In the Linux kernel, the following vulnerability has been resolved:<\/p>\n
ext4: drain in-flight DIO before buffered write fallback<\/p>\n
generic\/746 started failing intermittently on ext3 (no-extent inodes).
\nThe test triggers ‘Page cache invalidation failure on direct I\/O’
\nwarnings and subsequent fsync returns -EIO. Adding a 50ms delay
\nbetween ext4_buffered_write_iter() and filemap_write_and_wait_range()
\nin ext4_dio_write_iter() makes the race almost always reproducible.<\/p>\n
On no-extent inodes, DIO writes to holes cannot use unwritten extents,
\nso ext4_iomap_alloc() leaves m_flags=0 and ext4_map_blocks() returns 0.
\nThe iomap layer then returns -ENOTBLK, causing fallback to buffered I\/O.<\/p>\n
The fallback path in ext4_dio_write_iter() calls
\next4_buffered_write_iter() which dirties pages, then does flush and
\ninvalidate. However, there’s an unprotected window between
\next4_buffered_write_iter() returning (with inode lock released) and
\nthe subsequent flush+invalidate.<\/p>\n
Concurrent async DIO completions from other threads can run
\nkiocb_invalidate_post_direct_write() during this window. If pages have
\nbeen re-dirtied, post-invalidation finds dirty pages and triggers the
\nwarning, setting -EIO in the error sequence.<\/p>\n
Consider a file with two 4k extents: [hole][written]. Thread A does
\nDIO to the written extent, while thread B does DIO spanning both:<\/p>\n
kworker A (4k DIO, allocated block) kworker B (8k DIO, fallback)
\n ———————————– —————————-
\n inode_lock_shared() inode_lock_shared()
\n iomap_dio_rw(): iomap_dio_rw():
\n kiocb_invalidate_pages -> clean iomap_begin -> -ENOTBLK
\n submit_bio (async) dio->size = 0
\n inode_unlock_shared() inode_unlock_shared()<\/p>\n
[bio pending in block layer] \/* fallback: lock released *\/
\n ext4_buffered_write_iter()
\n inode_lock(exclusive)
\n generic_perform_write()
\n -> dirty pages [0, 8k]
\n inode_unlock(exclusive)<\/p>\n
\/* pages dirty, no lock *\/
\n [bio completes] filemap_write_and_wait_range()
\n iomap_dio_complete() -> flush dirty pages
\n kiocb_invalidate_post_direct_write() invalidate_mapping_pages()
\n invalidate_inode_pages2_range()
\n -> finds dirty page!
\n -> dio_warn_stale_pagecache()
\n -> errseq_set(-EIO)<\/p>\n
This issue can be triggered through normal I\/O paths, not just
\nintentionally overlapping DIO writes from userspace. For example,
\ngeneric\/746 uses a loop device where multiple kworkers issue concurrent
\nI\/O to the backing file. Additionally, when block_size < folio_size,
\nnon-overlapping DIO writes that share a large folio can also trigger
\nthe race.<\/p>\n
Add inode_dio_wait() in ext4_buffered_write_iter() before
\next4_write_checks() to drain all in-flight DIO. This ensures that
\nall DIO clears existing pages before submitting IO (via
\nkiocb_invalidate_pages()), all BIO waits for all DIO to complete
\n(via inode_dio_wait()), and ext4_write_checks() observes the inode
\nsize after all completed DIO so that ext4_block_zero_eof() does not
\nrace with in-flight DIO, thus eliminating the race.<\/p>\n","protected":false},"excerpt":{"rendered":"
In the Linux kernel, the following vulnerability has been resolved:<\/p>\n
ext4: drain in-flight DIO before buffered write fallback<\/p>\n
generic\/746 started failing intermittently on ext3 (no-extent inodes).
\nTh<\/p>\n","protected":false},"template":"","meta":[],"categories":[],"tags":[],"class_list":["post-25870","vuln","type-vuln","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/vuln\/25870","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/vuln"}],"about":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/types\/vuln"}],"wp:attachment":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=25870"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=25870"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=25870"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}