IB/uverbs: Fix ordering of ucontext check in ib_uverbs_write

During disassociation the ucontext will become NULL, however due to how
the SRCU locking works the ucontext must only be examined after looking
at the ib_dev, which governs the RCU control flow.

With the wrong ordering userspace will see EINVAL instead of EIO for a
disassociated uverbs FD, which breaks rdma-core.

Cc: stable@vger.kernel.org
Fixes: 491d5c6 ("RDMA/uverbs: Move uncontext check before SRCU read lock")
Reported-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>

drivers/infiniband/core/uverbs_main.c

@@ -736,10 +736,6 @@ static ssize_t ib_uverbs_write(struct file *filp, const char __user *buf,
 	if (ret)
 		return ret;
 
-	if (!file->ucontext &&
-	    (command != IB_USER_VERBS_CMD_GET_CONTEXT || extended))
-		return -EINVAL;
-
 	if (extended) {
 		if (count < (sizeof(hdr) + sizeof(ex_hdr)))
 			return -EINVAL;
@@ -759,6 +755,16 @@ static ssize_t ib_uverbs_write(struct file *filp, const char __user *buf,
 		goto out;
 	}
 
+	/*
+	 * Must be after the ib_dev check, as once the RCU clears ib_dev ==
+	 * NULL means ucontext == NULL
+	 */
+	if (!file->ucontext &&
+	    (command != IB_USER_VERBS_CMD_GET_CONTEXT || extended)) {
+		ret = -EINVAL;
+		goto out;
+	}
+
 	if (!verify_command_mask(ib_dev, command, extended)) {
 		ret = -EOPNOTSUPP;
 		goto out;