Re: [PATCH] io_uring/rsrc: hold uring_lock across index-alloc put_user

[Jens Axboe <axboe@kernel.dk>]

On 11/4/26 2:28 AM, BeaCox wrote:
> io_files_update_with_index_alloc() installs a fixed file via
> io_fixed_fd_install(), which drops ->uring_lock when the request runs on
> io-wq, then put_user()s the allocated slot back to userspace.  If that
> faults, the error path calls __io_close_fixed() by slot index after
> re-taking the lock.
> 
> The slot can be closed and reused between the install and that cleanup:
> another fixed CLOSE plus open/accept/pipe/socket, or a FILES_UPDATE, can
> put an unrelated file into the same slot.  The error path then removes
> that file instead of the one this request installed, ending an unrelated
> file's fixed-table reference early.  This is a correctness/DoS issue, not
> a use-after-free: fixed-file requests hold an io_rsrc_node reference, so
> the node and file stay alive until they are done.
> 
> Hold ->uring_lock across __io_fixed_fd_install() and put_user(), and do
> the failure cleanup with io_fixed_fd_remove() under the same lock.  This
> is the same class as io_pipe_fixed() (CVE-2026-99265); faulting while
> holding ->uring_lock is fine, the registration paths already do it.

Fix looks fine to me, but it's not a CVE kind of bug. Neither is the
one referenced, I'll get that one invalidated.

-- 
Jens Axboe