FAILED: patch "[PATCH] ubi: Fix race condition between ctrl_cdev_ioctl and" failed to apply to 4.9-stable tree

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 3cbf0e392f173ba0ce425968c8374a6aa3e90f2e Mon Sep 17 00:00:00 2001
From: Baokun Li <[email protected]>
Date: Fri, 5 Nov 2021 17:30:22 +0800
Subject: [PATCH] ubi: Fix race condition between ctrl_cdev_ioctl and
ubi_cdev_ioctl

Hulk Robot reported a KASAN report about use-after-free:
==================================================================

 BUG: KASAN: use-after-free in __list_del_entry_valid+0x13d/0x160
 Read of size 8 at addr ffff888035e37d98 by task ubiattach/1385
 [...]
 Call Trace:
  klist_dec_and_del+0xa7/0x4a0
  klist_put+0xc7/0x1a0
  device_del+0x4d4/0xed0
  cdev_device_del+0x1a/0x80
  ubi_attach_mtd_dev+0x2951/0x34b0 [ubi]
  ctrl_cdev_ioctl+0x286/0x2f0 [ubi]

 Allocated by task 1414:
  device_add+0x60a/0x18b0
  cdev_device_add+0x103/0x170
  ubi_create_volume+0x1118/0x1a10 [ubi]
  ubi_cdev_ioctl+0xb7f/0x1ba0 [ubi]

 Freed by task 1385:
  cdev_device_del+0x1a/0x80
  ubi_remove_volume+0x438/0x6c0 [ubi]
  ubi_cdev_ioctl+0xbf4/0x1ba0 [ubi]
 [...]
 ==================================================================

The lock held by ctrl_cdev_ioctl is ubi_devices_mutex, but the lock held by ubi_cdev_ioctl is ubi->device_mutex. Therefore, the two locks can be concurrent.

ctrl_cdev_ioctl contains two operations: ubi_attach and ubi_detach. ubi_detach is bug-free because it uses reference counting to prevent concurrency. However, uif_init and uif_close in ubi_attach may race with ubi_cdev_ioctl.

uif_init will race with ubi_cdev_ioctl as in the following stack.
cpu1 cpu2 cpu3
_______________________|________________________|______________________ ctrl_cdev_ioctl
ubi_attach_mtd_dev
uif_init
ubi_cdev_ioctl
ubi_create_volume
cdev_device_add
ubi_add_volume
// sysfs exist
kill_volumes

                                                    ubi_cdev_ioctl
                                                     ubi_remove_volume
                                                      cdev_device_del
                                                       // first free
    ubi_free_volume
     cdev_del
     // double free

cdev_device_del

And uif_close will race with ubi_cdev_ioctl as in the following stack.
cpu1 cpu2 cpu3
_______________________|________________________|______________________ ctrl_cdev_ioctl
ubi_attach_mtd_dev
uif_init
ubi_cdev_ioctl
ubi_create_volume
cdev_device_add
ubi_debugfs_init_dev
//error goto out_uif;
uif_close
kill_volumes

                                                    ubi_cdev_ioctl
                                                     ubi_remove_volume
                                                      cdev_device_del
                                                       // first free
    ubi_free_volume
    // double free

The cause of this problem is that commit 714fb87e8bc0 make device "available" before it becomes accessible via sysfs. Therefore, we roll back the modification. We will fix the race condition between ubi device creation and udev by removing ubi_get_device in vol_attribute_show and dev_attribute_show.This avoids accessing uninitialized ubi_devices[ubi_num].

ubi_get_device is used to prevent devices from being deleted during sysfs execution. However, now kernfs ensures that devices will not be deleted before all reference counting are released. The key process is shown in the following stack.

device_del
device_remove_attrs
device_remove_groups
sysfs_remove_groups
sysfs_remove_group
remove_files
kernfs_remove_by_name
kernfs_remove_by_name_ns
__kernfs_remove
kernfs_drain

index a7e3eb9befb6..a32050fecabf 100644
@@ -351,9 +351,6 @@ static ssize_t dev_attribute_show(struct device *dev,
 	 * we still can use 'ubi->ubi_num'.
 	 */
 	ubi = container_of(dev, struct ubi_device, dev);
-	ubi = ubi_get_device(ubi->ubi_num);
-	if (!ubi)
-		return -ENODEV;
 
 	if (attr == &dev_eraseblock_size)
 		ret = sprintf(buf, "%d\n", ubi->leb_size);

@@ -382,7 +379,6 @@ static ssize_t dev_attribute_show(struct device *dev,
 	else
 		ret = -EINVAL;
 
-	ubi_put_device(ubi);
 	return ret;
 }
 

@@ -979,9 +975,6 @@ int ubi_attach_mtd_dev(struct mtd_info *mtd, int ubi_num,
 			goto out_detach;
 	}
 
-	/* Make device "available" before it becomes accessible via sysfs */
-	ubi_devices[ubi_num] = ubi;
-
 	err = uif_init(ubi);
 	if (err)
 		goto out_detach;

@@ -1026,6 +1019,7 @@ int ubi_attach_mtd_dev(struct mtd_info *mtd, int ubi_num,
 	wake_up_process(ubi->bgt_thread);
 	spin_unlock(&ubi->wl_lock);
 
+	ubi_devices[ubi_num] = ubi;
 	ubi_notify_all(ubi, UBI_VOLUME_ADDED, NULL);
 	return ubi_num;
 

@@ -1034,7 +1028,6 @@ int ubi_attach_mtd_dev(struct mtd_info *mtd, int ubi_num,
 out_uif:
 	uif_close(ubi);
 out_detach:
-	ubi_devices[ubi_num] = NULL;
 	ubi_wl_close(ubi);
 	ubi_free_all_volumes(ubi);
 	vfree(ubi->vtbl);

index 139ee132bfbc..1bc7b3a05604 100644
@@ -56,16 +56,11 @@ static ssize_t vol_attribute_show(struct device *dev,
 {
 	int ret;
 	struct ubi_volume *vol = container_of(dev, struct ubi_volume, dev);
-	struct ubi_device *ubi;
-
-	ubi = ubi_get_device(vol->ubi->ubi_num);
-	if (!ubi)
-		return -ENODEV;
+	struct ubi_device *ubi = vol->ubi;
 
 	spin_lock(&ubi->volumes_lock);
 	if (!ubi->volumes[vol->vol_id]) {
 		spin_unlock(&ubi->volumes_lock);
-		ubi_put_device(ubi);
 		return -ENODEV;
 	}
 	/* Take a reference to prevent volume removal */

@@ -103,7 +98,6 @@ static ssize_t vol_attribute_show(struct device *dev,
 	vol->ref_count -= 1;
 	ubi_assert(vol->ref_count >= 0);
 	spin_unlock(&ubi->volumes_lock);
-	ubi_put_device(ubi);
 	return ret;
 }