From 36ebddca56b9d8993d6dc984fdecc748ee1f5e06 Mon Sep 17 00:00:00 2001 From: Kyle Zeng Date: Fri, 2 Oct 2026 05:59:02 +0000 Subject: [PATCH] fsnotify: prevent mark additions during connector teardown fsnotify_destroy_marks() keeps a reference to its current mark while it drops conn->lock to destroy the mark. This stabilizes the forward-walk cursor, but does not prevent a new mark from being inserted before it. For example, an inotify watch re-added after IN_IGNORED sorts before the old, detached mark for the same group. The walk can miss the replacement and then detach the entire connector from the object. The replacement is still on its group's marks_list and, for inotify, in the watch IDR. The group mutex no longer protects the connector's object from disappearing. An unprivileged fdinfo read racing the final detach can consequently call igrab(NULL): BUG: KASAN: null-ptr-deref in igrab+0x24/0x100 Write of size 4 at addr 0000000000000148 Call Trace: igrab+0x24/0x100 inotify_show_fdinfo+0x1dc/0x5e0 seq_show+0x397/0x690 Set a DESTROYING flag under conn->lock before starting the walk, and reject new marks on that connector with -ENOENT. The existing add-mark error paths undo the temporary group-list entry and the backend's watch allocation. Do not wait for teardown while holding the group mutex, which the destroyer needs. Once the old connector is detached, an add can install a fresh connector as before. This closes the insertion race for all object types using the common connector teardown, without changing the mark reference or SRCU lifetime rules. Fixes: 6b3f05d24d35 ("fsnotify: Detach mark from object list when last reference is dropped") Cc: stable@vger.kernel.org Signed-off-by: Kyle Zeng Signed-off-by: Christian Brauner (Amutable) --- fs/notify/mark.c | 11 +++++++++++ include/linux/fsnotify_backend.h | 1 + 2 files changed, 12 insertions(+) diff --git a/fs/notify/mark.c b/fs/notify/mark.c index d17628580a57..cb07f42d3422 100644 --- a/fs/notify/mark.c +++ b/fs/notify/mark.c @@ -929,6 +929,11 @@ static int fsnotify_add_mark_list(struct fsnotify_mark *mark, void *obj, goto restart; } + if (conn->flags & FSNOTIFY_CONN_FLAG_DESTROYING) { + err = -ENOENT; + goto out_err; + } + /* is mark the first mark? */ if (hlist_empty(&conn->list)) { hlist_add_head_rcu(&mark->obj_list, &conn->list); @@ -1116,6 +1121,12 @@ void fsnotify_destroy_marks(fsnotify_connp_t *connp) conn = fsnotify_grab_connector(connp); if (!conn) return; + /* + * New marks could be inserted before our cursor and survive the final + * detachment of the connector from the object. Prevent additions to + * this connector before dropping its lock. + */ + conn->flags |= FSNOTIFY_CONN_FLAG_DESTROYING; /* * We have to be careful since we can race with e.g. * fsnotify_clear_marks_by_group() and once we drop the conn->lock, the diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h index 618eed4d6d72..a6d5c1a89cff 100644 --- a/include/linux/fsnotify_backend.h +++ b/include/linux/fsnotify_backend.h @@ -566,6 +566,7 @@ struct fsnotify_mark_connector { unsigned char prio; /* Highest priority group */ #define FSNOTIFY_CONN_FLAG_IS_WATCHED 0x01 #define FSNOTIFY_CONN_FLAG_HAS_IREF 0x02 +#define FSNOTIFY_CONN_FLAG_DESTROYING 0x04 unsigned short flags; /* flags [lock] */ union { /* Object pointer [lock] */