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>

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index 6b9c275..63c6bb1 100644 (file)
--- 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 d593d42..22acb0a 100644 (file)
--- 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);
 }
 
 /*