CVE-2023-53511: Missing Release of Memory after Effective Lifetime

In the Linux kernel, the following vulnerability has been resolved:

io_uring: fix fget leak when fs don't support nowait buffered read

Heming reported a BUG when using io_uring doing link-cp on ocfs2. [1]

Do the following steps can reproduce this BUG: mount -t ocfs2 /dev/vdc /mnt/ocfs2 cp testfile /mnt/ocfs2/ ./link-cp /mnt/ocfs2/testfile /mnt/ocfs2/testfile.1 umount /mnt/ocfs2

Then umount will fail, and it outputs: umount: /mnt/ocfs2: target is busy.

While tracing umount, it blames mnt_get_count() not return as expected. Do a deep investigation for fget()/fput() on related code flow, I've finally found that fget() leaks since ocfs2 doesn't support nowait buffered read.

io_issue_sqe |-io_assign_file // do fget() first |-io_read |-io_iter_do_read |-ocfs2_file_read_iter // return -EOPNOTSUPP |-kiocb_done |-io_rw_done |-__io_complete_rw_common // set REQ_F_REISSUE |-io_resubmit_prep |-io_req_prep_async // override req->file, leak happens

This was introduced by commit a196c78b5443 in v5.18. Fix it by don't re-assign req->file if it has already been assigned.

[1] https://lore.kernel.org/ocfs2-devel/[email protected]/T/#t