scsi: core: Fix a scsi_show_rq() NULL pointer dereference

Avoid that scsi_show_rq() triggers a NULL pointer dereference if called
after sd_uninit_command(). Swap the NULL pointer assignment and the
mempool_free() call in sd_uninit_command() to make it less likely that
scsi_show_rq() triggers a use-after-free. Note: even with these changes
scsi_show_rq() can trigger a use-after-free but that's a lesser evil
than e.g. suppressing debug information for T10 PI Type 2 commands
completely. This patch fixes the following oops:

BUG: unable to handle kernel NULL pointer dereference at (null)
IP: scsi_format_opcode_name+0x1a/0x1c0
CPU: 1 PID: 1881 Comm: cat Not tainted 4.14.0-rc2.blk_mq_io_hang+ #516
Call Trace:
 __scsi_format_command+0x27/0xc0
 scsi_show_rq+0x5c/0xc0
 __blk_mq_debugfs_rq_show+0x116/0x130
 blk_mq_debugfs_rq_show+0xe/0x10
 seq_read+0xfe/0x3b0
 full_proxy_read+0x54/0x90
 __vfs_read+0x37/0x160
 vfs_read+0x96/0x130
 SyS_read+0x55/0xc0
 entry_SYSCALL_64_fastpath+0x1a/0xa5

[mkp: added Type 2]

Fixes: 0eebd005 ("scsi: Implement blk_mq_ops.show_rq()")
Reported-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: James E.J. Bottomley <jejb@linux.vnet.ibm.com>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Cc: stable@vger.kernel.org
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/scsi_debugfs.c

@@ -8,9 +8,11 @@ void scsi_show_rq(struct seq_file *m, struct request *rq)
 {
 	struct scsi_cmnd *cmd = container_of(scsi_req(rq), typeof(*cmd), req);
 	int msecs = jiffies_to_msecs(jiffies - cmd->jiffies_at_alloc);
-	char buf[80];
+	const u8 *const cdb = READ_ONCE(cmd->cmnd);
+	char buf[80] = "(?)";
 
-	__scsi_format_command(buf, sizeof(buf), cmd->cmnd, cmd->cmd_len);
+	if (cdb)
+		__scsi_format_command(buf, sizeof(buf), cdb, cmd->cmd_len);
 	seq_printf(m, ", .cmd=%s, .retries=%d, allocated %d.%03d s ago", buf,
 		   cmd->retries, msecs / 1000, msecs % 1000);
 }

drivers/scsi/sd.c

@@ -1312,6 +1312,7 @@ static int sd_init_command(struct scsi_cmnd *cmd)
 static void sd_uninit_command(struct scsi_cmnd *SCpnt)
 {
 	struct request *rq = SCpnt->request;
+	u8 *cmnd;
 
 	if (SCpnt->flags & SCMD_ZONE_WRITE_LOCK)
 		sd_zbc_write_unlock_zone(SCpnt);
@@ -1320,9 +1321,10 @@ static void sd_uninit_command(struct scsi_cmnd *SCpnt)
 		__free_page(rq->special_vec.bv_page);
 
 	if (SCpnt->cmnd != scsi_req(rq)->cmd) {
-		mempool_free(SCpnt->cmnd, sd_cdb_pool);
+		cmnd = SCpnt->cmnd;
 		SCpnt->cmnd = NULL;
 		SCpnt->cmd_len = 0;
+		mempool_free(cmnd, sd_cdb_pool);
 	}
 }