[PATCH RFC v2 2/4] procfs: add "pidns" mount option

[Aleksa Sarai <cyphar@cyphar.com>]

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.

So, provide a "pidns" mount option which allows such users to just
explicitly state which pid namespace they want that procfs instance to
use. This interface can be used with fsconfig(2) either with a file
descriptor or a path:

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

or with 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");

As this new API is effectively shorthand for setns(2) followed by
mount(2), the permission model for this mirrors pidns_install() to avoid
opening up new attack surfaces by loosening the existing permission
model.

Note that the mount infrastructure also allows userspace to reconfigure
the pidns of an existing procfs mount, which may or may not be useful to
some users.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 Documentation/filesystems/proc.rst |  6 +++
 fs/proc/root.c                     | 90 +++++++++++++++++++++++++++++++++++---
 2 files changed, 90 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..057c8a125c6e 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -38,12 +38,18 @@ enum proc_param {
 	Opt_gid,
 	Opt_hidepid,
 	Opt_subset,
+#ifdef CONFIG_PID_NS
+	Opt_pidns,
+#endif
 };
 
 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),
+#ifdef CONFIG_PID_NS
+	fsparam_file_or_string("pidns",	Opt_pidns),
+#endif
 	{}
 };
 
@@ -109,11 +115,67 @@ static int proc_parse_subset_param(struct fs_context *fc, char *value)
 	return 0;
 }
 
+#ifdef CONFIG_PID_NS
+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;
+}
+#endif /* CONFIG_PID_NS */
+
 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)
@@ -125,14 +187,24 @@ 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;
+
+#ifdef CONFIG_PID_NS
+	case Opt_pidns:
+		err = proc_parse_pidns_param(fc, param, &result);
+		if (err)
+			return err;
 		break;
+#endif
 
 	default:
 		return -EINVAL;
@@ -154,6 +226,12 @@ 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;
+#ifdef CONFIG_PID_NS
+	if (ctx->mask & (1 << Opt_pidns)) {
+		put_pid_ns(fs_info->pid_ns);
+		fs_info->pid_ns = get_pid_ns(ctx->pid_ns);
+	}
+#endif
 }
 
 static int proc_fill_super(struct super_block *s, struct fs_context *fc)

-- 
2.50.0