Re: Potential Bash Script Vulnerability

[Kerin Millar]

On Sun, 7 Apr 2024, at 5:17 AM, admin@osrc.rip wrote:
> Hello everyone!
>
> I've attached a minimal script which shows the issue, and my recommended 
> solution.
>
> Affected for sure:
> System1: 64 bit Ubuntu 22.04.4 LTS - Bash: 5.1.16(1)-release - Hardware: 
> HP Pavilion 14-ec0013nq (Ryzen 5 5500u, 32GB RAM, Radeon grapics, nvme 
> SSD.)
> System2: 64 bit Ubuntu 20.10 (No longer supported.) - Bash: 
> 5.0.17(1)-release - Hardware: DIY (AMD A10-5800k, 32GB RAM, Radeon 
> graphics, several SATA drives)
> and probably a lot more...
>
> Not sure whether or not this is a know issue, truth be told I discovered 
> it years ago (back around 2016) as I was learning bash scripting, and 
> accidentally appended a command to the running script, which got 
> executed immediately after the script but back then I didn't find it 
> important to report since I considered myself a noob. I figured someone 
> more experienced will probably find and fix it, or there must be a 
> reason for it. I forgotű it. Now watching a video about clever use of 
> shell in XZ stuff I remembered, tested it again and found it still 
> unpatched. :S So now I'm reporting it and hope it helps!

It is a known pitfall, though perhaps not as widely known as it ought to be. 
The reason that your usage of (GNU) sed fails as a self-modification technique 
is that sed -i behaves as follows.

1) it creates a temporary file
2) it sends its output to the temporary file
3) it renames the temporary file over the original file from which it read

The consequence of the third step is that the original file is unlinked. In its 
place will be a new hard link, bearing the same name, but otherwise quite 
distinct from the original. Such can be easily demonstrated:

$ touch file
$ stat -c %i file
1548822
$ strace -erename sed -i -e '' file
rename("./sedP2oQ5I", "file")           = 0
+++ exited with 0 +++
$ stat -c %i file
1548823

See how the revised file has an entirely new inode number? It proves that sed 
does not perform 'in-place' editing at all. For more information regarding that 
particular topic, take a look at 
https://backreference.org/2011/01/29/in-place-editing-of-files/index.html.

Now, at the point that the original file is unlinked, its contents will remain 
available until such time as its reference count drops to 0. This is a 
characteristic of unix and unix-like operating systems in general. Let's assume 
that the file in question is a bash script, that bash had the file open and 
that it was still reading from it. Bash will not yet 'see' your modifications. 
However, once bash closes the file and exits, should you then instruct bash to 
execute the script again, it will follow the new hard link and thereby read the 
new file. Further, assuming that no other processes also had the original file 
open at the time of bash exiting, its reference count will drop to 0, and the 
backing filesystem will free its associated data.

>From this, we may reason that the pitfall you stumbled upon applies where the 
>file is modified in such a way that its inode number does not change e.g. by 
>truncating and re-writing the file. One way to demonstrate this distinction is 
>to apply your edit with an editor that behaves in this way, such as nano. 
>Consider the following script.

#!/bin/bash
echo begin
sleep 10
: do nothing
echo end

You can try opening this script with nano before executing it. While the sleep 
command is still running, replace ": do nothing" with a command of your 
choosing, then instruct nano to save the amended script. You will find that the 
replacement command ends up being executed. Repeat the experiment with vim and 
you will find that the outcome is different. That's because the method by which 
vim amends files is similar to that of sed -i.

You propose a method by which bash might implicitly work around this pitfall 
but it would not suffice. If you perform an in-place edit upon any portion of a 
script that bash has not yet read and/or buffered - while bash is still 
executing said script - then the behaviour of the script will be affected. If 
you consider this to be a genuine nuisance, a potential defence is to compose 
your scripts using compound commands. For example:

#!/bin/bash
{
   : various commands here
   exit
}

Alternatively, use functions - which are really just compound commands attached 
to names:

#!/bin/bash
main() {
   : various commands here.
   exit
}
main "$@"

Doing so helps somewhat because bash is compelled to read all the way to the 
end of a compound command at the point that it encounters one, prior to its 
contents being executed.

Ultimately, the best defence against the potentially adverse consequences of 
performing an in-place edit is to to refrain entirely from performing in-place 
edits.

-- 
Kerin Millar