Re: [PATCH 0/8] ns: fixes for namespace iteration and active reference counting

[Hillf Danton <hdanton@sina.com>]

On Sun, 09 Nov 2025 22:11:21 +0100 Christian Brauner wrote:
> * Make sure to initialize the active reference count for the initial
>   network namespace and prevent __ns_common_init() from returning too
>   early.
> 
> * Make sure that passive reference counts are dropped outside of rcu
>   read locks as some namespaces such as the mount namespace do in fact
>   sleep when putting the last reference.
> 
> * The setns() system call supports:
> 
>   (1) namespace file descriptors (nsfd)
>   (2) process file descriptors (pidfd)
> 
>   When using nsfds the namespaces will remain active because they are
>   pinned by the vfs. However, when pidfds are used things are more
>   complicated.
> 
>   When the target task exits and passes through exit_nsproxy_namespaces()
>   or is reaped and thus also passes through exit_cred_namespaces() after
>   the setns()'ing task has called prepare_nsset() but before the active
>   reference count of the set of namespaces it wants to setns() to might
>   have been dropped already:
> 
>     P1                                                              P2
> 
>     pid_p1 = clone(CLONE_NEWUSER | CLONE_NEWNET | CLONE_NEWNS)
>                                                                     pidfd = pidfd_open(pid_p1)
>                                                                     setns(pidfd, CLONE_NEWUSER | CLONE_NEWNET | CLONE_NEWNS)
>                                                                     prepare_nsset()
> 
>     exit(0)
>     // ns->__ns_active_ref        == 1
>     // parent_ns->__ns_active_ref == 1
>     -> exit_nsproxy_namespaces()
>     -> exit_cred_namespaces()
> 
>     // ns_active_ref_put() will also put
>     // the reference on the owner of the
>     // namespace. If the only reason the
>     // owning namespace was alive was
>     // because it was a parent of @ns
>     // it's active reference count now goes
>     // to zero... --------------------------------
>     //                                           |
>     // ns->__ns_active_ref        == 0           |
>     // parent_ns->__ns_active_ref == 0           |
>                                                  |                  commit_nsset()
>                                                  -----------------> // If setns()
>                                                                     // now manages to install the namespaces
>                                                                     // it will call ns_active_ref_get()
>                                                                     // on them thus bumping the active reference
>                                                                     // count from zero again but without also
>                                                                     // taking the required reference on the owner.
>                                                                     // Thus we get:
>                                                                     //
>                                                                     // ns->__ns_active_ref        == 1
>                                                                     // parent_ns->__ns_active_ref == 0
> 
>     When later someone does ns_active_ref_put() on @ns it will underflow
>     parent_ns->__ns_active_ref leading to a splat from our asserts
>     thinking there are still active references when in fact the counter
>     just underflowed.
> 
>   So resurrect the ownership chain if necessary as well. If the caller
>   succeeded to grab passive references to the set of namespaces the
>   setns() should simply succeed even if the target task exists or gets
>   reaped in the meantime.
> 
>   The race is rare and can only be triggered when using pidfs to setns()
>   to namespaces. Also note that active reference on initial namespaces are
>   nops.
> 
>   Since we now always handle parent references directly we can drop
>   ns_ref_active_get_owner() when adding a namespace to a namespace tree.
>   This is now all handled uniformly in the places where the new namespaces
>   actually become active.
> 
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
>
FYI namespace-6.19.fixes failed to survive the syzbot test [1].

[1] Subject: Re: [syzbot] [lsm?] WARNING in put_cred_rcu
https://lore.kernel.org/lkml/690eedba.a70a0220.22f260.0075.GAE@google.com/