udmabuf: validate ubuf->pagecount

Syzbot has reported GPF in sg_alloc_append_table_from_pages(). The
problem was in ubuf->pages == ZERO_PTR.

ubuf->pagecount is calculated from arguments passed from user-space. If
user creates udmabuf with list.size == 0 then ubuf->pagecount will be
also equal to zero; it causes kmalloc_array() to return ZERO_PTR.

Fix it by validating ubuf->pagecount before passing it to
kmalloc_array().

Fixes: fbb0de79 ("Add udmabuf misc device")
Reported-and-tested-by: syzbot+2c56b725ec547fa9cb29@syzkaller.appspotmail.com
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20211230142649.23022-1-paskripkin@gmail.comSigned-off-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/dma-buf/udmabuf.c

@@ -190,6 +190,10 @@ static long udmabuf_create(struct miscdevice *device,
 		if (ubuf->pagecount > pglimit)
 			goto err;
 	}
+
+	if (!ubuf->pagecount)
+		goto err;
+
 	ubuf->pages = kmalloc_array(ubuf->pagecount, sizeof(*ubuf->pages),
 				    GFP_KERNEL);
 	if (!ubuf->pages) {