fanotify: allow userspace to override max marks

Some fanotify groups, especially those like AV scanners, will need to place
lots of marks, particularly ignore marks.  Since ignore marks do not pin
inodes in cache and are cleared if the inode is removed from core (usually
under memory pressure) we expose an interface for listeners, with
CAP_SYS_ADMIN, to override the maximum number of marks and be allowed to
set and 'unlimited' number of marks.  Programs which make use of this
feature will be able to OOM a machine.

Signed-off-by: Eric Paris <eparis@redhat.com>

2 files changed, 11 insertions, 2 deletions

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index 1d33d7db277a..f9216102b426 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -707,7 +707,14 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
                 group->max_events = FANOTIFY_DEFAULT_MAX_EVENTS;
         }
 
-        group->fanotify_data.max_marks = FANOTIFY_DEFAULT_MAX_MARKS;
+        if (flags & FAN_UNLIMITED_MARKS) {
+                fd = -EPERM;
+                if (!capable(CAP_SYS_ADMIN))
+                        goto out_put_group;
+                group->fanotify_data.max_marks = UINT_MAX;
+        } else {
+                group->fanotify_data.max_marks = FANOTIFY_DEFAULT_MAX_MARKS;
+        }
 
         fd = anon_inode_getfd("[fanotify]", &fanotify_fops, group, f_flags);
         if (fd < 0)
diff --git a/include/linux/fanotify.h b/include/linux/fanotify.h
index e37f559c95e1..7592a366a57b 100644
--- a/include/linux/fanotify.h
+++ b/include/linux/fanotify.h
@@ -32,9 +32,11 @@
                                  FAN_CLASS_PRE_CONTENT)
 
 #define FAN_UNLIMITED_QUEUE     0x00000010
+#define FAN_UNLIMITED_MARKS     0x00000020
 
 #define FAN_ALL_INIT_FLAGS      (FAN_CLOEXEC | FAN_NONBLOCK | \
-                                 FAN_ALL_CLASS_BITS | FAN_UNLIMITED_QUEUE)
+                                 FAN_ALL_CLASS_BITS | FAN_UNLIMITED_QUEUE |\
+                                 FAN_UNLIMITED_MARKS)
 
 /* flags used for fanotify_modify_mark() */
 #define FAN_MARK_ADD            0x00000001