[PATCH v2] iov_iter: validate extracted segment for bvec alignment

[Daehyeon Ko <4ncienth@gmail.com>]

iov_iter_extract_bvecs() validates iter_iov_addr() and iter_iov_len()
before iov_iter_extract_pages() selects the first nonempty segment and
limits it by iter->count.  The checks can therefore cover a different range
after a leading empty segment or iov_iter_truncate().

An unprivileged O_DIRECT preadv() reproduced the empty-segment case: an
aligned empty segment followed by a misaligned one completed 4096 bytes
instead of returning -EINVAL.  A truncated iterator has the same mismatch
when its full segment length is aligned but its remaining count is not.

Bound the normal fast-path length by iter->count.  Keep
first_iovec_segment() for leading empty segments so validation covers the
remaining portion of the segment available for extraction.

Closes: https://lore.kernel.org/r/20261012065520.1089147-1-4ncienth@gmail.com
Fixes: 14b007e17881 ("block: validate user space vectors during extraction")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
---
Changes in v2:
- Bound the normal fast-path length by iter->count while retaining
  first_iovec_segment() for leading empty segments.
- Cover the truncated-iterator case raised by Andrew Morton and Sashiko.

v1: https://lore.kernel.org/r/20261012065520.1089147-1-4ncienth@gmail.com

 lib/iov_iter.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 2072c04e99d052..d21082fb251bf7 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1022,7 +1022,7 @@ static ssize_t iter_xarray_get_pages(struct iov_iter *i,
 	return maxsize;
 }
 
-/* must be done on non-empty ITER_UBUF or ITER_IOVEC one */
+/* must be used on a non-empty ITER_UBUF, ITER_IOVEC or ITER_KVEC */
 static unsigned long first_iovec_segment(const struct iov_iter *i, size_t *size)
 {
 	size_t skip;
@@ -1938,8 +1938,13 @@ ssize_t iov_iter_extract_bvecs(struct iov_iter *iter, struct bio_vec *bv,
 		   iter_is_iovec(iter) ||
 		   iov_iter_is_kvec(iter))) {
 		unsigned long start = (unsigned long)iter_iov_addr(iter);
+		size_t seg_size = min(iter_iov_len(iter), iter->count);
 
-		if ((start | iter_iov_len(iter)) & mem_align_mask)
+		if (unlikely(!seg_size && iter->count)) {
+			seg_size = iter->count;
+			start = first_iovec_segment(iter, &seg_size);
+		}
+		if ((start | seg_size) & mem_align_mask)
 			return -EINVAL;
 	} else if (iov_iter_alignment(iter) & mem_align_mask) {
 		return -EINVAL;

base-commit: 72d3fcf802c45d00b300f25b848a93c3a2bd7c7e
-- 
2.55.0