Re: [PATCH v4 0/3] Linux: Fix posix_spawn when user with time namespaces

[Andrei Vagin <avagin@gmail.com>]

On Fri, May 27, 2022 at 08:53:54AM -0700, Andrei Vagin wrote:
> On Wed, May 25, 2022 at 5:24 AM Florian Weimer <fweimer@redhat.com> wrote:
> >
> > * Christian Brauner:
> >
> > > On Tue, May 10, 2022 at 09:18:46PM +0200, Florian Weimer wrote:
> > >> * Adhemerval Zanella:
> > >>
> > >> > The patchset adds some support to tests the fallback code to
> > >> > use only use CLONE_VFORK.  It uses unshare directly because
> > >> > it simpler than add container support.
> > >> >
> > >> > Adhemerval Zanella (3):
> > >> >   linux: Add CLONE_NEWTIME from Linux 5.6 to bits/sched.h
> > >> >   support: Add support_enter_time_namespace
> > >> >   linux: Add fallback for clone failure on posix_spawn (BZ #29115)
> > >>
> > >> Christan, how likely is it that we'd get another time namespace variant
> > >> that would only become effective after execve (when the DSO is remapped
> > >> anyway)?
> > >
> > > Not unlikely if it helps you avoid a lot of complexity. I will need some
> > > time to track down Andrei and others to discuss though.
> >
> > Any progress with that?  (I hope I guessed the right Andrei.)
> 
> I think this is the right me. Have I missed something?
> 
> >
> > Breaking vfork is really a bit of a hassle for us, and the workaround
> > code is quite non-trivial and will have to implemented across many
> > projects (not just glibc).  An unshare request that takes effect on
> > execve only would really help.
> 
> Is the problem that vfork fails if a process has half-entered a time namespace?

I like the idea of entering a time namespace on exec and don't fail
vfork.  The only worry is that the behavior becomes more complicated and
less obvious.

Here is the untested patch:

diff --git a/fs/exec.c b/fs/exec.c
index 14b4b3755580..96f650ec7533 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -65,6 +65,7 @@
 #include <linux/io_uring.h>
 #include <linux/syscall_user_dispatch.h>
 #include <linux/coredump.h>
+#include <linux/time_namespace.h>
 
 #include <linux/uaccess.h>
 #include <asm/mmu_context.h>
@@ -1266,6 +1267,7 @@ int begin_new_exec(struct linux_binprm * bprm)
 	if (retval)
 		goto out;
 
+	timens_on_fork(me->nsproxy, me);
 	/*
 	 * Cancel any io_uring activity across execve
 	 */
diff --git a/kernel/fork.c b/kernel/fork.c
index 124829ed0163..653b80524a54 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2030,15 +2030,6 @@ static __latent_entropy struct task_struct
*copy_process(
 			return ERR_PTR(-EINVAL);
 	}
 
-	/*
-	 * If the new process will be in a different time namespace
-	 * do not allow it to share VM or a thread group with the
	 forking task.
-	 */
-	if (clone_flags & (CLONE_THREAD | CLONE_VM)) {
-		if (nsp->time_ns != nsp->time_ns_for_children)
-			return ERR_PTR(-EINVAL);
-	}
-
 	if (clone_flags & CLONE_PIDFD) {
 		/*
 		 * - CLONE_DETACHED is blocked so that we can
 		 * potentially
diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
index eec72ca962e2..b4cbb406bc28 100644
--- a/kernel/nsproxy.c
+++ b/kernel/nsproxy.c
@@ -179,7 +179,8 @@ int copy_namespaces(unsigned long flags, struct
task_struct *tsk)
 	if (IS_ERR(new_ns))
 		return  PTR_ERR(new_ns);
 
-	timens_on_fork(new_ns, tsk);
+	if ((flags & CLONE_VM) == 0)
+		timens_on_fork(new_ns, tsk);
 
 	tsk->nsproxy = new_ns;
 	return 0;

If this is what we want, I can prepare a final patch. But I will be on
vacation next week, so it will happen after it.

Thanks,
Andrei