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, 10 insertions, 0 deletions

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 9d048fa2d902..3d56fe7570da 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -176,6 +176,9 @@ 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;
@@ -1325,6 +1328,13 @@ 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,