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

[Christian Brauner <brauner@kernel.org>]

On Fri, May 27, 2022 at 05:03:14PM -0700, Andrei Vagin wrote:
> 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.

No problem. It's the merge window anyway and it's May which means
bank holiday galore in parts of Europe.