** Description changed: [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. + There is no direct upstream commit addressing this issue, so the patch + needs to be reworked to apply to version 5.4. + + The patch introduces the io_commit_needs_flush function to determine if flushing the cqring while holding ctx->completion_lock is necessary. + However, implementing this logic in Focal requires backporting commits that introduce missing io_ring_ctx struct member variables. + This approach is not ideal, as those commits are unrelated to the CVE. + Additionally, introducing new member variables raises concerns, particularly since Focal is nearing the end of its standard support period. + To address this tradeoff, the SUSE commit [1] serves as a suitable reference. + + To address this issue in the simplest way, we can wrap the ctx->completion_lock spinlock around io_commit_cqring in the io_iopoll_complete function. + However, this approach has surfaced an existing issue related to invoking kill_fasync under completion_lock as highlighted in commit deadlock (4aa84f2ffa81 io_uring: don't kill fasync under completion_lock). + The following patches can help us align the code more closely with the upstream flow while also addressing the deadlock issue. + 0791015837f1 io_uring: remove extra check in __io_commit_cqring + 4aa84f2ffa81 io_uring: dont kill fasync under completion_lock + + After backporting these two commits, we can adopt SUSE's approach to + align the code more closely with the Linux stable branch. [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. + + [1] + https://github.com/SUSE/kernel/commit/a16c5d2daa1cf45f8694ff72bd2665d474a763a7
-- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/2078659 Title: Hold IOPOLL locks when triggering io_uring's deferred work To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2078659/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
