A WARN_ON_ONCE(!buffer_uptodate(bh)) in mark_buffer_dirty() is reachable from the buffered write path on a block device when the underlying device returns I/O errors at high density. Reproduced by fuzzing an NVMe controller (FEMU) that returns crafted error completions for a sustained workload from /dev/nvme0n1. The contract documented at set_buffer_uptodate() in include/linux/buffer_head.h reads: Any other serialization (with IO errors or whatever that might clear the bit) has to come from other state (eg BH_Lock). In fs/buffer.c, BH_Uptodate can be cleared from four I/O completion callbacks: __end_buffer_read_notouch, end_buffer_write_sync, end_buffer_async_read, end_buffer_async_write. end_buffer_async_read() runs with BH_Lock held throughout, so its clear is already serialized against any caller that also holds BH_Lock around set_buffer_uptodate(); the call may in fact be redundant, but addressing that is independent of this fix. end_buffer_write_sync() likewise holds BH_Lock while it clears BH_Uptodate on the write-error path. Removing that clear would change long-standing buffer-cache I/O-error semantics and is out of scope here. The race is therefore between block_commit_write() and end_buffer_write_sync(): CPU A: block_commit_write CPU B: end_buffer_write_sync (folio lock held, BH_Lock NOT) (BH_Lock held) set_buffer_uptodate(bh); clear_buffer_uptodate(bh); unlock_buffer(bh); mark_buffer_dirty(bh); /* WARN */ CPU B observes the contract; CPU A does not. With one side unlocked the serialization is one-sided and ineffective: CPU A's set can be immediately followed by CPU B's clear, tripping the WARN_ON_ONCE. In the fuzzing reproducer, write-error completions are frequent (visible as repeated "lost async page write" and per-LBA write failures); buffer I/O completion callbacks on the write-error path (e.g. end_buffer_write_sync, end_buffer_async_write) clear BH_Uptodate while holding BH_Lock. The bug is not benign: a not-uptodate buffer can be marked dirty and subsequently written back; depending on whether the buffer was fully or partially covered by the user write, this can leave on-disk content that does not match the intended buffered write state. Fix this by taking BH_Lock around set_buffer_uptodate() + mark_buffer_dirty() in block_commit_write(), so both sides of the contract use the documented serialization. Found by FuzzNvme (Syzkaller with FEMU fuzzing framework). Acked-by: Sungwoo Kim Acked-by: Dave Tian Acked-by: Weidong Zhu Signed-off-by: Chao Shi --- Hi Matthew, Hi Jan, Thanks for the review on v1. v2 takes the feedback, quick notes below. To Matthew: You were right that the v1's timing diagram named the wrong racer. The actual race is with end_buffer_write_sync() on the write-error path, as Jan pointed out -- it also clears BH_Uptodate under BH_Lock, but block_commit_write()'s else branch was reaching set_buffer_uptodate without BH_Lock, leaving the serialization one-sided. v2's commit message and in-code comment now name end_buffer_write_sync() as the racer. To Jan: Thanks for confirming the racer and for the historical context on the dirty + !uptodate state question. v2 keeps the fix scoped to taking BH_Lock in block_commit_write(); the broader semantic question and any change to end_buffer_write_sync()'s clear is left out of scope here. The redundant lock_buffer/unlock_buffer that v1 added to the buffer_new branch of __block_write_begin_int() is also dropped in v2 -- that bh has no in-flight async I/O so no race exists there. v1 thread for context: https://lore.kernel.org/all/20260426020137.1221985-1-coshi036@gmail.com/ Thanks, Chao Changes in v2: - Drop the lock_buffer/unlock_buffer added in v1 to the buffer_new branch of __block_write_begin_int(): that bh is freshly BH_New and has no in-flight async I/O on it, so no race exists at that site. - Rewrite the commit message and the in-code comment to identify end_buffer_write_sync() as the actual racer, not end_buffer_async_read() as v1 claimed; end_buffer_async_read() holds BH_Lock across its clear so a caller that also holds BH_Lock would already be serialized. - Reference the BH_Lock contract at set_buffer_uptodate() in include/linux/buffer_head.h explicitly. - Drop verbose line-number citations and the WARN stack dump from the commit message; tighten wording around reproducer evidence and on-disk impact. v1: https://lore.kernel.org/all/20260426020137.1221985-1-coshi036@gmail.com/ fs/buffer.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/fs/buffer.c b/fs/buffer.c index 4d7f84e77d2..6fddb2f1e7c 100644 --- a/fs/buffer.c +++ b/fs/buffer.c @@ -2104,8 +2104,22 @@ void block_commit_write(struct folio *folio, size_t from, size_t to) if (!buffer_uptodate(bh)) partial = true; } else { + /* + * Per the contract documented at set_buffer_uptodate() + * in include/linux/buffer_head.h, callers must hold + * BH_Lock to serialize against concurrent clears of + * BH_Uptodate. Holding only the folio lock is not + * sufficient: a concurrent end_buffer_write_sync() on + * the write-error path clears BH_Uptodate while + * holding BH_Lock; without BH_Lock here the clear can + * land between set_buffer_uptodate() and + * mark_buffer_dirty(), tripping the WARN_ON_ONCE in + * mark_buffer_dirty(). + */ + lock_buffer(bh); set_buffer_uptodate(bh); mark_buffer_dirty(bh); + unlock_buffer(bh); } if (buffer_new(bh)) clear_buffer_new(bh); base-commit: ffe69af1f87fa77da975ad4b0b093d48c3cbe6c3 -- 2.43.0