io_uring/waitid: clear waitid info before copying it to userspace

commit 93d93f5f8da791e98159795c6ef683f45bd95d13 upstream.

IORING_OP_WAITID stores its result fields in struct io_waitid::info and
later copies them to userspace siginfo. The prep path initializes the
request arguments, but it does not initialize info itself.

If the wait operation completes without reporting a child event, the common
wait code can return without writing wo_info. In that case io_waitid_finish()
still copies iw->info to userspace, exposing stale bytes from the reused
io_kiocb command storage.

Clear the result storage during prep so the io_uring path matches the
regular waitid syscall, which uses a zero-initialized struct waitid_info.

BUG=b/522217835
TEST=presubmit
RELEASE_NOTE=Fixed CVE-2026-46315 in the Linux kernel.

cos-patch: security-high
Fixes: f31ecf671ddc ("io_uring: add IORING_OP_WAITID support")
Cc: stable@vger.kernel.org # 6.7+
Change-Id: Ia094a1293b4c8dca9d559a7f77f27577a0d426f3
Signed-off-by: Heechan Kang <gganji11@naver.com>
Link: https://patch.msgid.link/20260516184709.852814-1-gganji11@naver.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/160264
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Daniel Velasquez <rdvelasquez@google.com>