CVE-2024-43855

Summary

CVE-2024-43855 is a Linux kernel vulnerability affecting the md (Multiple Devices) driver. This issue involves a deadlock between mddev_suspend and flush bio processes. The root cause is a non-atomic increment/decrement of the active_io variable during the flush process. The deadlock occurs when mddev is being suspended while some flush bio is in progress. The first flush clears 'mddev->flush_bio' and tries to submit data, but is blocked because mddev is suspended. The second flush sets 'mddev->flush_bio' and attempts to queue md_submit_flush_data(), which is already running and won't execute again if on the same CPU as the first flush. The third flush increments active_io and tries to flush, but is blocked because 'mddev->flush_bio' is not NULL, set by the second flush. Finally, mddev_suspend() is called and waits for active_io to decrease to 0, which is incremented by the third flush. The fix involves ensuring that active_io is decremented before md_handle_request() in md_submit_flush_data(). This change prevents make_request() from being called directly and active_io only incrementing and decrementing once during the entire flush process, eliminating the deadlock. Additionally, the only difference between the fix and the previous patch is that make_request() no longer returns an error. Since data and flush operations are split into two separate I/O requests in dm, the size of the flush submitted by dm is always zero, and make_request() will not be called in md_submit_flush_data(). To prevent future modifications from introducing similar issues, a WARN_ON is added to ensure make_request() does not return an error in this context.

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