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 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.

Acked-by: Caleb Sander Mateos <csander@purestorage.com>
Fixes: 2299ceec364e ("ublk: use copy_{to,from}_iter() for user copy")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://patch.msgid.link/20260415230246.808176-1-tom.leiming@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
index 603a98a30989f72653e3aaaa2779812fff22e143..ef8a0705e68b515e5564bc06755a3f6e92f271d5 100644 (file)
--- a/drivers/block/ublk_drv.c
+++ b/drivers/block/ublk_drv.c
@@ -1319,10 +1319,18 @@ 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);