When a buffered read fails, iomap_finish_folio_read() reports the error with fserror_report_io(folio->mapping->host, ...). This is called after ifs->read_bytes_pending has been decremented by the bytes attempted to be read. For a folio split across multiple read completions, the folio is only guaranteed to stay locked while read_bytes_pending > 0. Once iomap_finish_folio_read() decrements read_bytes_pending, another in-flight read can complete and end the read on the folio, which unlocks it. This allows truncate logic to run and detach the folio (set folio->mapping to NULL). The error reporting path then can dereference a NULL folio->mapping. As reported by Sam Sun, this is the race that can occur: CPU0: failed completion CPU1: final completion CPU2: truncate ----------------------- ---------------------- -------------- read_bytes_pending -= len finished = false /* preempted before fserror_report_io() */ read_bytes_pending -= len finished = true folio_end_read() truncate clears folio->mapping fserror_report_io( folio->mapping->host, ...) ^ NULL deref Fix this by reporting the error first before decrementing ifs->read_bytes_pending. Fixes: a9d573ee88af ("iomap: report file I/O errors to the VFS") Cc: stable@vger.kernel.org Reported-by: Sam Sun Closes: https://lore.kernel.org/linux-fsdevel/CAEkJfYPhWdd59RKmuNLJg-bkypHz7xiOwaWyNVu3A8CUqQCnvg@mail.gmail.com/ Signed-off-by: Joanne Koong --- fs/iomap/buffered-io.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c index d7b648421a70..d55b936e6986 100644 --- a/fs/iomap/buffered-io.c +++ b/fs/iomap/buffered-io.c @@ -400,6 +400,11 @@ void iomap_finish_folio_read(struct folio *folio, size_t off, size_t len, bool uptodate = !error; bool finished = true; + if (error) + fserror_report_io(folio->mapping->host, FSERR_BUFFERED_READ, + folio_pos(folio) + off, len, error, + GFP_ATOMIC); + if (ifs) { unsigned long flags; @@ -411,11 +416,6 @@ void iomap_finish_folio_read(struct folio *folio, size_t off, size_t len, spin_unlock_irqrestore(&ifs->state_lock, flags); } - if (error) - fserror_report_io(folio->mapping->host, FSERR_BUFFERED_READ, - folio_pos(folio) + off, len, error, - GFP_ATOMIC); - if (finished) folio_end_read(folio, uptodate); } -- 2.52.0