Probable bug in auditd

Probable bug in auditd

[Shourya Jaiswal]

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Hi,

I have found a weird behavior in auditd. File "/abc" does not exist.

audit.rules:

-a always,exit -F arch=b32 -S open -S openat

-a always,exit -F arch=b64 -S open -S openat

A non-root user executes "echo > /abc", it doesn't get logged in audit.log. Same with "echo > /etc/abc"
A non-root user executes "cat /abc", it gets logged in audit.log

Since auditd is monitoring all the open and openat syscalls, ideally both the cases (i.e. read and write) should have be logged.

After I execute "chmod a+w /" then "chmod a-w /", if a non-root user executes "echo > /abc", then it gets logged in audit.log.

This looks like a bug to me. Kindly let me know if it's a bug or an intended feature.

System used to test: Linux 5.4.0-56-generic #62-Ubuntu SMP x86_64 x86_64 x86_64 GNU/Linux

Regards,
Shourya Jaiswal


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Re: Probable bug in auditd

[Steve Grubb]

Hello,

On Wednesday, January 20, 2021 3:54:45 PM EST Shourya Jaiswal wrote:
> I have found a weird behavior in auditd. File "/abc" does not exist.
> 
> audit.rules:
> 
> -a always,exit -F arch=b32 -S open -S openat
> 
> -a always,exit -F arch=b64 -S open -S openat
> 
> A non-root user executes "echo > /abc", it doesn't get logged in audit.log.

This is because name resolution fails before it gets to audit hooks inside 
the kernel. The audit hooks want to collect device, inode, permission, owner, 
group, etc. IOW, things that do not exist.



> Same with "echo > /etc/abc" A non-root user executes "cat /abc", it gets
> logged in audit.log
> 
> Since auditd is monitoring all the open and openat syscalls, ideally both
> the cases (i.e. read and write) should have be logged.

It's the kernel doing it. This comes up from time to time. It is logged here:

https://github.com/linux-audit/audit-kernel/issues/118

> After I execute "chmod a+w /" then "chmod a-w /", if a non-root user
> executes "echo > /abc", then it gets logged in audit.log.
> 
> This looks like a bug to me. Kindly let me know if it's a bug or an
> intended feature.

This is essentially how it works. But, there will be an improvement at some 
future point when issue 118 is resolved.

-Steve

> System used to test: Linux 5.4.0-56-generic #62-Ubuntu SMP x86_64 x86_64
> x86_64 GNU/Linux
> 


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