[PATCH v3 0/5] block: validate direct I/O memory alignment

[Keith Busch <kbusch@meta.com>]

From: Keith Busch <kbusch@kernel.org>

This addresses the misaligned direct-io problem behind various threads:

 https://lore.kernel.org/linux-xfs/20260610145218.141369-1-cem@kernel.org/
 https://lore.kernel.org/all/CAC_j7i1R7oy+nRhxEjCTba=DUgn02w9X+p94DCu0aHv5+5tKnQ@mail.gmail.com/
 https://lore.kernel.org/linux-block/ai7rnH20IYeSmY8s@gallifrey/
 https://lore.kernel.org/linux-block/20260616154009.2123183-1-kbusch@meta.com/

The previously tested fixes are correct as far as they go, but they
treat the symptom: they only matter because an invalid bio reaches those
drivers in the first place.

The reason it reaches them is an assumption I made when I removed
direct-io alignment checks in 5ff3f74e145a ("block: simplify direct io
validity check") and 7eac331869575 ("iomap: simplify direct io validity
check"): every bio is eventually split to the device limits, and the
upper layers cope with resulting errors once the bio has formed. Both
were optimistic assumptions. Drivers with their own ->submit_bio may
never pass through blk_mq_submit_bio()'s split, so the check never runs
for them, and as numerous threads showed, the consumers don't uniformly
handle this condition.

This series stops the invalid bio at the source instead. It validates
the buffer's alignment against the alignment limits when the bio is
built from the iov_iter. The check is folded into the bvec extraction
that already walks the vectors, so it adds only a comparison on a path
that is pinning direct-io pages anyway. Misalignment is now uniformly
rejected with EINVAL before submission for every direct-io path.

v2->v3:

- Dropped the bio_endio_errno helper and open-coded its two users.
- Documented the ITER_BVEC alignment expectation in uio.h and reworded
  the bvec check comment; the exhaustive per-segment validation stays
  behind CONFIG_DEBUG_KERNEL as a contract assertion.
- Reworked zloop_get_block_size() to mirror loop's structure.
- loop/zloop only ever tighten dma_alignment beyond the default.  I
  think these could use more relaxed alignments, but I'm just being
  extra conservative against introducing new changes here.

Previous version:

  https://lore.kernel.org/linux-block/20260622174241.2299563-1-kbusch@meta.com/

Keith Busch (5):
  block: use blkdev_iov_iter_get_pages status for errors
  block: fix dio leak on metadata mapping error
  loop: set dma_alignment from the backing file for direct I/O
  zloop: set dma_alignment from the backing files for direct I/O
  block: validate user space vectors during extraction

 block/bio.c           | 56 ++++++++++++++++++++++++++++++++++++++++---
 block/blk-map.c       |  2 +-
 block/fops.c          | 10 ++++----
 drivers/block/loop.c  | 46 ++++++++++++++++++++++++++++-------
 drivers/block/zloop.c | 35 +++++++++++++++++++--------
 fs/iomap/direct-io.c  |  1 +
 include/linux/bio.h   |  2 +-
 include/linux/uio.h   | 10 +++++++-
 lib/iov_iter.c        |  9 ++++++-
 9 files changed, 142 insertions(+), 29 deletions(-)


base-commit: 5c7804e3279c9bdc36e5eac743b4000633b25f65
-- 
2.53.0-Meta