nvme: identify-namespace without CAP_SYS_ADMIN

Allow all identify-namespace variants (CNS 00h, 05h and 08h) without
requiring CAP_SYS_ADMIN. The information (retrieved using id-ns) is
needed to form IO commands for passthrough interface.
Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/host/ioctl.c

@@ -22,9 +22,23 @@ static bool nvme_cmd_allowed(struct nvme_ns *ns, struct nvme_command *c,
 	    c->common.opcode == nvme_fabrics_command)
 		return false;
 
-	/* do not allow unprivileged admin commands */
-	if (!ns)
+	/*
+	 * Do not allow unprivileged passthrough of admin commands except
+	 * for a subset of identify commands that contain information required
+	 * to form proper I/O commands in userspace and do not expose any
+	 * potentially sensitive information.
+	 */
+	if (!ns) {
+		if (c->common.opcode == nvme_admin_identify) {
+			switch (c->identify.cns) {
+			case NVME_ID_CNS_NS:
+			case NVME_ID_CNS_CS_NS:
+			case NVME_ID_CNS_NS_CS_INDEP:
+				return true;
+			}
+		}
 		return false;
+	}
 
 	/*
 	 * Only allow I/O commands that transfer data to the controller if the