inotify: fix locking around inotify watching in the idr

The are races around the idr storage of inotify watches.  It's possible
that a watch could be found from sys_inotify_rm_watch() in the idr, but it
could be removed from the idr before that code does it's removal.  Move the
locking and the refcnt'ing so that these have to happen atomically.

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

1 files changed, 40 insertions, 10 deletions

diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c
index d8f73c253073..ce1f5823e2c2 100644
--- a/fs/notify/inotify/inotify_user.c
+++ b/fs/notify/inotify/inotify_user.c
@@ -364,20 +364,53 @@ static int inotify_find_inode(const char __user *dirname, struct path *path, uns
         return error;
 }
 
+/*
+ * Remove the mark from the idr (if present) and drop the reference
+ * on the mark because it was in the idr.
+ */
 static void inotify_remove_from_idr(struct fsnotify_group *group,
                                     struct inotify_inode_mark_entry *ientry)
 {
         struct idr *idr;
+        struct fsnotify_mark_entry *entry;
+        struct inotify_inode_mark_entry *found_ientry;
+        int wd;
 
         spin_lock(&group->inotify_data.idr_lock);
         idr = &group->inotify_data.idr;
-        idr_remove(idr, ientry->wd);
-        spin_unlock(&group->inotify_data.idr_lock);
+        wd = ientry->wd;
+
+        if (wd == -1)
+                goto out;
+
+        entry = idr_find(&group->inotify_data.idr, wd);
+        if (unlikely(!entry))
+                goto out;
+
+        found_ientry = container_of(entry, struct inotify_inode_mark_entry, fsn_entry);
+        if (unlikely(found_ientry != ientry)) {
+                /* We found an entry in the idr with the right wd, but it's
+                 * not the entry we were told to remove.  eparis seriously
+                 * fucked up somewhere. */
+                WARN_ON(1);
+                ientry->wd = -1;
+                goto out;
+        }
+
+        /* One ref for being in the idr, one ref held by the caller */
+        BUG_ON(atomic_read(&entry->refcnt) < 2);
+
+        idr_remove(idr, wd);
         ientry->wd = -1;
+
+        /* removed from the idr, drop that ref */
+        fsnotify_put_mark(entry);
+out:
+        spin_unlock(&group->inotify_data.idr_lock);
 }
+
 /*
- * Send IN_IGNORED for this wd, remove this wd from the idr, and drop the
- * internal reference help on the mark because it is in the idr.
+ * Send IN_IGNORED for this wd, remove this wd from the idr.
  */
 void inotify_ignored_and_remove_idr(struct fsnotify_mark_entry *entry,
                                     struct fsnotify_group *group)
@@ -417,9 +450,6 @@ skip_send_ignore:
         /* remove this entry from the idr */
         inotify_remove_from_idr(group, ientry);
 
-        /* removed from idr, drop that reference */
-        fsnotify_put_mark(entry);
-
         atomic_dec(&group->inotify_data.user->inotify_watches);
 }
 
@@ -535,6 +565,9 @@ retry:
                 goto out_err;
         }
 
+        /* we put the mark on the idr, take a reference */
+        fsnotify_get_mark(&tmp_ientry->fsn_entry);
+
         /* we are on the idr, now get on the inode */
         ret = fsnotify_add_mark(&tmp_ientry->fsn_entry, group, inode);
         if (ret) {
@@ -543,9 +576,6 @@ retry:
                 goto out_err;
         }
 
-        /* we put the mark on the idr, take a reference */
-        fsnotify_get_mark(&tmp_ientry->fsn_entry);
-
         /* update the idr hint, who cares about races, it's just a hint */
         group->inotify_data.last_wd = tmp_ientry->wd;