io_uring/rsrc: hold uring_lock across index-alloc put_user

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 the io_pipe_fixed() fix in a3d7ee11289f. Faulting
while holding ->uring_lock is fine, the registration paths already do
it.

Fixes: a7c41b4687f5 ("io_uring: let IORING_OP_FILES_UPDATE support choosing fixed file slots")
Signed-off-by: Yuansheng Yang <0xbeac@gmail.com>
Link: https://patch.msgid.link/20261104154655.81876-1-0xbeac@gmail.com
[axboe: various touch-ups]
Signed-off-by: Jens Axboe <axboe@kernel.dk>

0 files changed, 0 insertions, 0 deletions