Patch "block: fix del_gendisk() vs blkdev_ioctl crash" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

block: fix del_gendisk() vs blkdev_ioctl crash

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
block-fix-del_gendisk-vs-blkdev_ioctl-crash.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Dan Williams <[email protected]>
Date: Tue, 29 Dec 2015 14:02:29 -0800
Subject: block: fix del_gendisk() vs blkdev_ioctl crash

From: Dan Williams <[email protected]>

commit ac34f15e0c6d2fd58480052b6985f6991fb53bcc upstream.

When tearing down a block device early in its lifetime, userspace may still be performing discovery actions like blkdev_ioctl() to re-read partitions.

The nvdimm_revalidate_disk() implementation depends on disk->driverfs_dev to be valid at entry. However, it is set to NULL in del_gendisk() and fatally this is happening *before* the disk device is deleted from userspace view.

There's no reason for del_gendisk() to clear ->driverfs_dev. That device is the parent of the disk. It is guaranteed to not be freed until the disk, as a child, drops its ->parent reference.

We could also fix this issue locally in nvdimm_revalidate_disk() by using disk_to_dev(disk)->parent, but lets fix it globally since ->driverfs_dev follows the lifetime of the parent. Longer term we should probably just add a @parent parameter to add_disk(), and stop carrying this pointer in the gendisk.

 BUG: unable to handle kernel NULL pointer dereference at           (null)
 IP: [<ffffffffa00340a8>] nvdimm_revalidate_disk+0x18/0x90 [libnvdimm]
 CPU: 2 PID: 538 Comm: systemd-udevd Tainted: G           O    4.4.0-rc5 #2257
 [..]
 Call Trace:
  [<ffffffff8143e5c7>] rescan_partitions+0x87/0x2c0
  [<ffffffff810f37f9>] ? __lock_is_held+0x49/0x70
  [<ffffffff81438c62>] __blkdev_reread_part+0x72/0xb0
  [<ffffffff81438cc5>] blkdev_reread_part+0x25/0x40
  [<ffffffff8143982d>] blkdev_ioctl+0x4fd/0x9c0
  [<ffffffff811246c9>] ? current_kernel_time64+0x69/0xd0
  [<ffffffff812916dd>] block_ioctl+0x3d/0x50
  [<ffffffff81264c38>] do_vfs_ioctl+0x308/0x560
  [<ffffffff8115dbd1>] ? __audit_syscall_entry+0xb1/0x100
  [<ffffffff810031d6>] ? do_audit_syscall_entry+0x66/0x70
  [<ffffffff81264f09>] SyS_ioctl+0x79/0x90
  [<ffffffff81902672>] entry_SYSCALL_64_fastpath+0x12/0x76

---
 block/genhd.c |    1 -
 1 file changed, 1 deletion(-)

@@ -664,7 +664,6 @@ void del_gendisk(struct gendisk *disk)
 
 	kobject_put(disk->part0.holder_dir);
 	kobject_put(disk->slave_dir);
-	disk->driverfs_dev = NULL;
 	if (!sysfs_deprecated)
 		sysfs_remove_link(block_depr, dev_name(disk_to_dev(disk)));
 	pm_runtime_set_memalloc_noio(disk_to_dev(disk), false);

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/block-fix-del_gendisk-vs-blkdev_ioctl-crash.patch queue-4.4/x86-pmem-fix-broken-__copy_user_nocache-cache-bypass-assumptions.patch