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fsnotify: prevent mark additions during connector teardown

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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: 6b3f05d24d ("fsnotify: Detach mark from object list when last reference is dropped")
Cc: stable@vger.kernel.org
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
This commit is contained in:
Kyle Zeng 2026-10-02 05:59:02 +00:00 committed by Christian Brauner
parent 40d38f58fc
commit 36ebddca56
No known key found for this signature in database
GPG Key ID: 91C61BC06578DCA2
2 changed files with 12 additions and 0 deletions
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11
fs/notify/mark.c
View File

@ -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

1
include/linux/fsnotify_backend.h
View File

@ -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] */