Messages in this thread | | | Subject | Re: [RFC PATCH v2 2/2] blk-mq: Lockout tagset iter when freeing rqs | From | John Garry <> | Date | Mon, 21 Dec 2020 18:47:24 +0000 |
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On 21/12/2020 18:09, Bart Van Assche wrote: > On 12/21/20 4:06 AM, John Garry wrote: >> On 18/12/2020 22:43, Bart Van Assche wrote: >>> Does this mean that we do not yet have >>> a full explanation about why the above call stack can be triggered? >> >> We understand it, and I'll describe my experiment in detail: >> a. fio runs on 2x disks, sda (root partition disk) and sdb. >> b. for sda, userpace triggers blk_mq_queue_tag_busy_iter(), as in >> stackframe above. Since its request queue is not frozen, it will iter >> the busy tags. >> c. on sdb, I continuously change the IO scheduler. >> >> So sdb request queue gets frozen as we switch IO sched, but we could >> have this sequence of events: >> - blk_mq_queue_tag_busy_iter() on sda takes reference to a sdb request >> - Getting a tag and updating ->rqs[] in tagset is not atomic >> - requests for sdb cleared in tagset and request memory is freed >> - blk_mq_queue_tag_busy_iter() on sda still holds reference to sdb >> request and dereferences it -> UAF >> >> Hope it's clear. It is a bit unlikely, I will admit, but it still can >> happen and UAF is never good. So please let me know if other idea to solve. > > Hi John, > > Do you agree that all partitions (struct block_device) of a disk share a > request queue (block_device.bd_disk->queue)? I'm asking this because it > seems like in the above explanation it has been assumed that different > partitions use different request queues. > > Thanks, > > Bart. >
Hi Bart,
Yes, I agree, and I'm not sure what I wrote to give that impression.
About "root partition", above, I'm just saying that / is mounted on a sda partition:
root@ubuntu:/home/john# mount | grep sda /dev/sda2 on / type ext4 (rw,relatime,errors=remount-ro,stripe=32) /dev/sda1 on /boot/efi type vfat (rw,relatime,fmask=0077,dmask=0077,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro)
Thanks, John
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