[PATCH] tools/xenstored: Don't crash xenstored when Live-Update is cancelled

[Julien Grall <julien@xxxxxxx>]

From: Julien GralL <jgrall@xxxxxxxxxx>

As Live-Update is asynchronous, it is possible to receive a request to
cancel it (either on the same connection or from a different one).

Currently, this will crash xenstored because do_lu_start() assumes
lu_status will be valid. This is not the case when Live-Update has been
cancelled. This will result to dereference a NULL pointer and
crash Xenstored.

Rework do_lu_start() to check if lu_status is NULL and return an
error in this case.

Fixes: af216a99fb ("tools/xenstore: add the basic framework for doing the live 
update")
Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>

----

This is currently based on top of:

https://lore.kernel.org/xen-devel/20210616144324.31652-1-julien@xxxxxxx

This can be re-ordered if necessary.
---
 tools/xenstore/xenstored_control.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/tools/xenstore/xenstored_control.c 
b/tools/xenstore/xenstored_control.c
index a045f102a420..37a3d39f20b5 100644
--- a/tools/xenstore/xenstored_control.c
+++ b/tools/xenstore/xenstored_control.c
@@ -696,7 +696,18 @@ static bool do_lu_start(struct delayed_request *req)
        time_t now = time(NULL);
        const char *ret;
        struct buffered_data *saved_in;
-       struct connection *conn = lu_status->conn;
+       struct connection *conn = req->data;
+
+       /*
+        * Cancellation may have been requested asynchronously. In this
+        * case, lu_status will be NULL.
+        */
+       if (!lu_status) {
+               ret = "Cancellation was requested";
+               conn = req->data;
+               goto out;
+       } else
+               assert(lu_status->conn == conn);
 
        if (!lu_check_lu_allowed()) {
                if (now < lu_status->started_at + lu_status->timeout)
@@ -747,7 +758,7 @@ static const char *lu_start(const void *ctx, struct 
connection *conn,
        lu_status->timeout = to;
        lu_status->started_at = time(NULL);
 
-       errno = delay_request(conn, conn->in, do_lu_start, NULL, false);
+       errno = delay_request(conn, conn->in, do_lu_start, conn, false);
 
        return NULL;
 }
-- 
2.17.1