fork+exec which is *very* costly if necessary for every
> > > > > filesystem operation involving a container.
> > > >
> > > > Wait. fork() I understand, but why exec? And actually, you don't need
> > > > a full fork() either, clone() lets you do this with some process parts
> > > > shared. And then you also shouldn't need to use SCM_RIGHTS, just keep
> > > > the file descriptor table shared. And why chroot()/pivot_root(),
> > > > wouldn't you want to use setns()?
> > >
> > > You're right about this -- for C runtimes. In Go we cannot do a raw
> > > clone() or fork() (if you do it manually with RawSyscall you'll end with
> > > broken runtime state). So you're forced to do fork+exec (which then
> > > means that you can't use CLONE_FILES and must use SCM_RIGHTS). Same goes
> > > for CLONE_VFORK.
> > >
> > > (It should be noted that multi-threaded C runtimes have somewhat similar
> > > issues -- AFAIK you can technically only use AS-Safe glibc functions
> > > after a fork() but that's more of a theoretical concern here. If you
> > > just use raw syscalls there isn't an issue.)
> > >
> > > As for why use setns() rather than pivot_root(), there are cases where
> > > you're operating on a container's image without a running container
> > > (think image extraction or snapshotting tools). In those cases, you
> > > would need to set up a dummy container process in order to setns() into
> > > its namespaces. You are right that setns() would be a better option if
> > > you want the truthful state of what mounts the container sees.
> > >
> > > [I also don't like the idea of joining the user namespace of a malicious
> > > container unless it's necessary but that's probably just needless
> > > paranoia more than anything -- since you're not joining the pidns you
> > > aren't trivially addressable by a malicious container.]
> > >
> > > > // Ensure that we are non-dumpable. Together with
> > > > // commit bfedb589252c, this ensures that container root
> > > > // can't trace our child once it enters the container.
> > > > // My patch
> > > > // https://lore.kernel.org/lkml/1451098351-8917-1-git-send-email-jann@xxxxxxxxx/
> > > > // would make this unnecessary, but that patch didn't
> > > > // land because Eric nacked it (for political reasons,
> > > > // because people incorrectly claimed that this was a
> > > > // security fix):
> > >
> > > Unless I'm very much mistaken this was fixed by bfedb589252c ("mm: Add a
> > > user_ns owner to mm_struct and fix ptrace permission checks"). If you
> > > join a user namespace then processes within that user namespace won't
> > > have ptrace_may_access() permissions because your mm is owned by an
> > > ancestor user namespace -- only after exec() will you be traceable.
> >
> > That is not _completely_ true.
> > Iirc (Please someone do yell at me if I'm wrong!), this is as follows.
> > You will in fact be dumpable as long as you don't set{g,u}id() to an
> > effective uid that is different from the effective uid of the process
> > that created the task. For example, if you clone(CLONE_NEWUSER) as an
> > unprivileged user with uid and euid 1000 you are in fact dumpable and
> > thus traceable *but* if you do a setuid(0) in the new task then you will
> > end up with old->euid = 1000 and new->euid = 0 at which point the kernel
> > will remove the dumpable flag and the creating process cannot trace you
> > anymore (which has funny consequences for lsm isolation and sending fds
> > around). Iiuc, The same logic applies when you do a setns() to another
> > user namespace.
> 
> (Note that this is only true if your un-namespaced UID actually
> changes. If you create a user namespace and then write to its uid_map
> such that your namespaced UID is zero, that won't trigger this logic.)

The way I figured this works is that it actually only applies to the
case where you're creating a user namespace as an unprivileged user. And
even in that case you will retain dumpability in two cases:
1. mapping userns id 0 to <my-unpriv-host-uid>
and any identity mapping, i.e.
2. mapping <my-unpriv-host-uid> to <my-unpriv-host-uid>

If you're creating a user namespace as root and set up mappings you
should always retain the dumpable flag (I might not be remembering all
corner-cases atm.) if you don't muck with capability sets and so on.