From: SeongJae Park sjpark@amazon.de
mainline inclusion from mainline-v5.11-rc1 commit fed1755b118147721f2c87b37b9d66e62c39b668 category: bugfix bugzilla: NA CVE: CVE-2020-29568
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If handling logics of watch events are slower than the events enqueue logic and the events can be created from the guests, the guests could trigger memory pressure by intensively inducing the events, because it will create a huge number of pending events that exhausting the memory.
Fortunately, some watch events could be ignored, depending on its handler callback. For example, if the callback has interest in only one single path, the watch wouldn't want multiple pending events. Or, some watches could ignore events to same path.
To let such watches to volutarily help avoiding the memory pressure situation, this commit introduces new watch callback, 'will_handle'. If it is not NULL, it will be called for each new event just before enqueuing it. Then, if the callback returns false, the event will be discarded. No watch is using the callback for now, though.
This is part of XSA-349
Cc: stable@vger.kernel.org Signed-off-by: SeongJae Park sjpark@amazon.de Reported-by: Michael Kurth mku@amazon.de Reported-by: Pawel Wieczorkiewicz wipawel@amazon.de Reviewed-by: Juergen Gross jgross@suse.com Signed-off-by: Juergen Gross jgross@suse.com Signed-off-by: Yang Yingliang yangyingliang@huawei.com Reviewed-by: Jason Yan yanaijie@huawei.com Signed-off-by: Yang Yingliang yangyingliang@huawei.com --- drivers/net/xen-netback/xenbus.c | 4 ++++ drivers/xen/xenbus/xenbus_client.c | 1 + drivers/xen/xenbus/xenbus_xs.c | 5 ++++- include/xen/xenbus.h | 7 +++++++ 4 files changed, 16 insertions(+), 1 deletion(-)
diff --git a/drivers/net/xen-netback/xenbus.c b/drivers/net/xen-netback/xenbus.c index cd51492ae6c2..14273a431d99 100644 --- a/drivers/net/xen-netback/xenbus.c +++ b/drivers/net/xen-netback/xenbus.c @@ -777,12 +777,14 @@ static int xen_register_credit_watch(struct xenbus_device *dev, return -ENOMEM; snprintf(node, maxlen, "%s/rate", dev->nodename); vif->credit_watch.node = node; + vif->credit_watch.will_handle = NULL; vif->credit_watch.callback = xen_net_rate_changed; err = register_xenbus_watch(&vif->credit_watch); if (err) { pr_err("Failed to set watcher %s\n", vif->credit_watch.node); kfree(node); vif->credit_watch.node = NULL; + vif->credit_watch.will_handle = NULL; vif->credit_watch.callback = NULL; } return err; @@ -829,6 +831,7 @@ static int xen_register_mcast_ctrl_watch(struct xenbus_device *dev, snprintf(node, maxlen, "%s/request-multicast-control", dev->otherend); vif->mcast_ctrl_watch.node = node; + vif->mcast_ctrl_watch.will_handle = NULL; vif->mcast_ctrl_watch.callback = xen_mcast_ctrl_changed; err = register_xenbus_watch(&vif->mcast_ctrl_watch); if (err) { @@ -836,6 +839,7 @@ static int xen_register_mcast_ctrl_watch(struct xenbus_device *dev, vif->mcast_ctrl_watch.node); kfree(node); vif->mcast_ctrl_watch.node = NULL; + vif->mcast_ctrl_watch.will_handle = NULL; vif->mcast_ctrl_watch.callback = NULL; } return err; diff --git a/drivers/xen/xenbus/xenbus_client.c b/drivers/xen/xenbus/xenbus_client.c index f7b553faadb1..5a8bd3baa6e5 100644 --- a/drivers/xen/xenbus/xenbus_client.c +++ b/drivers/xen/xenbus/xenbus_client.c @@ -120,6 +120,7 @@ int xenbus_watch_path(struct xenbus_device *dev, const char *path, int err;
watch->node = path; + watch->will_handle = NULL; watch->callback = callback;
err = register_xenbus_watch(watch); diff --git a/drivers/xen/xenbus/xenbus_xs.c b/drivers/xen/xenbus/xenbus_xs.c index 3a06eb699f33..e8bdbd0a1e26 100644 --- a/drivers/xen/xenbus/xenbus_xs.c +++ b/drivers/xen/xenbus/xenbus_xs.c @@ -705,7 +705,10 @@ int xs_watch_msg(struct xs_watch_event *event)
spin_lock(&watches_lock); event->handle = find_watch(event->token); - if (event->handle != NULL) { + if (event->handle != NULL && + (!event->handle->will_handle || + event->handle->will_handle(event->handle, + event->path, event->token))) { spin_lock(&watch_events_lock); list_add_tail(&event->list, &watch_events); wake_up(&watch_events_waitq); diff --git a/include/xen/xenbus.h b/include/xen/xenbus.h index 869c816d5f8c..55f543fe0bd8 100644 --- a/include/xen/xenbus.h +++ b/include/xen/xenbus.h @@ -59,6 +59,13 @@ struct xenbus_watch /* Path being watched. */ const char *node;
+ /* + * Called just before enqueing new event while a spinlock is held. + * The event will be discarded if this callback returns false. + */ + bool (*will_handle)(struct xenbus_watch *, + const char *path, const char *token); + /* Callback (executed in a process context with no locks held). */ void (*callback)(struct xenbus_watch *, const char *path, const char *token);