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SubjectRe: [PATCH 1/2] kernel: show current values of user namespace counters
On Tue, Aug 16, 2016 at 03:05:29PM -0500, Serge E. Hallyn wrote:
> Quoting Kees Cook (keescook@chromium.org):
> > On Mon, Aug 15, 2016 at 1:10 PM, Andrei Vagin <avagin@openvz.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@canonical.com>
> > > Cc: Kees Cook <keescook@chromium.org>
> > > Cc: "Eric W. Biederman" <ebiederm@xmission.com>
> > > Signed-off-by: Andrei Vagin <avagin@openvz.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?

seq_file works like you described here. It allocates a buffer, then
calls ->start(), fills the buffer and calls ->stop().

seq_file calls only ->next() and ->show() between ->start and stop().

ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
{
if (!m->buf) {
m->buf = seq_buf_alloc(m->size = PAGE_SIZE);
...
p = m->op->start(m, &pos);
...
while (m->count < size) {
...
p = m->op->next(m, p, &next);
if (!p || IS_ERR(p)) {
err = PTR_ERR(p);
break;
}

err = m->op->show(m, p);
if (seq_has_overflowed(m) || err) {
...
break;
}
...
}
m->op->stop(m, p);
}

In our case, a buffer size will not be bigger then one page, because a
bigger buffer is allocated only if it isn't enough to place one record.

Thanks,
Andrei

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