ublk: use unchecked copy helpers for bio page data

Bio pages may originate from slab caches that lack a usercopy region
(e.g. jbd2 frozen metadata buffers allocated via jbd2_alloc()).
When CONFIG_HARDENED_USERCOPY is enabled, copy_to_iter() calls
check_copy_size() which rejects these slab pages, triggering a
kernel BUG in usercopy_abort().

This is a false positive: the data is ordinary block I/O content —
the same data the loop/nbd driver writes to its backing file via
vfs_iter_write().  The bvec length is always trusted, so the size
check in check_copy_size() is not needed either.

Switch to _copy_to_iter()/_copy_from_iter() which skip the
check_copy_size() wrapper while the underlying copy_to_user()
remains unchanged.

Fixes: 2299cee ("ublk: use copy_{to,from}_iter() for user copy")
Signed-off-by: Ming Lei <[email protected]>

Author: Ming Lei

drivers/block/ublk_drv.c

@@ -1281,10 +1281,18 @@ static bool ublk_copy_user_bvec(const struct bio_vec *bv, unsigned *offset,
 
 	len = bv->bv_len - *offset;
 	bv_buf = kmap_local_page(bv->bv_page) + bv->bv_offset + *offset;
+	/*
+	 * Bio pages may originate from slab caches without a usercopy region
+	 * (e.g. jbd2 frozen metadata buffers).  This is the same data that
+	 * the loop driver writes to its backing file — no exposure risk.
+	 * The bvec length is always trusted, so the size check in
+	 * check_copy_size() is not needed either.  Use the unchecked
+	 * helpers to avoid false positives on slab pages.
+	 */
 	if (dir == ITER_DEST)
-		copied = copy_to_iter(bv_buf, len, uiter);
+		copied = _copy_to_iter(bv_buf, len, uiter);
 	else
-		copied = copy_from_iter(bv_buf, len, uiter);
+		copied = _copy_from_iter(bv_buf, len, uiter);
 
 	kunmap_local(bv_buf);