[patch 1/1] block: restore the iterator when rejecting a misaligned bio

[Andrew Morton <akpm@linux-foundation.org>]

From: Andrew Morton <akpm@linux-foundation.org>
Subject: block: restore the iterator when rejecting a misaligned bio
Date: Sun, 18 Oct 2026 17:30:00 -0700

bio_iov_iter_get_pages() can extract some iovecs before encountering a
misaligned one.  It releases the pages and rejects the entire bio, but
leaves the supplied iterator advanced.

An iomap direct-I/O caller may subsequently reexpand the iterator's
byte count and fall back to buffered I/O.  The reexpanded count then
includes bytes before the iterator's current segment, potentially
causing iterate_iovec() to read beyond the imported iovec array.

In a reproducer using an initial empty iovec followed by 10-byte and
1015-byte segments, writev() returned 1015 instead of 1025 and KASAN
reported an 8-byte read beyond the nine-element iovec allocation.
Saving and restoring the iterator on the rejected-bio path makes
writev() return 1025 and eliminates the KASAN report.

Restore the entire iterator rather than reverting a byte count:
advancing past an empty segment changes its position without consuming
any bytes.

Leave successful and partially accepted bio paths unchanged.

Fixes: 14b007e17881 ("block: validate user space vectors during extraction")
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Haocheng Yu <l1za0.sec@gmail.com>
Closes: https://lore.kernel.org/20261018081203.2431035-1-l1za0.sec@gmail.com
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Keith Busch <kbusch@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Tal Zussman <tz2294@columbia.edu>
Cc: Daehyeon Ko <4ncienth@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Hannes Reinecke <hare@kernel.org>
Cc: <stable@vger.kernel.org>
---
Testing on x86-64 QEMU/KVM with generic KASAN:
- Before: the ext4 writev() reproducer returned 1015 instead of 1025
  and triggered an 8-byte slab out-of-bounds read.
- After: 100 consecutive reproducer runs returned 1025 without KASAN;
  60-second buffered and direct ext4 fio runs passed.
- Daehyeon's alignment patch independently avoids the reproducer.

Tal Zussman's pending atomic-write changes introduce another rejected-bio
cleanup path. If both patches are combined, restore the iterator there too.
 block/bio.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/block/bio.c~block-restore-the-iterator-when-rejecting-a-misaligned-bio
+++ a/block/bio.c
@@ -1285,6 +1285,7 @@ int bio_iov_iter_get_pages(struct bio *b
 			   unsigned mem_align_mask, unsigned len_align_mask)
 {
 	iov_iter_extraction_t flags = 0;
+	struct iov_iter saved_iter;
 
 	if (WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED)))
 		return -EIO;
@@ -1303,6 +1304,7 @@ int bio_iov_iter_get_pages(struct bio *b
 	if (bio->bi_bdev && blk_queue_pci_p2pdma(bio->bi_bdev->bd_disk->queue))
 		flags |= ITER_ALLOW_P2PDMA;
 
+	saved_iter = *iter;
 	do {
 		ssize_t ret;
 
@@ -1320,6 +1322,7 @@ int bio_iov_iter_get_pages(struct bio *b
 				bio_release_pages(bio, false);
 				bio_clear_flag(bio, BIO_PAGE_PINNED);
 				bio->bi_vcnt = 0;
+				*iter = saved_iter;
 			}
 			if (!bio->bi_vcnt)
 				return ret;
_