A race condition exists between SMB request handling in `ksmbd_conn_handler_loop()` and the freeing of `ksmbd_conn` in the workqueue handler `handle_ksmbd_work()`. This leads to a UAF. - KASAN: slab-use-after-free Read in handle_ksmbd_work - KASAN: slab-use-after-free in rtlock_slowlock_locked

This race condition arises as follows: - `ksmbd_conn_handler_loop()` waits for `conn->r_count` to reach zero: `wait_event(conn->r_count_q, atomic_read(&conn->r_count) == 0);` - Meanwhile, `handle_ksmbd_work()` decrements `conn->r_count` using `atomic_dec_return(&conn->r_count)`, and if it reaches zero, calls `ksmbd_conn_free()`, which frees `conn`. - However, after `handle_ksmbd_work()` decrements `conn->r_count`, it may still access `conn->r_count_q` in the following line: `waitqueue_active(&conn->r_count_q)` or `wake_up(&conn->r_count_q)` This results in a UAF, as `conn` has already been freed.

The discovery of this UAF can be referenced in the following PR for syzkaller's support for SMB requests. Link: https://github.com/google/syzkaller/pull/5524

Fixes: ee426bfb9d09 ("ksmbd: add refcnt to ksmbd_conn struct") Cc: linux-cifs@vger.kernel.org Cc: stable@vger.kernel.org # v6.6.55+, v6.10.14+, v6.11.3+ Cc: syzkaller@googlegroups.com Signed-off-by: Yunseong Kim yskelg@gmail.com --- fs/smb/server/server.c | 4 ++++ 1 file changed, 4 insertions(+)

diff --git a/fs/smb/server/server.c b/fs/smb/server/server.c index e6cfedba9992..c8cc6fa6fc3e 100644 --- a/fs/smb/server/server.c +++ b/fs/smb/server/server.c @@ -276,8 +276,12 @@ static void handle_ksmbd_work(struct work_struct *wk) * disconnection. waitqueue_active is safe because it * uses atomic operation for condition. */ + atomic_inc(&conn->refcnt); if (!atomic_dec_return(&conn->r_count) && waitqueue_active(&conn->r_count_q)) wake_up(&conn->r_count_q); + + if (atomic_dec_and_test(&conn->refcnt)) + kfree(conn); }

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