Date: Mon, 27 Apr 2020 21:48:15 +0200 From: Christian Brauner <christian.brauner@...ntu.com> To: Jann Horn <jannh@...gle.com> Cc: kernel list <linux-kernel@...r.kernel.org>, Alexander Viro <viro@...iv.linux.org.uk>, Stéphane Graber <stgraber@...ntu.com>, Linux Containers <containers@...ts.linux-foundation.org>, "Eric W . Biederman" <ebiederm@...ssion.com>, Serge Hallyn <serge@...lyn.com>, Aleksa Sarai <cyphar@...har.com>, linux-security-module <linux-security-module@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Linux API <linux-api@...r.kernel.org> Subject: Re: [PATCH] nsproxy: attach to namespaces via pidfds On Mon, Apr 27, 2020 at 09:41:20PM +0200, Jann Horn wrote: > On Mon, Apr 27, 2020 at 8:15 PM Christian Brauner > <christian.brauner@...ntu.com> wrote: > > On Mon, Apr 27, 2020 at 07:28:56PM +0200, Jann Horn wrote: > > > On Mon, Apr 27, 2020 at 4:47 PM Christian Brauner > > > <christian.brauner@...ntu.com> wrote: > [...] > > > > That means > > > > setns(nsfd, CLONE_NEWNET) equals setns(pidfd, CLONE_NEWNET). However, > > > > when a pidfd is passed, multiple namespace flags can be specified in the > > > > second setns() argument and setns() will attach the caller to all the > > > > specified namespaces all at once or to none of them. If 0 is specified > > > > together with a pidfd then setns() will interpret it the same way 0 is > > > > interpreted together with a nsfd argument, i.e. attach to any/all > > > > namespaces. > > > [...] > > > > Apart from significiantly reducing the number of syscalls from double > > > > digit to single digit which is a decent reason post-spectre/meltdown > > > > this also allows to switch to a set of namespaces atomically, i.e. > > > > either attaching to all the specified namespaces succeeds or we fail. > > > > > > Apart from the issues I've pointed out below, I think it's worth > > > calling out explicitly that with the current design, the switch will > > > not, in fact, be fully atomic - the process will temporarily be in > > > intermediate stages where the switches to some namespaces have > > > completed while the switches to other namespaces are still pending; > > > and while there will be less of these intermediate stages than before, > > > it also means that they will be less explicit to userspace. > > > > Right, that can be fixed by switching to the unshare model of getting a > > new set of credentials and committing it after the nsproxy has been > > installed? Then there shouldn't be an intermediate state anymore or > > rather an intermediate stage where we can still fail somehow. > > It still wouldn't be atomic (in the sense of parallelism, not in the > sense of intermediate error handling) though; for example, if task B > does setns(<pidfd_of_task_a>, 0) and task C concurrently does > setns(<pidfd_of_task_b>, 0), then task C may end up with the new mount > namespace of task B but the old user namespace, or something like > that. If C is more privileged than B, that may cause C to have more > privileges through its configuration of namespaces than B does (e.g. > by running in the &init_user_ns but with a mount namespace owned by an > unprivileged user), which C may not expect. Same thing for racing > between unshare() and setns(). > > [...] > > > > + put_user_ns(user_ns); > > > > + } > > > > +#else > > > > + if (flags & CLONE_NEWUSER) > > > > + ret = -EINVAL; > > > > +#endif > > > > + > > > > + if (!ret && wants_ns(flags, CLONE_NEWNS)) > > > > + ret = __ns_install(nsproxy, mnt_ns_to_common(nsp->mnt_ns)); > > > > > > And this one might be even worse, because the mount namespace change > > > itself is only stored in the nsproxy at this point, but the cwd and > > > root paths have already been overwritten on the task's fs_struct. > > > > > > To actually make sys_set_ns() atomic, I think you'd need some > > > moderately complicated prep work, splitting the ->install handlers up > > > into prep work and a commit phase that can't fail. > > > > Wouldn't it be sufficient to move to an unshare like model, i.e. > > creating a new set of creds, and passing the new user_ns to > > create_new_namespaces() as well as having a temporary new_fs struct? > > That should get rid of all intermediate stages. > > Ah, good point, I didn't realize that that already exists for unshare(). Let me try and switch the patch to that. Christian
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