Re: [PATCH 1/2] kernel: show current values of user namespace counters

["Serge E. Hallyn" <serge-A9i7LUbDfNHQT0dZR+AlfA@public.gmane.org>]

Quoting Kees Cook (keescook-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org):
> On Mon, Aug 15, 2016 at 1:10 PM, Andrei Vagin <avagin-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org> wrote:
> > Recently Eric added user namespace counters.  User namespace counters is
> > a feature that allows to limit the number of various kernel objects a
> > user can create. These limits are set via /proc/sys/user/ sysctls on a
> > per user namespace basis and are applicable to all users in that
> > namespace.
> >
> > This patch adds /proc/PID/userns_counts files to provide current usage
> > of user namespace counters.
> >
> >   > cat /proc/813/userns_counts
> >   user_namespaces          101000               1
> >   pid_namespaces           101000               1
> >   ipc_namespaces           101000               4
> >   net_namespaces           101000               2
> >   mnt_namespaces           101000               5
> >   mnt_namespaces           100000               1
> >
> > The meanings of the columns are as follows, from left to right:
> >
> >   Name         Object name
> >   UID          User ID
> >   Usage        Current usage
> >
> > Cc: Serge Hallyn <serge.hallyn-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org>
> > Cc: Kees Cook <keescook-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>
> > Cc: "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
> > Signed-off-by: Andrei Vagin <avagin-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>
> > ---
> >  fs/proc/array.c                |  57 +++++++++++++++++++++++
> >  fs/proc/base.c                 |   3 ++
> >  fs/proc/internal.h             |   1 +
> >  include/linux/user_namespace.h |   8 ++++
> >  kernel/ucount.c                | 102 +++++++++++++++++++++++++++++++++++++++++
> >  5 files changed, 171 insertions(+)
> >
> > diff --git a/fs/proc/array.c b/fs/proc/array.c
> > index 88c7de1..f186625 100644
> > --- a/fs/proc/array.c
> > +++ b/fs/proc/array.c
> > @@ -734,3 +734,60 @@ const struct file_operations proc_tid_children_operations = {
> >         .release = children_seq_release,
> >  };
> >  #endif /* CONFIG_PROC_CHILDREN */
> > +
> > +#ifdef CONFIG_USER_NS
> > +static int ucounts_open(struct inode *inode, struct file *filp)
> > +{
> > +       struct ucounts_iterator *iter;
> > +       struct seq_file *seq;
> > +       int ret;
> > +
> > +       struct task_struct *task;
> > +       struct user_namespace *ns;
> > +
> > +       task = get_proc_task(inode);
> > +       if (!task)
> > +               return -ESRCH;
> > +
> > +       rcu_read_lock();
> > +       ns = get_user_ns(__task_cred(task)->user_ns);
> > +       rcu_read_unlock();
> > +
> > +       put_task_struct(task);
> > +
> > +       if (ns == NULL)
> > +               return -ESRCH;
> > +
> > +       ret = seq_open_private(filp, &ucounts_seq_operations,
> > +                                       sizeof(struct ucounts_iterator));
> > +
> > +       if (ret) {
> > +               put_user_ns(ns);
> > +               return ret;
> > +       }
> > +
> > +       seq = filp->private_data;
> > +       iter = seq->private;
> > +       iter->ns = ns;
> > +
> > +       return 0;
> > +}
> > +
> > +int ucounts_release(struct inode *inode, struct file *file)
> > +{
> > +       struct seq_file *seq = file->private_data;
> > +       struct ucounts_iterator *iter = seq->private;
> > +
> > +       put_user_ns(iter->ns);
> > +
> > +       return seq_release_private(inode, file);
> > +}
> > +
> > +
> > +const struct file_operations proc_ucounts_operations = {
> > +       .open           = ucounts_open,
> > +       .read           = seq_read,
> > +       .llseek         = seq_lseek,
> > +       .release        = ucounts_release,
> > +};
> > +#endif /* CONFIG_USER_NS */
> > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > index 54e2702..4252f7a 100644
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> > @@ -2910,6 +2910,9 @@ static const struct pid_entry tgid_base_stuff[] = {
> >         REG("timers",     S_IRUGO, proc_timers_operations),
> >  #endif
> >         REG("timerslack_ns", S_IRUGO|S_IWUGO, proc_pid_set_timerslack_ns_operations),
> > +#ifdef CONFIG_USER_NS
> > +       REG("userns_counts",  S_IRUGO, proc_ucounts_operations),
> > +#endif
> >  };
> >
> >  static int proc_tgid_base_readdir(struct file *file, struct dir_context *ctx)
> > diff --git a/fs/proc/internal.h b/fs/proc/internal.h
> > index 7931c55..845cadb 100644
> > --- a/fs/proc/internal.h
> > +++ b/fs/proc/internal.h
> > @@ -298,6 +298,7 @@ extern const struct file_operations proc_pid_smaps_operations;
> >  extern const struct file_operations proc_tid_smaps_operations;
> >  extern const struct file_operations proc_clear_refs_operations;
> >  extern const struct file_operations proc_pagemap_operations;
> > +extern const struct file_operations proc_ucounts_operations;
> >
> >  extern unsigned long task_vsize(struct mm_struct *);
> >  extern unsigned long task_statm(struct mm_struct *,
> > diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> > index 30ffe10..5f824dd 100644
> > --- a/include/linux/user_namespace.h
> > +++ b/include/linux/user_namespace.h
> > @@ -106,6 +106,14 @@ extern ssize_t proc_setgroups_write(struct file *, const char __user *, size_t,
> >  extern int proc_setgroups_show(struct seq_file *m, void *v);
> >  extern bool userns_may_setgroups(const struct user_namespace *ns);
> >  extern bool current_in_userns(const struct user_namespace *target_ns);
> > +
> > +struct ucounts_iterator {
> > +       struct user_namespace *ns;
> > +       int     hash;
> > +};
> > +
> > +extern const struct seq_operations ucounts_seq_operations;
> > +
> >  #else
> >
> >  static inline struct user_namespace *get_user_ns(struct user_namespace *ns)
> > diff --git a/kernel/ucount.c b/kernel/ucount.c
> > index 6ebbe9b..cef09e3 100644
> > --- a/kernel/ucount.c
> > +++ b/kernel/ucount.c
> > @@ -9,6 +9,8 @@
> >  #include <linux/sysctl.h>
> >  #include <linux/slab.h>
> >  #include <linux/hash.h>
> > +#include <linux/seq_file.h>
> > +#include <linux/proc_fs.h>
> >  #include <linux/user_namespace.h>
> >
> >  #define UCOUNTS_HASHTABLE_BITS 10
> > @@ -232,4 +234,104 @@ static __init int user_namespace_sysctl_init(void)
> >  }
> >  subsys_initcall(user_namespace_sysctl_init);
> >
> > +#ifdef CONFIG_PROC_FS
> > +static void *ucounts_start(struct seq_file *f, loff_t *pos)
> > +{
> > +       struct ucounts_iterator *iter = f->private;
> > +       int h, i = 0;
> > +
> > +       spin_lock(&ucounts_lock);
> 
> This series is much improved, thanks! However, I still don't think
> it's a good idea to hold this spinlock across the start/stop lifetime.
> It creates too many opportunities for abuse. :( Perhaps Eric will have
> some better ideas about how to deal with this...

Would it be too limiting to pre-allocate a page and simply fill it out
right at ucounts_start, so that we can drop the spinlock and simply
return data from that page?