[CFT][PATCH] proc: Update /proc/net to point at the accessing threads network namespace

[Eric W. Biederman]

Since common apparmor policies don't allow access /proc/tgid/task/tid/net
point the code at /proc/tid/net instead.

Link: https://lkml.kernel.org/r/dacfc18d6667421d97127451eafe4f29@xxxxxxxxxxxxxxxx
Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---

I have only compile tested this.  All of the boiler plate is a copy of
/proc/self and /proc/thread-self, so it should work.

Can David or someone who cares and has access to the limited apparmor
configurations could test this to make certain this works?

 fs/proc/base.c          | 12 ++++++--
 fs/proc/internal.h      |  2 ++
 fs/proc/proc_net.c      | 68 ++++++++++++++++++++++++++++++++++++++++-
 fs/proc/root.c          |  7 ++++-
 include/linux/proc_fs.h |  1 +
 5 files changed, 85 insertions(+), 5 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 93f7e3d971e4..c205234f3822 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -3479,7 +3479,7 @@ static struct tgid_iter next_tgid(struct pid_namespace *ns, struct tgid_iter ite
 	return iter;
 }
 
-#define TGID_OFFSET (FIRST_PROCESS_ENTRY + 2)
+#define TGID_OFFSET (FIRST_PROCESS_ENTRY + 3)
 
 /* for the /proc/ directory itself, after non-process stuff has been done */
 int proc_pid_readdir(struct file *file, struct dir_context *ctx)
@@ -3492,18 +3492,24 @@ int proc_pid_readdir(struct file *file, struct dir_context *ctx)
 	if (pos >= PID_MAX_LIMIT + TGID_OFFSET)
 		return 0;
 
-	if (pos == TGID_OFFSET - 2) {
+	if (pos == TGID_OFFSET - 3) {
 		struct inode *inode = d_inode(fs_info->proc_self);
 		if (!dir_emit(ctx, "self", 4, inode->i_ino, DT_LNK))
 			return 0;
 		ctx->pos = pos = pos + 1;
 	}
-	if (pos == TGID_OFFSET - 1) {
+	if (pos == TGID_OFFSET - 2) {
 		struct inode *inode = d_inode(fs_info->proc_thread_self);
 		if (!dir_emit(ctx, "thread-self", 11, inode->i_ino, DT_LNK))
 			return 0;
 		ctx->pos = pos = pos + 1;
 	}
+	if (pos == TGID_OFFSET - 1) {
+		struct inode *inode = d_inode(fs_info->proc_net);
+		if (!dir_emit(ctx, "net", 11, inode->i_ino, DT_LNK))
+			return 0;
+		ctx->pos = pos = pos + 1;
+	}
 	iter.tgid = pos - TGID_OFFSET;
 	iter.task = NULL;
 	for (iter = next_tgid(ns, iter);
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index 06a80f78433d..9d13c24b80c8 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -232,8 +232,10 @@ extern const struct inode_operations proc_net_inode_operations;
 
 #ifdef CONFIG_NET
 extern int proc_net_init(void);
+extern int proc_setup_net_symlink(struct super_block *s);
 #else
 static inline int proc_net_init(void) { return 0; }
+static inline int proc_setup_net_symlink(struct super_block *s) { return 0; }
 #endif
 
 /*
diff --git a/fs/proc/proc_net.c b/fs/proc/proc_net.c
index 856839b8ae8b..99335e800c1c 100644
--- a/fs/proc/proc_net.c
+++ b/fs/proc/proc_net.c
@@ -408,9 +408,75 @@ static struct pernet_operations __net_initdata proc_net_ns_ops = {
 	.exit = proc_net_ns_exit,
 };
 
+/*
+ * /proc/net:
+ */
+static const char *proc_net_symlink_get_link(struct dentry *dentry,
+					     struct inode *inode,
+					     struct delayed_call *done)
+{
+	struct pid_namespace *ns = proc_pid_ns(inode->i_sb);
+	pid_t tid = task_pid_nr_ns(current, ns);
+	char *name;
+
+	if (!tid)
+		return ERR_PTR(-ENOENT);
+	name = kmalloc(10 + 4 + 1, dentry ? GFP_KERNEL : GFP_ATOMIC);
+	if (unlikely(!name))
+		return dentry ? ERR_PTR(-ENOMEM) : ERR_PTR(-ECHILD);
+	sprintf(name, "%u/net", tid);
+	set_delayed_call(done, kfree_link, name);
+	return name;
+}
+
+static const struct inode_operations proc_net_symlink_inode_operations = {
+	.get_link	= proc_net_symlink_get_link,
+};
+
+static unsigned net_symlink_inum __ro_after_init;
+
+int proc_setup_net_symlink(struct super_block *s)
+{
+	struct inode *root_inode = d_inode(s->s_root);
+	struct proc_fs_info *fs_info = proc_sb_info(s);
+	struct dentry *net_symlink;
+	int ret = -ENOMEM;
+
+	inode_lock(root_inode);
+	net_symlink = d_alloc_name(s->s_root, "net");
+	if (net_symlink) {
+		struct inode *inode = new_inode(s);
+		if (inode) {
+			inode->i_ino = net_symlink_inum;
+			inode->i_mtime = inode->i_atime = inode->i_ctime = current_time(inode);
+			inode->i_mode = S_IFLNK | S_IRWXUGO;
+			inode->i_uid = GLOBAL_ROOT_UID;
+			inode->i_gid = GLOBAL_ROOT_GID;
+			inode->i_op = &proc_net_symlink_inode_operations;
+			d_add(net_symlink, inode);
+			ret = 0;
+		} else {
+			dput(net_symlink);
+		}
+	}
+	inode_unlock(root_inode);
+
+	if (ret)
+		pr_err("proc_fill_super: can't allocate /proc/net\n");
+	else
+		fs_info->proc_net = net_symlink;
+
+	return ret;
+}
+
+void __init proc_net_symlink_init(void)
+{
+	proc_alloc_inum(&net_symlink_inum);
+}
+
 int __init proc_net_init(void)
 {
-	proc_symlink("net", NULL, "self/net");
+	proc_net_symlink_init();
 
 	return register_pernet_subsys(&proc_net_ns_ops);
 }
diff --git a/fs/proc/root.c b/fs/proc/root.c
index 3c2ee3eb1138..6e57e9a4acf9 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -207,7 +207,11 @@ static int proc_fill_super(struct super_block *s, struct fs_context *fc)
 	if (ret) {
 		return ret;
 	}
-	return proc_setup_thread_self(s);
+	ret = proc_setup_thread_self(s);
+	if (ret) {
+		return ret;
+	}
+	return proc_setup_net_symlink(s);
 }
 
 static int proc_reconfigure(struct fs_context *fc)
@@ -268,6 +272,7 @@ static void proc_kill_sb(struct super_block *sb)
 
 	dput(fs_info->proc_self);
 	dput(fs_info->proc_thread_self);
+	dput(fs_info->proc_net);
 
 	kill_anon_super(sb);
 	put_pid_ns(fs_info->pid_ns);
diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
index 81d6e4ec2294..65f4ef15c8bf 100644
--- a/include/linux/proc_fs.h
+++ b/include/linux/proc_fs.h
@@ -62,6 +62,7 @@ struct proc_fs_info {
 	struct pid_namespace *pid_ns;
 	struct dentry *proc_self;        /* For /proc/self */
 	struct dentry *proc_thread_self; /* For /proc/thread-self */
+	struct dentry *proc_net;	 /* For /proc/net */
 	kgid_t pid_gid;
 	enum proc_hidepid hide_pid;
 	enum proc_pidonly pidonly;
-- 
2.35.3