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** Changed in: linux (Ubuntu)
Status: New => Incomplete
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https://bugs.launchpad.net/bugs/1736808
Title:
Changes to overlay lowerdir produce kernel file-caps error
Status in linux package in Ubuntu:
Incomplete
Bug description:
In 4.13+ kernels, if you replace an executable file on the lowerdir of
an Overlay union (such that its inode changes), the system will no
longer allow you to execute the file via the upperdir. The changes
introduced to the kernel in this commit...
https://github.com/torvalds/linux/commit/8db6c34f1dbc8e06aa016a9b829b06902c3e1340
... cause it to report a file security capabilities error.
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Replication steps and result:
1. Set up an Overlay union containing some executable files. In my
case i have an ext4 lowerdir and a tmpfs upperdir, but i don't think
it matters.
2. Verify that executing some file (/bin/true for example) on the
upperdir works.
3. Replace that file on the lowerdir using mv, rsync, or similar.
4. Attempt to execute the file on the upperdir again — it will fail.
The shell will give either 126 or 127 as the return status.
5. Check the kernel log. A message like the following appears:
>kernel: Invalid argument reading file caps for /bin/true
I replicated this on Xenial using the HWE-edge kernel (4.13). The
error does NOT occur on the HWE kernel (4.10).
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NOTE: I am aware that the result of changing files on the lowerdir of
an Overlay union, per the documentation, is undefined — so this is
probably not a 'bug' per se. However, i wasn't sure it was deliberate,
either, and it seemed like maybe the previous undefined behaviour was
nicer than the new undefined behaviour, so i thought i'd report it
anyway.
---
Config information:
Ubuntu release: 16.04.3 (Xenial)
Kernel package: linux-image-generic-hwe-16.04-edge 4.13.0.17.24
Kernel version signature: Ubuntu 4.13.0-17.20~16.04.1-generic 4.13.8
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