[PATCH 0/1] direct-io: validate user space vectors during extraction

[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 various 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 patch 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 submission
path.

With this in place, the dm side changes under discussion are no longer
required to fix the bugs: the affected targets simply never see the
invalid bio. The tested patches remain reasonable as defense-in-depth if
desired, but they are not strictly necessary after this.

Keith Busch (1):
  block: validate user space vectors during extraction

 block/bio.c          | 19 ++++++++++++++++---
 block/blk-map.c      |  2 +-
 block/fops.c         |  3 ++-
 fs/iomap/direct-io.c |  3 ++-
 include/linux/bio.h  |  2 +-
 include/linux/uio.h  |  3 ++-
 lib/iov_iter.c       |  9 ++++++++-
 7 files changed, 32 insertions(+), 9 deletions(-)

-- 
2.52.0