[PATCH v4 5/8] block: fail a short atomic pin in bio_iov_iter_get_pages()

[Tal Zussman <tz2294@columbia.edu>]

On a partial page pin, __blkdev_direct_IO_simple() and
__blkdev_direct_IO_async() submit what was pinned with REQ_ATOMIC set
and leave the rest to the buffered fallback, tearing an IOCB_ATOMIC
write.

This can be triggered deterministically. A 16K pwritev2(RWF_ATOMIC)
whose last page is PROT_NONE, on a scsi_debug device with atomic_wr=1,
completes short with only three of the four pages written, violating
RWF_ATOMIC semantics.

Make bio_iov_iter_get_pages() release the pins and return -EINVAL when
a REQ_ATOMIC bio doesn't cover the whole iterator, since an atomic
write is submitted as a single bio and a short one would be torn. That
covers iomap as well, where a partially unmapped buffer could trip the
WARN_ON_ONCE() in iomap_dio_bio_iter_one(). The async block device path
currently sets REQ_ATOMIC after pinning, so set it before, and move
REQ_NOWAIT along with it.

Fixes: caf336f81b3a ("block: Add fops atomic write support")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Link: https://sashiko.dev/#/patchset/20260802-blkdev-fixes-v1-0-a82fc549fd74%40columbia.edu?part=2
Assisted-by: Claude:claude-fable-5
Signed-off-by: Tal Zussman <tz2294@columbia.edu>
---
 block/bio.c  | 29 ++++++++++++++++++++++-------
 block/fops.c | 12 ++++++------
 2 files changed, 28 insertions(+), 13 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index f95b63c0604a..14429a5d4e68 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1285,6 +1285,7 @@ int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter,
 			   unsigned mem_align_mask, unsigned len_align_mask)
 {
 	iov_iter_extraction_t flags = 0;
+	int ret;
 
 	if (WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED)))
 		return -EIO;
@@ -1304,34 +1305,48 @@ int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter,
 		flags |= ITER_ALLOW_P2PDMA;
 
 	do {
-		ssize_t ret;
+		ssize_t size;
 
-		ret = iov_iter_extract_bvecs(iter, bio->bi_io_vec,
+		size = iov_iter_extract_bvecs(iter, bio->bi_io_vec,
 				BIO_MAX_SIZE - bio->bi_iter.bi_size,
 				&bio->bi_vcnt, bio->bi_max_vecs,
 				mem_align_mask, flags);
-		if (ret <= 0) {
+		if (size <= 0) {
 			/*
 			 * A misaligned vector fails the whole I/O.  Release any
 			 * pages pinned by earlier iterations before returning
 			 * since this bio won't be submitted to release them.
 			 */
-			if (ret == -EINVAL) {
+			if (size == -EINVAL) {
 				bio_release_pages(bio, false);
 				bio_clear_flag(bio, BIO_PAGE_PINNED);
 				bio->bi_vcnt = 0;
 			}
 			if (!bio->bi_vcnt)
-				return ret;
+				return size;
 			break;
 		}
-		bio->bi_iter.bi_size += ret;
+		bio->bi_iter.bi_size += size;
 	} while (iov_iter_count(iter) && !bio_full(bio, 0));
 
 	if (is_pci_p2pdma_page(bio->bi_io_vec->bv_page))
 		bio->bi_opf |= REQ_NOMERGE;
-	return bio_iov_iter_align_down(bio, iter,
+	ret = bio_iov_iter_align_down(bio, iter,
 			&bio->bi_io_vec[bio->bi_vcnt - 1], len_align_mask);
+	if (ret)
+		return ret;
+
+	/*
+	 * An atomic write is submitted as a single bio, so it has to cover
+	 * the whole iterator or it would be torn.
+	 */
+	if ((bio->bi_opf & REQ_ATOMIC) && iov_iter_count(iter)) {
+		bio_release_pages(bio, false);
+		bio_clear_flag(bio, BIO_PAGE_PINNED);
+		bio->bi_vcnt = 0;
+		return -EINVAL;
+	}
+	return 0;
 }
 
 static struct folio *folio_alloc_greedy(gfp_t gfp, size_t *size,
diff --git a/block/fops.c b/block/fops.c
index a3a709697b40..90777e8a9a6c 100644
--- a/block/fops.c
+++ b/block/fops.c
@@ -342,6 +342,12 @@ static ssize_t __blkdev_direct_IO_async(struct kiocb *iocb,
 	bio->bi_end_io = blkdev_bio_end_io_async;
 	bio->bi_ioprio = iocb->ki_ioprio;
 
+	if (iocb->ki_flags & IOCB_ATOMIC)
+		bio->bi_opf |= REQ_ATOMIC;
+
+	if (iocb->ki_flags & IOCB_NOWAIT)
+		bio->bi_opf |= REQ_NOWAIT;
+
 	/*
 	 * Users don't rely on the iterator being in any particular
 	 * state for async I/O returning -EIOCBQUEUED, hence we can
@@ -371,12 +377,6 @@ static ssize_t __blkdev_direct_IO_async(struct kiocb *iocb,
 			goto out_bio_put;
 	}
 
-	if (iocb->ki_flags & IOCB_ATOMIC)
-		bio->bi_opf |= REQ_ATOMIC;
-
-	if (iocb->ki_flags & IOCB_NOWAIT)
-		bio->bi_opf |= REQ_NOWAIT;
-
 	if (iocb->ki_flags & IOCB_HIPRI) {
 		bio->bi_opf |= REQ_POLLED;
 		submit_bio(bio);

-- 
2.39.5