[PATCH RFC 0/4] procfs: make reference pidns more user-visible

[PATCH RFC 0/4] procfs: make reference pidns more user-visible

[Aleksa Sarai]

Ever since the introduction of pid namespaces, procfs has had very
implicit behaviour surrounding them (the pidns used by a procfs mount is
auto-selected based on the mounting process's active pidns, and the
pidns itself is basically hidden once the mount has been constructed).
This has historically meant that userspace was required to do some
special dances in order to configure the pidns of a procfs mount as
desired. Examples include:

 * In order to bypass the mnt_too_revealing() check, Kubernetes creates
   a procfs mount from an empty pidns so that user namespaced containers
   can be nested (without this, the nested containers would fail to
   mount procfs). But this requires forking off a helper process because
   you cannot just one-shot this using mount(2).

 * Container runtimes in general need to fork into a container before
   configuring its mounts, which can lead to security issues in the case
   of shared-pidns containers (a privileged process in the pidns can
   interact with your container runtime process). While
   SUID_DUMP_DISABLE and user namespaces make this less of an issue, the
   strict need for this due to a minor uAPI wart is kind of unfortunate.

Things would be much easier if there was a way for userspace to just
specify the pidns they want. Patch 1 implements a new "pidns" argument
which can be set using fsconfig(2):

    fsconfig(procfd, FSCONFIG_SET_FD, "pidns", NULL, nsfd);
    fsconfig(procfd, FSCONFIG_SET_STRING, "pidns", "/proc/self/ns/pid", 0);

or classic mount(2) / mount(8):

    // mount -t proc -o pidns=/proc/self/ns/pid proc /tmp/proc
    mount("proc", "/tmp/proc", "proc", MS_..., "pidns=/proc/self/ns/pid");

The initial security model I have in this RFC is to be as conservative
as possible and just mirror the security model for setns(2) -- which
means that you can only set pidns=... to pid namespaces that your
current pid namespace is a direct ancestor of. This fulfils the
requirements of container runtimes, but I suspect that this may be too
strict for some usecases.

The pidns argument is not displayed in mountinfo -- it's not clear to me
what value it would make sense to show (maybe we could just use ns_dname
to provide an identifier for the namespace, but this number would be
fairly useless to userspace). I'm open to suggestions.

In addition, being able to figure out what pid namespace is being used
by a procfs mount is quite useful when you have an administrative
process (such as a container runtime) which wants to figure out the
correct way of mapping PIDs between its own namespace and the namespace
for procfs (using NS_GET_{PID,TGID}_{IN,FROM}_PIDNS). There are
alternative ways to do this, but they all rely on ancillary information
that third-party libraries and tools do not necessarily have access to.

To make this easier, add a new ioctl (PROCFS_GET_PID_NAMESPACE) which
can be used to get a reference to the pidns that a procfs is using.

It's not quite clear what is the correct security model for this API,
but the current approach I've taken is to:

 * Make the ioctl only valid on the root (meaning that a process without
   access to the procfs root -- such as only having an fd to a procfs
   file or some open_tree(2)-like subset -- cannot use this API).

 * Require that the process requesting either has access to
   /proc/1/ns/pid anyway (i.e. has ptrace-read access to the pidns
   pid1), has CAP_SYS_ADMIN access to the pidns (i.e. has administrative
   access to it and can join it if they had a handle), or is in a pidns
   that is a direct ancestor of the target pidns (i.e. all of the pids
   are already visible in the procfs for the current process's pidns).

The security model for this is a little loose, as it seems to me that
all of the cases mentioned are valid cases to allow access, but I'm open
to suggestions for whether we need to make this stricter or looser.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
Aleksa Sarai (4):
      pidns: move is-ancestor logic to helper
      procfs: add pidns= mount option
      procfs: add PROCFS_GET_PID_NAMESPACE ioctl
      selftests/proc: add tests for new pidns APIs

 Documentation/filesystems/proc.rst        |  10 ++
 fs/proc/root.c                            | 132 +++++++++++++-
 include/linux/pid_namespace.h             |   9 +
 include/uapi/linux/fs.h                   |   3 +
 kernel/pid_namespace.c                    |  21 ++-
 tools/testing/selftests/proc/.gitignore   |   1 +
 tools/testing/selftests/proc/Makefile     |   1 +
 tools/testing/selftests/proc/proc-pidns.c | 286 ++++++++++++++++++++++++++++++
 8 files changed, 448 insertions(+), 15 deletions(-)
---
base-commit: 4c838c7672c39ec6ec48456c6ce22d14a68f4cda
change-id: 20250717-procfs-pidns-api-8ed1583431f0

Best regards,
-- 
Aleksa Sarai <cyphar@cyphar.com>

[PATCH RFC 1/4] pidns: move is-ancestor logic to helper

[Aleksa Sarai]

This check will be needed in later patches, and there's no point
open-coding it each time.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 include/linux/pid_namespace.h |  9 +++++++++
 kernel/pid_namespace.c        | 21 ++++++++++++++-------
 2 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
index 7c67a5811199..df421a1e3e0b 100644
--- a/include/linux/pid_namespace.h
+++ b/include/linux/pid_namespace.h
@@ -84,6 +84,9 @@ extern void zap_pid_ns_processes(struct pid_namespace *pid_ns);
 extern int reboot_pid_ns(struct pid_namespace *pid_ns, int cmd);
 extern void put_pid_ns(struct pid_namespace *ns);
 
+extern bool pidns_is_ancestor(struct pid_namespace *child,
+			      struct pid_namespace *ancestor);
+
 #else /* !CONFIG_PID_NS */
 #include <linux/err.h>
 
@@ -118,6 +121,12 @@ static inline int reboot_pid_ns(struct pid_namespace *pid_ns, int cmd)
 {
 	return 0;
 }
+
+bool pidns_is_ancestor(struct pid_namespace *child,
+		       struct pid_namespace *ancestor)
+{
+	return false;
+}
 #endif /* CONFIG_PID_NS */
 
 extern struct pid_namespace *task_active_pid_ns(struct task_struct *tsk);
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index 7098ed44e717..5719b1f679ad 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -390,6 +390,19 @@ static void pidns_put(struct ns_common *ns)
 	put_pid_ns(to_pid_ns(ns));
 }
 
+bool pidns_is_ancestor(struct pid_namespace *child,
+		       struct pid_namespace *ancestor)
+{
+	struct pid_namespace *ns;
+
+	if (child->level < ancestor->level)
+		return false;
+	for (ns = child; ns->level > ancestor->level; ns = ns->parent)
+		;
+	return ns == ancestor;
+}
+EXPORT_SYMBOL_GPL(pidns_is_ancestor);
+
 static int pidns_install(struct nsset *nsset, struct ns_common *ns)
 {
 	struct nsproxy *nsproxy = nsset->nsproxy;
@@ -408,13 +421,7 @@ static int pidns_install(struct nsset *nsset, struct ns_common *ns)
 	 * this maintains the property that processes and their
 	 * children can not escape their current pid namespace.
 	 */
-	if (new->level < active->level)
-		return -EINVAL;
-
-	ancestor = new;
-	while (ancestor->level > active->level)
-		ancestor = ancestor->parent;
-	if (ancestor != active)
+	if (!pidns_is_ancestor(new, active))
 		return -EINVAL;
 
 	put_pid_ns(nsproxy->pid_ns_for_children);

-- 
2.50.0

[PATCH RFC 2/4] procfs: add pidns= mount option

[Aleksa Sarai]

Since the introduction of pid namespaces, their interaction with procfs
has been entirely implicit in ways that require a lot of dancing around
by programs that need to construct sandboxes with different PID
namespaces.

Being able to explicitly specify the pid namespace to use when
constructing a procfs super block will allow programs to no longer need
to fork off a process which does then does unshare(2) / setns(2) and
forks again in order to construct a procfs in a pidns.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 Documentation/filesystems/proc.rst |  6 +++
 fs/proc/root.c                     | 80 +++++++++++++++++++++++++++++++++++---
 2 files changed, 80 insertions(+), 6 deletions(-)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 5236cb52e357..c520b9f8a3fd 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -2360,6 +2360,7 @@ The following mount options are supported:
 	hidepid=	Set /proc/<pid>/ access mode.
 	gid=		Set the group authorized to learn processes information.
 	subset=		Show only the specified subset of procfs.
+	pidns=		Specify a the namespace used by this procfs.
 	=========	========================================================
 
 hidepid=off or hidepid=0 means classic mode - everybody may access all
@@ -2392,6 +2393,11 @@ information about processes information, just add identd to this group.
 subset=pid hides all top level files and directories in the procfs that
 are not related to tasks.
 
+pidns= specifies a pid namespace (either as a string path to something like
+`/proc/$pid/ns/pid`, or a file descriptor when using `FSCONFIG_SET_FD`) that
+will be used by the procfs instance when translating pids. By default, procfs
+will use the calling process's active pid namespace.
+
 Chapter 5: Filesystem behavior
 ==============================
 
diff --git a/fs/proc/root.c b/fs/proc/root.c
index ed86ac710384..10ca94be0eef 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -38,12 +38,14 @@ enum proc_param {
 	Opt_gid,
 	Opt_hidepid,
 	Opt_subset,
+	Opt_pidns,
 };
 
 static const struct fs_parameter_spec proc_fs_parameters[] = {
-	fsparam_u32("gid",	Opt_gid),
+	fsparam_u32("gid",		Opt_gid),
 	fsparam_string("hidepid",	Opt_hidepid),
 	fsparam_string("subset",	Opt_subset),
+	fsparam_file_or_string("pidns",	Opt_pidns),
 	{}
 };
 
@@ -109,11 +111,65 @@ static int proc_parse_subset_param(struct fs_context *fc, char *value)
 	return 0;
 }
 
+static int proc_parse_pidns_param(struct fs_context *fc,
+				  struct fs_parameter *param,
+				  struct fs_parse_result *result)
+{
+	struct proc_fs_context *ctx = fc->fs_private;
+	struct pid_namespace *target, *active = task_active_pid_ns(current);
+	struct ns_common *ns;
+	struct file *ns_filp __free(fput) = NULL;
+
+	switch (param->type) {
+	case fs_value_is_file:
+		/* came throug fsconfig, steal the file reference */
+		ns_filp = param->file;
+		param->file = NULL;
+		break;
+	case fs_value_is_string:
+		ns_filp = filp_open(param->string, O_RDONLY, 0);
+		break;
+	default:
+		WARN_ON_ONCE(true);
+		break;
+	}
+	if (!ns_filp)
+		ns_filp = ERR_PTR(-EBADF);
+	if (IS_ERR(ns_filp)) {
+		errorfc(fc, "could not get file from pidns argument");
+		return PTR_ERR(ns_filp);
+	}
+
+	if (!proc_ns_file(ns_filp))
+		return invalfc(fc, "pidns argument is not an nsfs file");
+	ns = get_proc_ns(file_inode(ns_filp));
+	if (ns->ops->type != CLONE_NEWPID)
+		return invalfc(fc, "pidns argument is not a pidns file");
+	target = container_of(ns, struct pid_namespace, ns);
+
+	/*
+	 * pidns= is shorthand for joining the pidns to get a fsopen fd, so the
+	 * permission model should be the same as pidns_install().
+	 */
+	if (!ns_capable(target->user_ns, CAP_SYS_ADMIN)) {
+		errorfc(fc, "insufficient permissions to set pidns");
+		return -EPERM;
+	}
+	if (!pidns_is_ancestor(target, active))
+		return invalfc(fc, "cannot set pidns to non-descendant pidns");
+
+	put_pid_ns(ctx->pid_ns);
+	ctx->pid_ns = get_pid_ns(target);
+	put_user_ns(fc->user_ns);
+	fc->user_ns = get_user_ns(ctx->pid_ns->user_ns);
+	return 0;
+}
+
 static int proc_parse_param(struct fs_context *fc, struct fs_parameter *param)
 {
 	struct proc_fs_context *ctx = fc->fs_private;
 	struct fs_parse_result result;
-	int opt;
+	int opt, err;
 
 	opt = fs_parse(fc, proc_fs_parameters, param, &result);
 	if (opt < 0)
@@ -125,13 +181,21 @@ static int proc_parse_param(struct fs_context *fc, struct fs_parameter *param)
 		break;
 
 	case Opt_hidepid:
-		if (proc_parse_hidepid_param(fc, param))
-			return -EINVAL;
+		err = proc_parse_hidepid_param(fc, param);
+		if (err)
+			return err;
 		break;
 
 	case Opt_subset:
-		if (proc_parse_subset_param(fc, param->string) < 0)
-			return -EINVAL;
+		err = proc_parse_subset_param(fc, param->string);
+		if (err)
+			return err;
+		break;
+
+	case Opt_pidns:
+		err = proc_parse_pidns_param(fc, param, &result);
+		if (err)
+			return err;
 		break;
 
 	default:
@@ -154,6 +218,10 @@ static void proc_apply_options(struct proc_fs_info *fs_info,
 		fs_info->hide_pid = ctx->hidepid;
 	if (ctx->mask & (1 << Opt_subset))
 		fs_info->pidonly = ctx->pidonly;
+	if (ctx->mask & (1 << Opt_pidns)) {
+		put_pid_ns(fs_info->pid_ns);
+		fs_info->pid_ns = get_pid_ns(ctx->pid_ns);
+	}
 }
 
 static int proc_fill_super(struct super_block *s, struct fs_context *fc)

-- 
2.50.0

[PATCH RFC 3/4] procfs: add PROCFS_GET_PID_NAMESPACE ioctl

[Aleksa Sarai]

/proc has historically had very opaque semantics about PID namespaces,
which is a little unfortunate for container runtimes and other programs
that deal with switching namespaces very often. One common issue is that
of converting between PIDs in the process's namespace and PIDs in the
namespace of /proc.

In principle, it is possible to do this today by opening a pidfd with
pidfd_open(2) and then looking at /proc/self/fdinfo/$n (which will
contain a PID value translated to the pid namespace associated with that
procfs superblock).

However, allocating a new file for each PID to be converted is less than
ideal for programs that may need to scan procfs, and it is generally
useful for userspace to be able to finally get this information from
procfs. This also acts as a sister feature to the pidns= mount option,
finally allowing userspace full control of the pid namespaces associated
with /proc instances.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 Documentation/filesystems/proc.rst |  4 +++
 fs/proc/root.c                     | 52 ++++++++++++++++++++++++++++++++++++--
 include/uapi/linux/fs.h            |  3 +++
 3 files changed, 57 insertions(+), 2 deletions(-)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index c520b9f8a3fd..506383273c9d 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -2398,6 +2398,10 @@ pidns= specifies a pid namespace (either as a string path to something like
 will be used by the procfs instance when translating pids. By default, procfs
 will use the calling process's active pid namespace.
 
+Processes can check which pid namespace is used by a procfs instance by using
+the `PROCFS_GET_PID_NAMESPACE` ioctl() on the root directory of the procfs
+instance.
+
 Chapter 5: Filesystem behavior
 ==============================
 
diff --git a/fs/proc/root.c b/fs/proc/root.c
index 10ca94be0eef..ee90749ccd8e 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -23,8 +23,10 @@
 #include <linux/cred.h>
 #include <linux/magic.h>
 #include <linux/slab.h>
+#include <linux/ptrace.h>
 
 #include "internal.h"
+#include "../internal.h"
 
 struct proc_fs_context {
 	struct pid_namespace	*pid_ns;
@@ -408,15 +410,61 @@ static int proc_root_readdir(struct file *file, struct dir_context *ctx)
 	return proc_pid_readdir(file, ctx);
 }
 
+static long int proc_root_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
+{
+	switch (cmd) {
+	case PROCFS_GET_PID_NAMESPACE: {
+		struct pid_namespace *active = task_active_pid_ns(current);
+		struct pid_namespace *ns = proc_pid_ns(file_inode(filp)->i_sb);
+		bool can_access_pidns = false;
+
+		/*
+		 * If we are in an ancestors of the pidns, or have join
+		 * privileges (CAP_SYS_ADMIN), then it makes sense that we
+		 * would be able to grab a handle to the pidns.
+		 *
+		 * Otherwise, if there is a root process, then being able to
+		 * access /proc/$pid/ns/pid is equivalent to this ioctl and so
+		 * we should probably match the permission model. For empty
+		 * namespaces it seems unlikely for there to be a downside to
+		 * allowing unprivileged users to open a handle to it (setns
+		 * will fail for unprivileged users anyway).
+		 */
+		can_access_pidns = pidns_is_ancestor(ns, active) ||
+				   ns_capable(ns->user_ns, CAP_SYS_ADMIN);
+		if (!can_access_pidns) {
+			bool cannot_ptrace_pid1 = false;
+
+			read_lock(&tasklist_lock);
+			if (ns->child_reaper)
+				cannot_ptrace_pid1 = ptrace_may_access(ns->child_reaper,
+								       PTRACE_MODE_READ_FSCREDS);
+			read_unlock(&tasklist_lock);
+			can_access_pidns = !cannot_ptrace_pid1;
+		}
+		if (!can_access_pidns)
+			return -EPERM;
+
+		/* open_namespace() unconditionally consumes the reference. */
+		get_pid_ns(ns);
+		return open_namespace(to_ns_common(ns));
+	}
+	default:
+		return -ENOIOCTLCMD;
+	}
+}
+
 /*
  * The root /proc directory is special, as it has the
  * <pid> directories. Thus we don't use the generic
  * directory handling functions for that..
  */
 static const struct file_operations proc_root_operations = {
-	.read		 = generic_read_dir,
-	.iterate_shared	 = proc_root_readdir,
+	.read		= generic_read_dir,
+	.iterate_shared	= proc_root_readdir,
 	.llseek		= generic_file_llseek,
+	.unlocked_ioctl = proc_root_ioctl,
+	.compat_ioctl   = compat_ptr_ioctl,
 };
 
 /*
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 0bd678a4a10e..aa642cb48feb 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -437,6 +437,9 @@ typedef int __bitwise __kernel_rwf_t;
 
 #define PROCFS_IOCTL_MAGIC 'f'
 
+/* procfs root ioctls */
+#define PROCFS_GET_PID_NAMESPACE	_IO(PROCFS_IOCTL_MAGIC, 1)
+
 /* Pagemap ioctl */
 #define PAGEMAP_SCAN	_IOWR(PROCFS_IOCTL_MAGIC, 16, struct pm_scan_arg)
 

-- 
2.50.0

[PATCH RFC 4/4] selftests/proc: add tests for new pidns APIs

[Aleksa Sarai]

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 tools/testing/selftests/proc/.gitignore   |   1 +
 tools/testing/selftests/proc/Makefile     |   1 +
 tools/testing/selftests/proc/proc-pidns.c | 286 ++++++++++++++++++++++++++++++
 3 files changed, 288 insertions(+)

diff --git a/tools/testing/selftests/proc/.gitignore b/tools/testing/selftests/proc/.gitignore
index 973968f45bba..2dced03e9e0e 100644
--- a/tools/testing/selftests/proc/.gitignore
+++ b/tools/testing/selftests/proc/.gitignore
@@ -17,6 +17,7 @@
 /proc-tid0
 /proc-uptime-001
 /proc-uptime-002
+/proc-pidns
 /read
 /self
 /setns-dcache
diff --git a/tools/testing/selftests/proc/Makefile b/tools/testing/selftests/proc/Makefile
index b12921b9794b..c6f7046b9860 100644
--- a/tools/testing/selftests/proc/Makefile
+++ b/tools/testing/selftests/proc/Makefile
@@ -27,5 +27,6 @@ TEST_GEN_PROGS += setns-sysvipc
 TEST_GEN_PROGS += thread-self
 TEST_GEN_PROGS += proc-multiple-procfs
 TEST_GEN_PROGS += proc-fsconfig-hidepid
+TEST_GEN_PROGS += proc-pidns
 
 include ../lib.mk
diff --git a/tools/testing/selftests/proc/proc-pidns.c b/tools/testing/selftests/proc/proc-pidns.c
new file mode 100644
index 000000000000..e7e34c78d383
--- /dev/null
+++ b/tools/testing/selftests/proc/proc-pidns.c
@@ -0,0 +1,286 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Author: Aleksa Sarai <cyphar@cyphar.com>
+ * Copyright (C) 2025 SUSE LLC.
+ */
+
+#include <assert.h>
+#include <errno.h>
+#include <sched.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <stdio.h>
+#include <sys/mount.h>
+#include <sys/stat.h>
+#include <sys/ioctl.h>
+#include <sys/prctl.h>
+
+#include "../kselftest_harness.h"
+
+#define bail(fmt, ...)							\
+	do {								\
+		fprintf(stderr, fmt ": %m", __VA_ARGS__);		\
+		exit(1);						\
+	} while (0)
+
+#define ASSERT_SUCCESS	ASSERT_FALSE
+#define ASSERT_FAIL	ASSERT_TRUE
+
+int touch(char *path)
+{
+	int fd = open(path, O_WRONLY|O_CREAT|O_CLOEXEC, 0644);
+	if (fd < 0 || close(fd) < 0)
+		return -errno;
+	return 0;
+}
+
+FIXTURE(ns)
+{
+	int host_mntns, host_pidns;
+	int dummy_pidns;
+};
+
+FIXTURE_SETUP(ns)
+{
+	/* Stash the old mntns. */
+	self->host_mntns = open("/proc/self/ns/mnt", O_RDONLY|O_CLOEXEC);
+	ASSERT_GE(self->host_mntns, 0);
+
+	/* Create a new mount namespace and make it private. */
+	ASSERT_SUCCESS(unshare(CLONE_NEWNS));
+	ASSERT_SUCCESS(mount(NULL, "/", NULL, MS_PRIVATE|MS_REC, NULL));
+
+	/*
+	 * Create a proper tmpfs that we can use and will disappear once we
+	 * leave this mntns.
+	 */
+	ASSERT_SUCCESS(mount("tmpfs", "/tmp", "tmpfs", 0, NULL));
+
+	/*
+	 * Create a pidns we can use for later tests. We need to fork off a
+	 * child so that we get a usable nsfd that we can bind-mount and open.
+	 */
+	ASSERT_SUCCESS(touch("/tmp/dummy-pidns"));
+
+	self->host_pidns = open("/proc/self/ns/pid", O_RDONLY|O_CLOEXEC);
+	ASSERT_GE(self->host_pidns, 0);
+	ASSERT_SUCCESS(unshare(CLONE_NEWPID));
+
+	pid_t pid = fork();
+	ASSERT_GE(pid, 0);
+	if (!pid) {
+		prctl(PR_SET_PDEATHSIG, SIGKILL);
+		ASSERT_SUCCESS(mount("/proc/self/ns/pid", "/tmp/dummy-pidns", NULL, MS_BIND, 0));
+		exit(0);
+	}
+
+	int wstatus;
+	ASSERT_EQ(waitpid(pid, &wstatus, 0), pid);
+	ASSERT_TRUE(WIFEXITED(wstatus));
+	ASSERT_EQ(WEXITSTATUS(wstatus), 0);
+
+	ASSERT_SUCCESS(setns(self->host_pidns, CLONE_NEWPID));
+
+	self->dummy_pidns = open("/tmp/dummy-pidns", O_RDONLY|O_CLOEXEC);
+	ASSERT_GE(self->dummy_pidns, 0);
+}
+
+FIXTURE_TEARDOWN(ns)
+{
+	ASSERT_SUCCESS(setns(self->host_mntns, CLONE_NEWNS));
+	ASSERT_SUCCESS(close(self->host_mntns));
+
+	ASSERT_SUCCESS(close(self->host_pidns));
+	ASSERT_SUCCESS(close(self->dummy_pidns));
+}
+
+TEST_F(ns, pidns_mount_string_path)
+{
+	ASSERT_SUCCESS(mkdir("/tmp/proc-host", 0755));
+	ASSERT_SUCCESS(mount("proc", "/tmp/proc-host", "proc", 0, "pidns=/proc/self/ns/pid"));
+	ASSERT_SUCCESS(access("/tmp/proc-host/self/", X_OK));
+
+	ASSERT_SUCCESS(mkdir("/tmp/proc-dummy", 0755));
+	ASSERT_SUCCESS(mount("proc", "/tmp/proc-dummy", "proc", 0, "pidns=/tmp/dummy-pidns"));
+	ASSERT_FAIL(access("/tmp/proc-dummy/1/", X_OK));
+	ASSERT_FAIL(access("/tmp/proc-dummy/self/", X_OK));
+}
+
+TEST_F(ns, pidns_fsconfig_string_path)
+{
+	int fsfd = fsopen("proc", FSOPEN_CLOEXEC);
+	ASSERT_GE(fsfd, 0);
+
+	ASSERT_SUCCESS(fsconfig(fsfd, FSCONFIG_SET_STRING, "pidns", "/tmp/dummy-pidns", 0));
+	ASSERT_SUCCESS(fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0));
+
+	int mountfd = fsmount(fsfd, FSMOUNT_CLOEXEC, 0);
+	ASSERT_GE(mountfd, 0);
+
+	ASSERT_FAIL(faccessat(mountfd, "1/", X_OK, 0));
+	ASSERT_FAIL(faccessat(mountfd, "self/", X_OK, 0));
+
+	ASSERT_SUCCESS(close(fsfd));
+	ASSERT_SUCCESS(close(mountfd));
+}
+
+TEST_F(ns, pidns_fsconfig_fd)
+{
+	int fsfd = fsopen("proc", FSOPEN_CLOEXEC);
+	ASSERT_GE(fsfd, 0);
+
+	ASSERT_SUCCESS(fsconfig(fsfd, FSCONFIG_SET_FD, "pidns", NULL, self->dummy_pidns));
+	ASSERT_SUCCESS(fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0));
+
+	int mountfd = fsmount(fsfd, FSMOUNT_CLOEXEC, 0);
+	ASSERT_GE(mountfd, 0);
+
+	ASSERT_FAIL(faccessat(mountfd, "1/", X_OK, 0));
+	ASSERT_FAIL(faccessat(mountfd, "self/", X_OK, 0));
+
+	ASSERT_SUCCESS(close(fsfd));
+	ASSERT_SUCCESS(close(mountfd));
+}
+
+TEST_F(ns, pidns_reconfigure_remount)
+{
+	ASSERT_SUCCESS(mkdir("/tmp/proc", 0755));
+	ASSERT_SUCCESS(mount("proc", "/tmp/proc", "proc", 0, ""));
+	ASSERT_SUCCESS(access("/tmp/proc/self/", X_OK));
+
+	ASSERT_SUCCESS(mount(NULL, "/tmp/proc", NULL, MS_REMOUNT, "pidns=/tmp/dummy-pidns"));
+	ASSERT_FAIL(access("/tmp/proc/self/", X_OK));
+}
+
+TEST_F(ns, pidns_reconfigure_fsconfig_string_path)
+{
+	int fsfd = fsopen("proc", FSOPEN_CLOEXEC);
+	ASSERT_GE(fsfd, 0);
+
+	ASSERT_SUCCESS(fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0));
+
+	int mountfd = fsmount(fsfd, FSMOUNT_CLOEXEC, 0);
+	ASSERT_GE(mountfd, 0);
+
+	ASSERT_SUCCESS(faccessat(mountfd, "1/", X_OK, 0));
+	ASSERT_SUCCESS(faccessat(mountfd, "self/", X_OK, 0));
+
+	ASSERT_SUCCESS(fsconfig(fsfd, FSCONFIG_SET_STRING, "pidns", "/tmp/dummy-pidns", 0));
+	ASSERT_SUCCESS(fsconfig(fsfd, FSCONFIG_CMD_RECONFIGURE, NULL, NULL, 0));
+
+	ASSERT_FAIL(faccessat(mountfd, "1/", X_OK, 0));
+	ASSERT_FAIL(faccessat(mountfd, "self/", X_OK, 0));
+
+	ASSERT_SUCCESS(close(fsfd));
+	ASSERT_SUCCESS(close(mountfd));
+}
+
+TEST_F(ns, pidns_reconfigure_fsconfig_fd)
+{
+	int fsfd = fsopen("proc", FSOPEN_CLOEXEC);
+	ASSERT_GE(fsfd, 0);
+
+	ASSERT_SUCCESS(fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0));
+
+	int mountfd = fsmount(fsfd, FSMOUNT_CLOEXEC, 0);
+	ASSERT_GE(mountfd, 0);
+
+	ASSERT_SUCCESS(faccessat(mountfd, "1/", X_OK, 0));
+	ASSERT_SUCCESS(faccessat(mountfd, "self/", X_OK, 0));
+
+	ASSERT_SUCCESS(fsconfig(fsfd, FSCONFIG_SET_FD, "pidns", NULL, self->dummy_pidns));
+	ASSERT_SUCCESS(fsconfig(fsfd, FSCONFIG_CMD_RECONFIGURE, NULL, NULL, 0));
+
+	ASSERT_FAIL(faccessat(mountfd, "1/", X_OK, 0));
+	ASSERT_FAIL(faccessat(mountfd, "self/", X_OK, 0));
+
+	ASSERT_SUCCESS(close(fsfd));
+	ASSERT_SUCCESS(close(mountfd));
+}
+
+int is_same_inode(int fd1, int fd2)
+{
+	struct stat stat1, stat2;
+
+	assert(fstat(fd1, &stat1) == 0);
+	assert(fstat(fd2, &stat2) == 0);
+
+	return stat1.st_ino == stat2.st_ino && stat1.st_dev == stat2.st_dev;
+}
+
+#define PROCFS_IOCTL_MAGIC 'f'
+#define PROCFS_GET_PID_NAMESPACE	_IO(PROCFS_IOCTL_MAGIC, 1)
+
+TEST_F(ns, get_pidns_ioctl)
+{
+	int fsfd = fsopen("proc", FSOPEN_CLOEXEC);
+	ASSERT_GE(fsfd, 0);
+
+	ASSERT_SUCCESS(fsconfig(fsfd, FSCONFIG_SET_FD, "pidns", NULL, self->dummy_pidns));
+	ASSERT_SUCCESS(fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0));
+
+	int mountfd = fsmount(fsfd, FSMOUNT_CLOEXEC, 0);
+	ASSERT_GE(mountfd, 0);
+
+	/* fsmount returns an O_PATH, which ioctl(2) doesn't accept. */
+	int new_mountfd = openat(mountfd, ".", O_RDONLY|O_DIRECTORY|O_CLOEXEC);
+	ASSERT_GE(new_mountfd, 0);
+
+	ASSERT_SUCCESS(close(mountfd));
+	mountfd = -EBADF;
+
+	int procfs_pidns = ioctl(new_mountfd, PROCFS_GET_PID_NAMESPACE);
+	ASSERT_GE(procfs_pidns, 0);
+
+	ASSERT_NE(self->dummy_pidns, procfs_pidns);
+	ASSERT_FALSE(is_same_inode(self->host_pidns, procfs_pidns));
+	ASSERT_TRUE(is_same_inode(self->dummy_pidns, procfs_pidns));
+
+	ASSERT_SUCCESS(close(fsfd));
+	ASSERT_SUCCESS(close(new_mountfd));
+	ASSERT_SUCCESS(close(procfs_pidns));
+}
+
+TEST_F(ns, reconfigure_get_pidns_ioctl)
+{
+	int fsfd = fsopen("proc", FSOPEN_CLOEXEC);
+	ASSERT_GE(fsfd, 0);
+
+	ASSERT_SUCCESS(fsconfig(fsfd, FSCONFIG_CMD_CREATE, NULL, NULL, 0));
+
+	int mountfd = fsmount(fsfd, FSMOUNT_CLOEXEC, 0);
+	ASSERT_GE(mountfd, 0);
+
+	/* fsmount returns an O_PATH, which ioctl(2) doesn't accept. */
+	int new_mountfd = openat(mountfd, ".", O_RDONLY|O_DIRECTORY|O_CLOEXEC);
+	ASSERT_GE(new_mountfd, 0);
+
+	ASSERT_SUCCESS(close(mountfd));
+	mountfd = -EBADF;
+
+	int procfs_pidns1 = ioctl(new_mountfd, PROCFS_GET_PID_NAMESPACE);
+	ASSERT_GE(procfs_pidns1, 0);
+
+	ASSERT_NE(self->dummy_pidns, procfs_pidns1);
+	ASSERT_TRUE(is_same_inode(self->host_pidns, procfs_pidns1));
+	ASSERT_FALSE(is_same_inode(self->dummy_pidns, procfs_pidns1));
+
+	ASSERT_SUCCESS(fsconfig(fsfd, FSCONFIG_SET_STRING, "pidns", "/tmp/dummy-pidns", 0));
+	ASSERT_SUCCESS(fsconfig(fsfd, FSCONFIG_CMD_RECONFIGURE, NULL, NULL, 0));
+
+	int procfs_pidns2 = ioctl(new_mountfd, PROCFS_GET_PID_NAMESPACE);
+	ASSERT_GE(procfs_pidns2, 0);
+
+	ASSERT_NE(self->dummy_pidns, procfs_pidns2);
+	ASSERT_FALSE(is_same_inode(self->host_pidns, procfs_pidns2));
+	ASSERT_TRUE(is_same_inode(self->dummy_pidns, procfs_pidns2));
+
+	ASSERT_SUCCESS(close(fsfd));
+	ASSERT_SUCCESS(close(new_mountfd));
+	ASSERT_SUCCESS(close(procfs_pidns1));
+	ASSERT_SUCCESS(close(procfs_pidns2));
+}
+
+TEST_HARNESS_MAIN

-- 
2.50.0

Re: [PATCH RFC 0/4] procfs: make reference pidns more user-visible

[Andy Lutomirski]

On Mon, Jul 21, 2025 at 1:44 AM Aleksa Sarai <cyphar@cyphar.com> wrote:
>
> Ever since the introduction of pid namespaces, procfs has had very
> implicit behaviour surrounding them (the pidns used by a procfs mount is
> auto-selected based on the mounting process's active pidns, and the
> pidns itself is basically hidden once the mount has been constructed).
> This has historically meant that userspace was required to do some
> special dances in order to configure the pidns of a procfs mount as
> desired. Examples include:
>
>  * In order to bypass the mnt_too_revealing() check, Kubernetes creates
>    a procfs mount from an empty pidns so that user namespaced containers
>    can be nested (without this, the nested containers would fail to
>    mount procfs). But this requires forking off a helper process because
>    you cannot just one-shot this using mount(2).
>
>  * Container runtimes in general need to fork into a container before
>    configuring its mounts, which can lead to security issues in the case
>    of shared-pidns containers (a privileged process in the pidns can
>    interact with your container runtime process). While
>    SUID_DUMP_DISABLE and user namespaces make this less of an issue, the
>    strict need for this due to a minor uAPI wart is kind of unfortunate.
>
> Things would be much easier if there was a way for userspace to just
> specify the pidns they want. Patch 1 implements a new "pidns" argument
> which can be set using fsconfig(2):
>
>     fsconfig(procfd, FSCONFIG_SET_FD, "pidns", NULL, nsfd);
>     fsconfig(procfd, FSCONFIG_SET_STRING, "pidns", "/proc/self/ns/pid", 0);
>
> or classic mount(2) / mount(8):
>
>     // mount -t proc -o pidns=/proc/self/ns/pid proc /tmp/proc
>     mount("proc", "/tmp/proc", "proc", MS_..., "pidns=/proc/self/ns/pid");
>
> The initial security model I have in this RFC is to be as conservative
> as possible and just mirror the security model for setns(2) -- which
> means that you can only set pidns=... to pid namespaces that your
> current pid namespace is a direct ancestor of. This fulfils the
> requirements of container runtimes, but I suspect that this may be too
> strict for some usecases.
>
> The pidns argument is not displayed in mountinfo -- it's not clear to me
> what value it would make sense to show (maybe we could just use ns_dname
> to provide an identifier for the namespace, but this number would be
> fairly useless to userspace). I'm open to suggestions.
>
> In addition, being able to figure out what pid namespace is being used
> by a procfs mount is quite useful when you have an administrative
> process (such as a container runtime) which wants to figure out the
> correct way of mapping PIDs between its own namespace and the namespace
> for procfs (using NS_GET_{PID,TGID}_{IN,FROM}_PIDNS). There are
> alternative ways to do this, but they all rely on ancillary information
> that third-party libraries and tools do not necessarily have access to.
>
> To make this easier, add a new ioctl (PROCFS_GET_PID_NAMESPACE) which
> can be used to get a reference to the pidns that a procfs is using.
>
> It's not quite clear what is the correct security model for this API,
> but the current approach I've taken is to:
>
>  * Make the ioctl only valid on the root (meaning that a process without
>    access to the procfs root -- such as only having an fd to a procfs
>    file or some open_tree(2)-like subset -- cannot use this API).
>
>  * Require that the process requesting either has access to
>    /proc/1/ns/pid anyway (i.e. has ptrace-read access to the pidns
>    pid1), has CAP_SYS_ADMIN access to the pidns (i.e. has administrative
>    access to it and can join it if they had a handle), or is in a pidns
>    that is a direct ancestor of the target pidns (i.e. all of the pids
>    are already visible in the procfs for the current process's pidns).

What's the motivation for the ptrace-read option?  While I don't see
an attack off the top of my head, it seems like creating a procfs
mount may give write-ish access to things in the pidns (because the
creator is likely to have CAP_DAC_OVERRIDE, etc) and possibly even
access to namespace-wide things that aren't inherently visible to
PID1.

Even the ancestor check seems dicey.  Imagine that uid 1000 makes an
unprivileged container complete with a userns.  Then uid 1001 (outside
the container) makes its own userns and mountns but stays in the init
pidns and then mounts (and owns, with all filesystem-related
capabilities) that mount.  Is this really safe?

CAP_SYS_ADMIN seems about right.

--Andy

Re: [PATCH RFC 0/4] procfs: make reference pidns more user-visible

[Aleksa Sarai]

[-- Attachment #1: Type: text/plain, Size: 6114 bytes --]

On 2025-07-21, Andy Lutomirski <luto@amacapital.net> wrote:
> On Mon, Jul 21, 2025 at 1:44 AM Aleksa Sarai <cyphar@cyphar.com> wrote:
> >
> > Ever since the introduction of pid namespaces, procfs has had very
> > implicit behaviour surrounding them (the pidns used by a procfs mount is
> > auto-selected based on the mounting process's active pidns, and the
> > pidns itself is basically hidden once the mount has been constructed).
> > This has historically meant that userspace was required to do some
> > special dances in order to configure the pidns of a procfs mount as
> > desired. Examples include:
> >
> >  * In order to bypass the mnt_too_revealing() check, Kubernetes creates
> >    a procfs mount from an empty pidns so that user namespaced containers
> >    can be nested (without this, the nested containers would fail to
> >    mount procfs). But this requires forking off a helper process because
> >    you cannot just one-shot this using mount(2).
> >
> >  * Container runtimes in general need to fork into a container before
> >    configuring its mounts, which can lead to security issues in the case
> >    of shared-pidns containers (a privileged process in the pidns can
> >    interact with your container runtime process). While
> >    SUID_DUMP_DISABLE and user namespaces make this less of an issue, the
> >    strict need for this due to a minor uAPI wart is kind of unfortunate.
> >
> > Things would be much easier if there was a way for userspace to just
> > specify the pidns they want. Patch 1 implements a new "pidns" argument
> > which can be set using fsconfig(2):
> >
> >     fsconfig(procfd, FSCONFIG_SET_FD, "pidns", NULL, nsfd);
> >     fsconfig(procfd, FSCONFIG_SET_STRING, "pidns", "/proc/self/ns/pid", 0);
> >
> > or classic mount(2) / mount(8):
> >
> >     // mount -t proc -o pidns=/proc/self/ns/pid proc /tmp/proc
> >     mount("proc", "/tmp/proc", "proc", MS_..., "pidns=/proc/self/ns/pid");
> >
> > The initial security model I have in this RFC is to be as conservative
> > as possible and just mirror the security model for setns(2) -- which
> > means that you can only set pidns=... to pid namespaces that your
> > current pid namespace is a direct ancestor of. This fulfils the
> > requirements of container runtimes, but I suspect that this may be too
> > strict for some usecases.
> >
> > The pidns argument is not displayed in mountinfo -- it's not clear to me
> > what value it would make sense to show (maybe we could just use ns_dname
> > to provide an identifier for the namespace, but this number would be
> > fairly useless to userspace). I'm open to suggestions.
> >
> > In addition, being able to figure out what pid namespace is being used
> > by a procfs mount is quite useful when you have an administrative
> > process (such as a container runtime) which wants to figure out the
> > correct way of mapping PIDs between its own namespace and the namespace
> > for procfs (using NS_GET_{PID,TGID}_{IN,FROM}_PIDNS). There are
> > alternative ways to do this, but they all rely on ancillary information
> > that third-party libraries and tools do not necessarily have access to.
> >
> > To make this easier, add a new ioctl (PROCFS_GET_PID_NAMESPACE) which
> > can be used to get a reference to the pidns that a procfs is using.
> >
> > It's not quite clear what is the correct security model for this API,
> > but the current approach I've taken is to:
> >
> >  * Make the ioctl only valid on the root (meaning that a process without
> >    access to the procfs root -- such as only having an fd to a procfs
> >    file or some open_tree(2)-like subset -- cannot use this API).
> >
> >  * Require that the process requesting either has access to
> >    /proc/1/ns/pid anyway (i.e. has ptrace-read access to the pidns
> >    pid1), has CAP_SYS_ADMIN access to the pidns (i.e. has administrative
> >    access to it and can join it if they had a handle), or is in a pidns
> >    that is a direct ancestor of the target pidns (i.e. all of the pids
> >    are already visible in the procfs for the current process's pidns).
> 
> What's the motivation for the ptrace-read option?  While I don't see
> an attack off the top of my head, it seems like creating a procfs
> mount may give write-ish access to things in the pidns (because the
> creator is likely to have CAP_DAC_OVERRIDE, etc) and possibly even
> access to namespace-wide things that aren't inherently visible to
> PID1.

This latter section is about the privilege model for
ioctl(PROCFS_GET_PID_NAMESPACE), not the pidns= mount flag. pidns=
requires CAP_SYS_ADMIN for pidns->user_ns, in addition to the same
restrictions as pidns_install() (must be a direct ancestor). Maybe I
should add some headers in this cover letter for v2...

For the ioctl -- if the user can ptrace-read pid1 in the pidns, they can
open a handle to /proc/1/ns/pid which is exactly the same thing they'd
get from PROCFS_GET_PID_NAMESPACE.

> Even the ancestor check seems dicey.  Imagine that uid 1000 makes an
> unprivileged container complete with a userns.  Then uid 1001 (outside
> the container) makes its own userns and mountns but stays in the init
> pidns and then mounts (and owns, with all filesystem-related
> capabilities) that mount.  Is this really safe?

As for the ancestor check (for the ioctl), the logic I had was that
being in an ancestor pidns means that you already can see all of the
subprocesses in your own pidns, so it seems strange to not be able to
get a handle to their pidns. Maybe this isn't quite right, idk.

Ultimately there isn't too much you can do with a pidns fd if you don't
have privileges to join it (the only thing I can think of is that you
could bind-mount it, which could maybe be used to trick an
administrative process if they trusted your mountns for some reason).

> CAP_SYS_ADMIN seems about right.

For pidns=, sure. For the ioctl, I think this is overkill.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/

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Re: [PATCH RFC 0/4] procfs: make reference pidns more user-visible

[Aleksa Sarai]

[-- Attachment #1: Type: text/plain, Size: 6534 bytes --]

On 2025-07-22, Aleksa Sarai <cyphar@cyphar.com> wrote:
> On 2025-07-21, Andy Lutomirski <luto@amacapital.net> wrote:
> > On Mon, Jul 21, 2025 at 1:44 AM Aleksa Sarai <cyphar@cyphar.com> wrote:
> > >
> > > Ever since the introduction of pid namespaces, procfs has had very
> > > implicit behaviour surrounding them (the pidns used by a procfs mount is
> > > auto-selected based on the mounting process's active pidns, and the
> > > pidns itself is basically hidden once the mount has been constructed).
> > > This has historically meant that userspace was required to do some
> > > special dances in order to configure the pidns of a procfs mount as
> > > desired. Examples include:
> > >
> > >  * In order to bypass the mnt_too_revealing() check, Kubernetes creates
> > >    a procfs mount from an empty pidns so that user namespaced containers
> > >    can be nested (without this, the nested containers would fail to
> > >    mount procfs). But this requires forking off a helper process because
> > >    you cannot just one-shot this using mount(2).
> > >
> > >  * Container runtimes in general need to fork into a container before
> > >    configuring its mounts, which can lead to security issues in the case
> > >    of shared-pidns containers (a privileged process in the pidns can
> > >    interact with your container runtime process). While
> > >    SUID_DUMP_DISABLE and user namespaces make this less of an issue, the
> > >    strict need for this due to a minor uAPI wart is kind of unfortunate.
> > >
> > > Things would be much easier if there was a way for userspace to just
> > > specify the pidns they want. Patch 1 implements a new "pidns" argument
> > > which can be set using fsconfig(2):
> > >
> > >     fsconfig(procfd, FSCONFIG_SET_FD, "pidns", NULL, nsfd);
> > >     fsconfig(procfd, FSCONFIG_SET_STRING, "pidns", "/proc/self/ns/pid", 0);
> > >
> > > or classic mount(2) / mount(8):
> > >
> > >     // mount -t proc -o pidns=/proc/self/ns/pid proc /tmp/proc
> > >     mount("proc", "/tmp/proc", "proc", MS_..., "pidns=/proc/self/ns/pid");
> > >
> > > The initial security model I have in this RFC is to be as conservative
> > > as possible and just mirror the security model for setns(2) -- which
> > > means that you can only set pidns=... to pid namespaces that your
> > > current pid namespace is a direct ancestor of. This fulfils the
> > > requirements of container runtimes, but I suspect that this may be too
> > > strict for some usecases.
> > >
> > > The pidns argument is not displayed in mountinfo -- it's not clear to me
> > > what value it would make sense to show (maybe we could just use ns_dname
> > > to provide an identifier for the namespace, but this number would be
> > > fairly useless to userspace). I'm open to suggestions.
> > >
> > > In addition, being able to figure out what pid namespace is being used
> > > by a procfs mount is quite useful when you have an administrative
> > > process (such as a container runtime) which wants to figure out the
> > > correct way of mapping PIDs between its own namespace and the namespace
> > > for procfs (using NS_GET_{PID,TGID}_{IN,FROM}_PIDNS). There are
> > > alternative ways to do this, but they all rely on ancillary information
> > > that third-party libraries and tools do not necessarily have access to.
> > >
> > > To make this easier, add a new ioctl (PROCFS_GET_PID_NAMESPACE) which
> > > can be used to get a reference to the pidns that a procfs is using.
> > >
> > > It's not quite clear what is the correct security model for this API,
> > > but the current approach I've taken is to:
> > >
> > >  * Make the ioctl only valid on the root (meaning that a process without
> > >    access to the procfs root -- such as only having an fd to a procfs
> > >    file or some open_tree(2)-like subset -- cannot use this API).
> > >
> > >  * Require that the process requesting either has access to
> > >    /proc/1/ns/pid anyway (i.e. has ptrace-read access to the pidns
> > >    pid1), has CAP_SYS_ADMIN access to the pidns (i.e. has administrative
> > >    access to it and can join it if they had a handle), or is in a pidns
> > >    that is a direct ancestor of the target pidns (i.e. all of the pids
> > >    are already visible in the procfs for the current process's pidns).
> > 
> > What's the motivation for the ptrace-read option?  While I don't see
> > an attack off the top of my head, it seems like creating a procfs
> > mount may give write-ish access to things in the pidns (because the
> > creator is likely to have CAP_DAC_OVERRIDE, etc) and possibly even
> > access to namespace-wide things that aren't inherently visible to
> > PID1.
> 
> This latter section is about the privilege model for
> ioctl(PROCFS_GET_PID_NAMESPACE), not the pidns= mount flag. pidns=
> requires CAP_SYS_ADMIN for pidns->user_ns, in addition to the same
> restrictions as pidns_install() (must be a direct ancestor). Maybe I
> should add some headers in this cover letter for v2...
> 
> For the ioctl -- if the user can ptrace-read pid1 in the pidns, they can
> open a handle to /proc/1/ns/pid which is exactly the same thing they'd
> get from PROCFS_GET_PID_NAMESPACE.
> 
> > Even the ancestor check seems dicey.  Imagine that uid 1000 makes an
> > unprivileged container complete with a userns.  Then uid 1001 (outside
> > the container) makes its own userns and mountns but stays in the init
> > pidns and then mounts (and owns, with all filesystem-related
> > capabilities) that mount.  Is this really safe?
> 
> As for the ancestor check (for the ioctl), the logic I had was that
> being in an ancestor pidns means that you already can see all of the
> subprocesses in your own pidns, so it seems strange to not be able to
> get a handle to their pidns. Maybe this isn't quite right, idk.
> 
> Ultimately there isn't too much you can do with a pidns fd if you don't
> have privileges to join it (the only thing I can think of is that you
> could bind-mount it, which could maybe be used to trick an
> administrative process if they trusted your mountns for some reason).
> 
> > CAP_SYS_ADMIN seems about right.
> 
> For pidns=, sure. For the ioctl, I think this is overkill.

My bad, I forgot to add you to Cc for v2 Andy. PTAL:

 <https://lore.kernel.org/all/20250723-procfs-pidns-api-v2-0-621e7edd8e40@cyphar.com/>

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/

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