[PATCH] powernv/elog: Fix the race while processing OPAL error log event.

[Mahesh Salgaonkar]

Every error log reported by OPAL is exported to userspace through a sysfs
interface and notified using kobject_uevent(). The userspace daemon
(opal_errd) then reads the error log and acknowledges it error log is saved
safely to disk. Once acknowledged the kernel removes the respective sysfs
file entry causing respective resources getting released including kobject.

However there are chances where user daemon may already be scanning elog
entries while new sysfs elog entry is being created by kernel. User daemon
may read this new entry and ack it even before kernel can notify userspace
about it through kobject_uevent() call. If that happens then we have a
potential race between elog_ack_store->kobject_put() and kobject_uevent
which can lead to use-after-free issue of a kernfs object resulting into a
kernel crash. This patch fixes this race by protecting a sysfs file
creation/notification using mutex lock per elog record.

Reported-by: Oliver O'Halloran <oohall at gmail.com>
Signed-off-by: Mahesh Salgaonkar <mahesh at linux.ibm.com>
---
 arch/powerpc/platforms/powernv/opal-elog.c |   17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/arch/powerpc/platforms/powernv/opal-elog.c b/arch/powerpc/platforms/powernv/opal-elog.c
index 62ef7ad995da..4bba8f968ced 100644
--- a/arch/powerpc/platforms/powernv/opal-elog.c
+++ b/arch/powerpc/platforms/powernv/opal-elog.c
@@ -24,6 +24,7 @@ struct elog_obj {
 	uint64_t type;
 	size_t size;
 	char *buffer;
+	struct mutex elog_mutex;
 };
 #define to_elog_obj(x) container_of(x, struct elog_obj, kobj)
 
@@ -73,7 +74,11 @@ static ssize_t elog_ack_store(struct elog_obj *elog_obj,
 			      size_t count)
 {
 	opal_send_ack_elog(elog_obj->id);
+
+	/* Wait until kobject_uevent() is called */
+	mutex_lock(&elog_obj->elog_mutex);
 	sysfs_remove_file_self(&elog_obj->kobj, &attr->attr);
+	mutex_unlock(&elog_obj->elog_mutex);
 	kobject_put(&elog_obj->kobj);
 	return count;
 }
@@ -193,6 +198,7 @@ static struct elog_obj *create_elog_obj(uint64_t id, size_t size, uint64_t type)
 	kobject_init(&elog->kobj, &elog_ktype);
 
 	sysfs_bin_attr_init(&elog->raw_attr);
+	mutex_init(&elog->elog_mutex);
 
 	elog->raw_attr.attr.name = "raw";
 	elog->raw_attr.attr.mode = 0400;
@@ -222,13 +228,24 @@ static struct elog_obj *create_elog_obj(uint64_t id, size_t size, uint64_t type)
 		return NULL;
 	}
 
+	/*
+	 * As soon as sysfs file for this elog is created/activated there is
+	 * chance opal_errd daemon might read and acknowledge this elog before
+	 * kobject_uevent() is called. If that happens then we have a potential
+	 * race between elog_ack_store->kobject_put() and kobject_uevent which
+	 * leads to use-after-free issue of a kernfs object resulting into
+	 * kernel crash. Take a mutex lock to avoid this race.
+	 */
+	mutex_lock(&elog->elog_mutex);
 	rc = sysfs_create_bin_file(&elog->kobj, &elog->raw_attr);
 	if (rc) {
 		kobject_put(&elog->kobj);
+		mutex_unlock(&elog->elog_mutex);
 		return NULL;
 	}
 
 	kobject_uevent(&elog->kobj, KOBJ_ADD);
+	mutex_unlock(&elog->elog_mutex);
 
 	return elog;
 }