Commit that causes the regression:
commit 33ec3e53e7b1869d7851e59e126bdb0fe0bd1982
Author: Jan Kara <[email protected]>
Date: Thu May 16 16:01:27 2019 +0200
loop: Don't change loop device under exclusive opener
Loop module allows calling LOOP_SET_FD while there are other openers of
the loop device. Even exclusive ones. This can lead to weird
consequences such as kernel deadlocks like:
mount_bdev() lo_ioctl()
udf_fill_super()
udf_load_vrs()
sb_set_blocksize() - sets desired block size B
udf_tread()
sb_bread()
__bread_gfp(bdev, block, B)
loop_set_fd()
set_blocksize()
- now __getblk_slow() indefinitely loops because B != bdev
block size
Fix the problem by disallowing LOOP_SET_FD ioctl when there are
exclusive openers of a loop device.
[Deliberately chosen not to CC stable as a user with priviledges to
trigger this race has other means of taking the system down and this
has a potential of breaking some weird userspace setup]
Seems like it solves a race so I'll raise the issue to the commit
author.
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https://bugs.launchpad.net/bugs/1836914
Title:
Doing multiple squashfs (and other loop?) mounts in parallel breaks
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