fsnotify: Fix NULL ptr deref in fanotify_get_fsid()

fanotify_get_fsid() is reading mark->connector->fsid under srcu. It can
happen that it sees mark not fully initialized or mark that is already
detached from the object list. In these cases mark->connector
can be NULL leading to NULL ptr dereference. Fix the problem by
being careful when reading mark->connector and check it for being NULL.
Also use WRITE_ONCE when writing the mark just to prevent compiler from
doing something stupid.

Reported-by: syzbot+15927486a4f1bfcbaf91@syzkaller.appspotmail.com
Fixes: 77115225acc6 ("fanotify: cache fsid in fsnotify_mark_connector")
Signed-off-by: Jan Kara <jack@suse.cz>

2 files changed, 18 insertions, 8 deletions

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index 6b9c27548997..63c6bb1f8c4d 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c
@@ -346,10 +346,16 @@ static __kernel_fsid_t fanotify_get_fsid(struct fsnotify_iter_info *iter_info)
         __kernel_fsid_t fsid = {};
 
         fsnotify_foreach_obj_type(type) {
+                struct fsnotify_mark_connector *conn;
+
                 if (!fsnotify_iter_should_report_type(iter_info, type))
                         continue;
 
-                fsid = iter_info->marks[type]->connector->fsid;
+                conn = READ_ONCE(iter_info->marks[type]->connector);
+                /* Mark is just getting destroyed or created? */
+                if (!conn)
+                        continue;
+                fsid = conn->fsid;
                 if (WARN_ON_ONCE(!fsid.val[0] && !fsid.val[1]))
                         continue;
                 return fsid;
@@ -408,8 +414,12 @@ static int fanotify_handle_event(struct fsnotify_group *group,
                         return 0;
         }
 
-        if (FAN_GROUP_FLAG(group, FAN_REPORT_FID))
+        if (FAN_GROUP_FLAG(group, FAN_REPORT_FID)) {
                 fsid = fanotify_get_fsid(iter_info);
+                /* Racing with mark destruction or creation? */
+                if (!fsid.val[0] && !fsid.val[1])
+                        return 0;
+        }
 
         event = fanotify_alloc_event(group, inode, mask, data, data_type,
                                      &fsid);
diff --git a/fs/notify/mark.c b/fs/notify/mark.c
index d593d4269561..22acb0a79b53 100644
--- a/fs/notify/mark.c
+++ b/fs/notify/mark.c
@@ -239,13 +239,13 @@ static void fsnotify_drop_object(unsigned int type, void *objp)
 
 void fsnotify_put_mark(struct fsnotify_mark *mark)
 {
-        struct fsnotify_mark_connector *conn;
+        struct fsnotify_mark_connector *conn = READ_ONCE(mark->connector);
         void *objp = NULL;
         unsigned int type = FSNOTIFY_OBJ_TYPE_DETACHED;
         bool free_conn = false;
 
         /* Catch marks that were actually never attached to object */
-        if (!mark->connector) {
+        if (!conn) {
                 if (refcount_dec_and_test(&mark->refcnt))
                         fsnotify_final_mark_destroy(mark);
                 return;
@@ -255,10 +255,9 @@ void fsnotify_put_mark(struct fsnotify_mark *mark)
          * We have to be careful so that traversals of obj_list under lock can
          * safely grab mark reference.
          */
-        if (!refcount_dec_and_lock(&mark->refcnt, &mark->connector->lock))
+        if (!refcount_dec_and_lock(&mark->refcnt, &conn->lock))
                 return;
 
-        conn = mark->connector;
         hlist_del_init_rcu(&mark->obj_list);
         if (hlist_empty(&conn->list)) {
                 objp = fsnotify_detach_connector_from_object(conn, &type);
@@ -266,7 +265,7 @@ void fsnotify_put_mark(struct fsnotify_mark *mark)
         } else {
                 __fsnotify_recalc_mask(conn);
         }
-        mark->connector = NULL;
+        WRITE_ONCE(mark->connector, NULL);
         spin_unlock(&conn->lock);
 
         fsnotify_drop_object(type, objp);
@@ -620,7 +619,7 @@ restart:
         /* mark should be the last entry.  last is the current last entry */
         hlist_add_behind_rcu(&mark->obj_list, &last->obj_list);
 added:
-        mark->connector = conn;
+        WRITE_ONCE(mark->connector, conn);
 out_err:
         spin_unlock(&conn->lock);
         spin_unlock(&mark->lock);
@@ -808,6 +807,7 @@ void fsnotify_init_mark(struct fsnotify_mark *mark,
         refcount_set(&mark->refcnt, 1);
         fsnotify_get_group(group);
         mark->group = group;
+        WRITE_ONCE(mark->connector, NULL);
 }
 
 /*