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Subject[patch 016/104] epoll: introduce resource usage limits
2.6.27-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Davide Libenzi <davidel@xmailserver.org>
commit 7ef9964e6d1b911b78709f144000aacadd0ebc21 upstream.

It has been thought that the per-user file descriptors limit would also
limit the resources that a normal user can request via the epoll
interface. Vegard Nossum reported a very simple program (a modified
version attached) that can make a normal user to request a pretty large
amount of kernel memory, well within the its maximum number of fds. To
solve such problem, default limits are now imposed, and /proc based
configuration has been introduced. A new directory has been created,
named /proc/sys/fs/epoll/ and inside there, there are two configuration
points:

max_user_instances = Maximum number of devices - per user

max_user_watches = Maximum number of "watched" fds - per user

The current default for "max_user_watches" limits the memory used by epoll
to store "watches", to 1/32 of the amount of the low RAM. As example, a
256MB 32bit machine, will have "max_user_watches" set to roughly 90000.
That should be enough to not break existing heavy epoll users. The
default value for "max_user_instances" is set to 128, that should be
enough too.
This also changes the userspace, because a new error code can now come out
from EPOLL_CTL_ADD (-ENOSPC). The EMFILE from epoll_create() was already
listed, so that should be ok.

[akpm@linux-foundation.org: use get_current_user()]
Signed-off-by: Davide Libenzi <davidel@xmailserver.org>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Reported-by: Vegard Nossum <vegardno@ifi.uio.no>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
Documentation/filesystems/proc.txt | 27 +++++++++++
fs/eventpoll.c | 85 +++++++++++++++++++++++++++++++++----
include/linux/sched.h | 4 +
kernel/sysctl.c | 10 ++++
4 files changed, 118 insertions(+), 8 deletions(-)
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -44,6 +44,7 @@ Table of Contents
2.14 /proc/<pid>/io - Display the IO accounting fields
2.15 /proc/<pid>/coredump_filter - Core dump filtering settings
2.16 /proc/<pid>/mountinfo - Information about mounts
+ 2.17 /proc/sys/fs/epoll - Configuration options for the epoll interface

------------------------------------------------------------------------------
Preface
@@ -2471,4 +2472,30 @@ For more information on mount propagatio

Documentation/filesystems/sharedsubtree.txt

+2.17 /proc/sys/fs/epoll - Configuration options for the epoll interface
+--------------------------------------------------------
+
+This directory contains configuration options for the epoll(7) interface.
+
+max_user_instances
+------------------
+
+This is the maximum number of epoll file descriptors that a single user can
+have open at a given time. The default value is 128, and should be enough
+for normal users.
+
+max_user_watches
+----------------
+
+Every epoll file descriptor can store a number of files to be monitored
+for event readiness. Each one of these monitored files constitutes a "watch".
+This configuration option sets the maximum number of "watches" that are
+allowed for each user.
+Each "watch" costs roughly 90 bytes on a 32bit kernel, and roughly 160 bytes
+on a 64bit one.
+The current default value for max_user_watches is the 1/32 of the available
+low memory, divided for the "watch" cost in bytes.
+
+
------------------------------------------------------------------------------
+
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -102,6 +102,8 @@

#define EP_UNACTIVE_PTR ((void *) -1L)

+#define EP_ITEM_COST (sizeof(struct epitem) + sizeof(struct eppoll_entry))
+
struct epoll_filefd {
struct file *file;
int fd;
@@ -200,6 +202,9 @@ struct eventpoll {
* holding ->lock.
*/
struct epitem *ovflist;
+
+ /* The user that created the eventpoll descriptor */
+ struct user_struct *user;
};

/* Wait structure used by the poll hooks */
@@ -227,9 +232,17 @@ struct ep_pqueue {
};

/*
+ * Configuration options available inside /proc/sys/fs/epoll/
+ */
+/* Maximum number of epoll devices, per user */
+static int max_user_instances __read_mostly;
+/* Maximum number of epoll watched descriptors, per user */
+static int max_user_watches __read_mostly;
+
+/*
* This mutex is used to serialize ep_free() and eventpoll_release_file().
*/
-static struct mutex epmutex;
+static DEFINE_MUTEX(epmutex);

/* Safe wake up implementation */
static struct poll_safewake psw;
@@ -240,6 +253,33 @@ static struct kmem_cache *epi_cache __re
/* Slab cache used to allocate "struct eppoll_entry" */
static struct kmem_cache *pwq_cache __read_mostly;

+#ifdef CONFIG_SYSCTL
+
+#include <linux/sysctl.h>
+
+static int zero;
+
+ctl_table epoll_table[] = {
+ {
+ .procname = "max_user_instances",
+ .data = &max_user_instances,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = &proc_dointvec_minmax,
+ .extra1 = &zero,
+ },
+ {
+ .procname = "max_user_watches",
+ .data = &max_user_watches,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = &proc_dointvec_minmax,
+ .extra1 = &zero,
+ },
+ { .ctl_name = 0 }
+};
+#endif /* CONFIG_SYSCTL */
+

/* Setup the structure that is used as key for the RB tree */
static inline void ep_set_ffd(struct epoll_filefd *ffd,
@@ -402,6 +442,8 @@ static int ep_remove(struct eventpoll *e
/* At this point it is safe to free the eventpoll item */
kmem_cache_free(epi_cache, epi);

+ atomic_dec(&ep->user->epoll_watches);
+
DNPRINTK(3, (KERN_INFO "[%p] eventpoll: ep_remove(%p, %p)\n",
current, ep, file));

@@ -449,6 +491,8 @@ static void ep_free(struct eventpoll *ep

mutex_unlock(&epmutex);
mutex_destroy(&ep->mtx);
+ atomic_dec(&ep->user->epoll_devs);
+ free_uid(ep->user);
kfree(ep);
}

@@ -532,10 +576,19 @@ void eventpoll_release_file(struct file

static int ep_alloc(struct eventpoll **pep)
{
- struct eventpoll *ep = kzalloc(sizeof(*ep), GFP_KERNEL);
+ int error;
+ struct user_struct *user;
+ struct eventpoll *ep;

- if (!ep)
- return -ENOMEM;
+ user = get_current_user();
+ error = -EMFILE;
+ if (unlikely(atomic_read(&user->epoll_devs) >=
+ max_user_instances))
+ goto free_uid;
+ error = -ENOMEM;
+ ep = kzalloc(sizeof(*ep), GFP_KERNEL);
+ if (unlikely(!ep))
+ goto free_uid;

spin_lock_init(&ep->lock);
mutex_init(&ep->mtx);
@@ -544,12 +597,17 @@ static int ep_alloc(struct eventpoll **p
INIT_LIST_HEAD(&ep->rdllist);
ep->rbr = RB_ROOT;
ep->ovflist = EP_UNACTIVE_PTR;
+ ep->user = user;

*pep = ep;

DNPRINTK(3, (KERN_INFO "[%p] eventpoll: ep_alloc() ep=%p\n",
current, ep));
return 0;
+
+free_uid:
+ free_uid(user);
+ return error;
}

/*
@@ -703,9 +761,11 @@ static int ep_insert(struct eventpoll *e
struct epitem *epi;
struct ep_pqueue epq;

- error = -ENOMEM;
+ if (unlikely(atomic_read(&ep->user->epoll_watches) >=
+ max_user_watches))
+ return -ENOSPC;
if (!(epi = kmem_cache_alloc(epi_cache, GFP_KERNEL)))
- goto error_return;
+ return -ENOMEM;

/* Item initialization follow here ... */
INIT_LIST_HEAD(&epi->rdllink);
@@ -735,6 +795,7 @@ static int ep_insert(struct eventpoll *e
* install process. Namely an allocation for a wait queue failed due
* high memory pressure.
*/
+ error = -ENOMEM;
if (epi->nwait < 0)
goto error_unregister;

@@ -765,6 +826,8 @@ static int ep_insert(struct eventpoll *e

spin_unlock_irqrestore(&ep->lock, flags);

+ atomic_inc(&ep->user->epoll_watches);
+
/* We have to call this outside the lock */
if (pwake)
ep_poll_safewake(&psw, &ep->poll_wait);
@@ -789,7 +852,7 @@ error_unregister:
spin_unlock_irqrestore(&ep->lock, flags);

kmem_cache_free(epi_cache, epi);
-error_return:
+
return error;
}

@@ -1074,6 +1137,7 @@ asmlinkage long sys_epoll_create1(int fl
flags & O_CLOEXEC);
if (fd < 0)
ep_free(ep);
+ atomic_inc(&ep->user->epoll_devs);

error_return:
DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_create(%d) = %d\n",
@@ -1295,7 +1359,12 @@ asmlinkage long sys_epoll_pwait(int epfd

static int __init eventpoll_init(void)
{
- mutex_init(&epmutex);
+ struct sysinfo si;
+
+ si_meminfo(&si);
+ max_user_instances = 128;
+ max_user_watches = (((si.totalram - si.totalhigh) / 32) << PAGE_SHIFT) /
+ EP_ITEM_COST;

/* Initialize the structure used to perform safe poll wait head wake ups */
ep_poll_safewake_init(&psw);
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -587,6 +587,10 @@ struct user_struct {
atomic_t inotify_watches; /* How many inotify watches does this user have? */
atomic_t inotify_devs; /* How many inotify devs does this user have opened? */
#endif
+#ifdef CONFIG_EPOLL
+ atomic_t epoll_devs; /* The number of epoll descriptors currently open */
+ atomic_t epoll_watches; /* The number of file descriptors currently watched */
+#endif
#ifdef CONFIG_POSIX_MQUEUE
/* protected by mq_lock */
unsigned long mq_bytes; /* How many bytes can be allocated to mqueue? */
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -179,6 +179,9 @@ extern struct ctl_table random_table[];
#ifdef CONFIG_INOTIFY_USER
extern struct ctl_table inotify_table[];
#endif
+#ifdef CONFIG_EPOLL
+extern struct ctl_table epoll_table[];
+#endif

#ifdef HAVE_ARCH_PICK_MMAP_LAYOUT
int sysctl_legacy_va_layout;
@@ -1313,6 +1316,13 @@ static struct ctl_table fs_table[] = {
.child = inotify_table,
},
#endif
+#ifdef CONFIG_EPOLL
+ {
+ .procname = "epoll",
+ .mode = 0555,
+ .child = epoll_table,
+ },
+#endif
#endif
{
.ctl_name = KERN_SETUID_DUMPABLE,


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