proc: add option to mount only a pids subset

This allows to hide all files and directories in the procfs that are not
related to tasks.

Signed-off-by: Alexey Gladkov <[e-mail]>
Reviewed-by: Alexey Dobriyan <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Signed-off-by: Eric W. Biederman <[e-mail]>

fs/proc/generic.c

···
 struct dentry *proc_lookup(struct inode *dir, struct dentry *dentry,
 		unsigned int flags)
 {
+	struct proc_fs_info *fs_info = proc_sb_info(dir->i_sb);
+
+	if (fs_info->pidonly == PROC_PIDONLY_ON)
+		return ERR_PTR(-ENOENT);
+
 	return proc_lookup_de(dir, dentry, PDE(dir));
 }
 
···
 int proc_readdir(struct file *file, struct dir_context *ctx)
 {
 	struct inode *inode = file_inode(file);
+	struct proc_fs_info *fs_info = proc_sb_info(inode->i_sb);
+
+	if (fs_info->pidonly == PROC_PIDONLY_ON)
+		return 1;
 
 	return proc_readdir_de(file, ctx, PDE(inode));
 }

fs/proc/inode.c

···
 		seq_printf(seq, ",gid=%u", from_kgid_munged(&init_user_ns, fs_info->pid_gid));
 	if (fs_info->hide_pid != HIDEPID_OFF)
 		seq_printf(seq, ",hidepid=%u", fs_info->hide_pid);
+	if (fs_info->pidonly != PROC_PIDONLY_OFF)
+		seq_printf(seq, ",subset=pid");
 
 	return 0;
 }
···
 
 static int proc_reg_open(struct inode *inode, struct file *file)
 {
+	struct proc_fs_info *fs_info = proc_sb_info(inode->i_sb);
 	struct proc_dir_entry *pde = PDE(inode);
 	int rv = 0;
 	typeof_member(struct proc_ops, proc_open) open;
···
 			rv = open(inode, file);
 		return rv;
 	}
+
+	if (fs_info->pidonly == PROC_PIDONLY_ON)
+		return -ENOENT;
 
 	/*
 	 * Ensure that

fs/proc/root.c

···
 	unsigned int		mask;
 	int			hidepid;
 	int			gid;
+	int			pidonly;
 };
 
 enum proc_param {
 	Opt_gid,
 	Opt_hidepid,
+	Opt_subset,
 };
 
 static const struct fs_parameter_spec proc_fs_parameters[] = {
 	fsparam_u32("gid",	Opt_gid),
 	fsparam_u32("hidepid",	Opt_hidepid),
+	fsparam_string("subset",	Opt_subset),
 	{}
 };
 
···
 		value == HIDEPID_NO_ACCESS ||
 		value == HIDEPID_INVISIBLE ||
 		value == HIDEPID_NOT_PTRACEABLE);
+}
+
+static int proc_parse_subset_param(struct fs_context *fc, char *value)
+{
+	struct proc_fs_context *ctx = fc->fs_private;
+
+	while (value) {
+		char *ptr = strchr(value, ',');
+
+		if (ptr != NULL)
+			*ptr++ = '\0';
+
+		if (*value != '\0') {
+			if (!strcmp(value, "pid")) {
+				ctx->pidonly = PROC_PIDONLY_ON;
+			} else {
+				return invalf(fc, "proc: unsupported subset option - %s\n", value);
+			}
+		}
+		value = ptr;
+	}
+
+	return 0;
 }
 
 static int proc_parse_param(struct fs_context *fc, struct fs_parameter *param)
···
 		ctx->hidepid = result.uint_32;
 		break;
 
+	case Opt_subset:
+		if (proc_parse_subset_param(fc, param->string) < 0)
+			return -EINVAL;
+		break;
+
 	default:
 		return -EINVAL;
 	}
···
 		fs_info->pid_gid = make_kgid(user_ns, ctx->gid);
 	if (ctx->mask & (1 << Opt_hidepid))
 		fs_info->hide_pid = ctx->hidepid;
+	if (ctx->mask & (1 << Opt_subset))
+		fs_info->pidonly = ctx->pidonly;
 }
 
 static int proc_fill_super(struct super_block *s, struct fs_context *fc)

include/linux/proc_fs.h

···
 	HIDEPID_NOT_PTRACEABLE = 4, /* Limit pids to only ptraceable pids */
 };
 
+/* definitions for proc mount option pidonly */
+enum {
+	PROC_PIDONLY_OFF = 0,
+	PROC_PIDONLY_ON  = 1,
+};
+
 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 */
 	kgid_t pid_gid;
 	int hide_pid;
+	int pidonly;
 };
 
 static inline struct proc_fs_info *proc_sb_info(struct super_block *sb)