From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Fabian Ebner <f.ebner@proxmox.com>
Date: Wed, 25 May 2022 13:59:39 +0200
Subject: [PATCH] PVE-Backup: avoid segfault issues upon backup-cancel

When canceling a backup in PVE via a signal it's easy to run into a
situation where the job is already failing when the backup_cancel QMP
command comes in. With a bit of unlucky timing on top, it can happen
that job_exit() runs between schedulung of job_cancel_bh() and
execution of job_cancel_bh(). But job_cancel_sync() does not expect
that the job is already finalized (in fact, the job might've been
freed already, but even if it isn't, job_cancel_sync() would try to
deref job->txn which would be NULL at that point).

It is not possible to simply use the job_cancel() (which is advertised
as being async but isn't in all cases) in qmp_backup_cancel() for the
same reason job_cancel_sync() cannot be used. Namely, because it can
invoke job_finish_sync() (which uses AIO_WAIT_WHILE and thus hangs if
called from a coroutine). This happens when there's multiple jobs in
the transaction and job->deferred_to_main_loop is true (is set before
scheduling job_exit()) or if the job was not started yet.

Fix the issue by selecting the job to cancel in job_cancel_bh() itself
using the first job that's not completed yet. This is not necessarily
the first job in the list, because pvebackup_co_complete_stream()
might not yet have removed a completed job when job_cancel_bh() runs.

An alternative would be to continue using only the first job and
checking against JOB_STATUS_CONCLUDED or JOB_STATUS_NULL to decide if
it's still necessary and possible to cancel, but the approach with
using the first non-completed job seemed more robust.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
[FE: adapt for new job lock mechanism replacing AioContext locks]
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
---
 pve-backup.c | 57 ++++++++++++++++++++++++++++++++++------------------
 1 file changed, 38 insertions(+), 19 deletions(-)

diff --git a/pve-backup.c b/pve-backup.c
index 0cf30e1ced..4067018dbe 100644
--- a/pve-backup.c
+++ b/pve-backup.c
@@ -354,12 +354,41 @@ static void pvebackup_complete_cb(void *opaque, int ret)
 
 /*
  * job_cancel(_sync) does not like to be called from coroutines, so defer to
- * main loop processing via a bottom half.
+ * main loop processing via a bottom half. Assumes that caller holds
+ * backup_mutex.
  */
 static void job_cancel_bh(void *opaque) {
     CoCtxData *data = (CoCtxData*)opaque;
-    Job *job = (Job*)data->data;
-    job_cancel_sync(job, true);
+
+    /*
+     * Be careful to pick a valid job to cancel:
+     * 1. job_cancel_sync() does not expect the job to be finalized already.
+     * 2. job_exit() might run between scheduling and running job_cancel_bh()
+     *    and pvebackup_co_complete_stream() might not have removed the job from
+     *    the list yet (in fact, cannot, because it waits for the backup_mutex).
+     * Requiring !job_is_completed() ensures that no finalized job is picked.
+     */
+    GList *bdi = g_list_first(backup_state.di_list);
+    while (bdi) {
+        if (bdi->data) {
+            BlockJob *bj = ((PVEBackupDevInfo *)bdi->data)->job;
+            if (bj) {
+                Job *job = &bj->job;
+                WITH_JOB_LOCK_GUARD() {
+                    if (!job_is_completed_locked(job)) {
+                        job_cancel_sync_locked(job, true);
+                        /*
+                         * It's enough to cancel one job in the transaction, the
+                         * rest will follow automatically.
+                         */
+                        break;
+                    }
+                }
+            }
+        }
+        bdi = g_list_next(bdi);
+    }
+
     aio_co_enter(data->ctx, data->co);
 }
 
@@ -380,22 +409,12 @@ void coroutine_fn qmp_backup_cancel(Error **errp)
         proxmox_backup_abort(backup_state.pbs, "backup canceled");
     }
 
-    /* it's enough to cancel one job in the transaction, the rest will follow
-     * automatically */
-    GList *bdi = g_list_first(backup_state.di_list);
-    BlockJob *cancel_job = bdi && bdi->data ?
-        ((PVEBackupDevInfo *)bdi->data)->job :
-        NULL;
-
-    if (cancel_job) {
-        CoCtxData data = {
-            .ctx = qemu_get_current_aio_context(),
-            .co = qemu_coroutine_self(),
-            .data = &cancel_job->job,
-        };
-        aio_bh_schedule_oneshot(data.ctx, job_cancel_bh, &data);
-        qemu_coroutine_yield();
-    }
+    CoCtxData data = {
+        .ctx = qemu_get_current_aio_context(),
+        .co = qemu_coroutine_self(),
+    };
+    aio_bh_schedule_oneshot(data.ctx, job_cancel_bh, &data);
+    qemu_coroutine_yield();
 
     qemu_co_mutex_unlock(&backup_state.backup_mutex);
 }
