Cmnt: [SRU][F][PATCH v2 0/3] CVE-2023-21400

[Chengen Du]

On Tue, Dec 10, 2024 at 1:46 PM Koichiro Den <koichiro.den at canonical.com> wrote:
>
> On Tue, Nov 26, 2024 at 12:14:25PM +0800, Chengen Du wrote:
> > CVE-2023-21400
> >
> > BugLink: https://bugs.launchpad.net/bugs/2078659
> >
> > SRU Justification:
> >
> > [Impact]
> > io_commit_cqring() writes the CQ ring tail to make it visible and also triggers any deferred work.
> > When a ring is set up with IOPOLL, it doesn't require locking around the CQ ring updates.
> > However, if there is deferred work that needs processing, io_queue_deferred() assumes that the completion_lock is held.
> > The io_uring subsystem does not properly handle locking for rings with IOPOLL, leading to a double-free vulnerability, which can be exploited as CVE-2023-21400.
> >
> > [Fix]
> > There is a commit that fixed this issue.
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=fb348857e7b67eefe365052f1423427b66dedbf3
> >
> > There is no direct upstream commit for this issue, and the patch needs to be reworked to apply to version 5.4.
> >
> > [Test Plan]
> > This is a timing issue that can be triggered by using the liburing library to implement a test program.
> > First, set the io_uring_params flag to IORING_SETUP_IOPOLL and open an XFS file with the O_RDWR | O_DIRECT flags, as the XFS filesystem implements the iopoll function hook.
> > After setting up io_uring, create two threads in the process: one thread will wait for completion queue events, and the other will continuously send readv and writev requests in sequence.
> > The writev requests should include the IOSQE_IO_DRAIN flag to ensure that previous submission queue events are completed first.
> >
> > The issue arises when writev requests add entries into the defer_list, but the io_iopoll_complete function consumes entries from defer_list without holding the appropriate lock.
> >
> > [Where problems could occur]
> > The problematic call path can be triggered under specific usage scenarios and only affects io_uring functionality.
> > If the patch contains any issues, it may lead to a deadlock.
> >
> > Jens Axboe (1):
> >   io_uring: ensure IOPOLL locks around deferred work
> >
> > Pavel Begunkov (2):
> >   io_uring: remove extra check in __io_commit_cqring
> >   io_uring: dont kill fasync under completion_lock
>
> [1/3] and [2/3] seem to be not only unrelated to CVE-2023-21400 but also
> unhelpful in reducing context conflicts for [3/3]. If there is still a
> compelling reason to include those two for CVE-2023-21400, could you let me
> know?

Here is my initial version of the fix for this CVE:
https://lists.ubuntu.com/archives/kernel-team/2024-November/155144.html

To address the issue within the current code structure, I have wrapped
io_commit_cqring in the io_iopoll_complete function with
completion_lock.
However, this approach has surfaced an existing issue related to
invoking kill_fasync under completion_lock as highlighted in commit
4aa84f2ffa81 (io_uring: don't kill fasync under completion_lock).

While this existing issue is not directly related to the CVE, I am
concerned that the fix could increase the likelihood of encountering
it, especially since io_iopoll_complete may be triggered more
frequently.

To avoid exacerbating the situation, I recommend addressing the
kill_fasync logic separately before implementing the fix for the CVE,
as well as backporting the prerequisite commit 0791015837f1 (io_uring:
remove extra check in __io_commit_cqring).