[RndTbl] creat() fails on non-root owned file when stickybit set on dir (tcsh) (new kernel bug?)

[Gilbert E. Detilllieux]

See also...

https://www.spinics.net/lists/fedora-devel/msg252452.html

Thanks, Trevor, for bringing this to our attention.  I was not even 
aware of these new sysctl settings and kernel features.  I can see why 
they'd be desirable from a security perspective, but it does break 
compatibility, possibly for some legitimate but obscure use cases.

Gilbert

On 2019-12-30 11:57 p.m., Trevor Cordes wrote:
> After much kernel bisecting by me that yielded nothing of value, it
> turns out the bug isn't the kernel, it's a change in Fedora's default
> sysctl.conf settings between F29 and F30 that enable a new-ish kernel
> "feature".  The "feature" turns on this behavior.
> 
> Thanks to Andrew Morton and especially Al Viro for figuring this out
> for me as I'm pretty sure a sysctl of some obscure feature would have
> been the last place I would have looked!
> 
> The solution is:
> echo 0 >> /proc/sys/fs/protected_regular
> 
> The new feature is (and it may be systemd deciding this):
> 
> * The fs.protected_regular and fs.protected_fifos sysctls, which were
>    added in Linux 4.19 to make some data spoofing attacks harder, are
>    now enabled by default. While this will hopefully improve the
>    security of most installations, it is technically a backwards
>    incompatible change; to disable these sysctls again, place the
>    following lines in /etc/sysctl.d/60-protected.conf or a similar file:
>      fs.protected_regular = 0
>      fs.protected_fifos = 0
> 
> The bz is:
> https://bugzilla.kernel.org/show_bug.cgi?id=205727

-- 
Gilbert E. Detillieux        E-mail:  <gedetil at cs.umanitoba.ca>
Dept. of Computer Science    Web:     http://www.cs.umanitoba.ca/~gedetil/
University of Manitoba       Phone:   (204)474-8161
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