Re: [PATCH v2 0/3] pidfd: add task path ioctls

[Jann Horn]

On Wed, Sep 2, 2026 at 8:47 AM Chen Linxuan <me@xxxxxxxxxxxxx> wrote:
> On Mon, Aug 31, 2026 at 4:49 PM Christian Brauner <brauner@xxxxxxxxxx> wrote:
> > On 2026-08-31 10:59 +0800, Chen Linxuan wrote:
> > > Obtaining a target task's executable, working directory, or root
> > > currently requires walking procfs symlinks such as /proc/<pid>/exe,
> > > /proc/<pid>/cwd, and /proc/<pid>/root.  That makes the operation depend
> > > on procfs being mounted and visible to the caller, even when it already
> > > holds a pidfd for the target.
> > >
> > > This series adds PIDFD_GET_EXE, PIDFD_GET_CWD, and PIDFD_GET_ROOT. Each
> > > ioctl takes no argument and returns a close-on-exec O_PATH file
> > > descriptor referencing the corresponding task path.  The new ioctls use
> > > the same ptrace permission check and nonzero-argument rejection as the
> > > existing pidfd namespace ioctls.
> > >
> > > The target task is sampled while holding its exec_update_lock. This
> > > keeps the access decision and the task-state read in the same exec
> > > critical section, preventing a concurrent execve() from changing the
> > > credentials or target state between the check and the use.
> > >
> > > The first patch factors out helpers for acquiring referenced task paths
> > > and reuses them in procfs and AppArmor.  The second patch introduces
> > > scoped cleanup for privileged pidfd task access and separates namespace
> > > lookup from namespace fd creation.  The final patch uses these pieces to
> > > implement the three new ioctls.
> > >
> > > Signed-off-by: Chen Linxuan <me@xxxxxxxxxxxxx>
> > > ---
> >
> > I really have difficulties forming an opinion on this. So this sounds
> > very useful but it has implications.
> >
> > Right now, pidfd ioctls are available even in situations where the task
> > in question would not be accessible via procfs, e.g., when procfs is
> > mounted with "hidepid" options or similar. So this would expand the
>
> One point regarding hidepid: for callers that can pass
> PTRACE_MODE_READ_FSCREDS, hidepid does not provide an additional
> restriction. With hidepid=1 or hidepid=2, has_pid_permissions() falls
> back to ptrace_may_access(..., PTRACE_MODE_READ_FSCREDS), and
> hidepid=ptraceable uses that check directly. In addition, the
> /proc/<pid>/{exe,cwd,root} links independently perform the same
> PTRACE_MODE_READ_FSCREDS check in call_proc_get_link(), regardless of
> the hidepid mode.
>
> So for these specific path lookups, hidepid does not block a caller who
> would already pass the check used by the proposed pidfd ioctls.

Agreed, I think with regards to hidepid there should be no issue here.

If I try to come up with scenarios in which this could introduce
additional danger, the main one I can think of would be: A task T1 is
running inside a pid namespace and has a unix domain socket connection
to a task T2 outside the namespace, with both running as the same
EUID; T1 sets SO_PASSPIDFD on its socket to obtain a pidfd pointing to
T2 on the next message sent by T2, then T1 uses that to get access to
the mount namespace of T2.

Christian, is there some mechanism that already protects against using
something like SO_PASSPIDFD to get a pidfd to a process in a parent
namespace?
Otherwise, should we add something like a "pid_vnr(pid) != 0" check
either in these new operations or in SO_PASSPIDFD?