io_uring/zcrx: disallow user selected dmabuf offset and size

zcrx shouldn't be so frivolous about cutting a dmabuf sgtable and taking
a subrange into it, the dmabuf layer might be not expecting that. It
shouldn't be a problem for now, but since the zcrx dmabuf support is new
and there shouldn't be any real users, let's play safe and reject user
provided ranges into dmabufs. Also, it shouldn't be needed as userspace
should size them appropriately.

Fixes: a5c98e9 ("io_uring/zcrx: dmabuf backed zerocopy receive")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/be899f1afed32053eb2e2079d0da241514674aca.1752443579.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: isilence

io_uring/zcrx.c

@@ -76,6 +76,8 @@ static int io_import_dmabuf(struct io_zcrx_ifq *ifq,
 	int dmabuf_fd = area_reg->dmabuf_fd;
 	int i, ret;
 
+	if (off)
+		return -EINVAL;
 	if (WARN_ON_ONCE(!ifq->dev))
 		return -EFAULT;
 	if (!IS_ENABLED(CONFIG_DMA_SHARED_BUFFER))
@@ -106,7 +108,7 @@ static int io_import_dmabuf(struct io_zcrx_ifq *ifq,
 	for_each_sgtable_dma_sg(mem->sgt, sg, i)
 		total_size += sg_dma_len(sg);
 
-	if (total_size < off + len) {
+	if (total_size != len) {
 		ret = -EINVAL;
 		goto err;
 	}