epoll: introduce resource usage limits

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: <stable@kernel.org>
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>

1 files changed, 77 insertions, 8 deletions

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index aec5c13f6341..96355d505347 100644
--- 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 __read_mostly;
 /* 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 *ep, struct epitem *epi)
         /* 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 *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 **pep)
         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 *ep, struct epoll_event *event,
         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 *ep, struct epoll_event *event,
          * 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 *ep, struct epoll_event *event,
 
         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;
 }
 
@@ -1078,6 +1141,7 @@ asmlinkage long sys_epoll_create1(int flags)
                               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",
@@ -1299,7 +1363,12 @@ asmlinkage long sys_epoll_pwait(int epfd, struct epoll_event __user *events,
 
 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);