[GTALUG] Adding all users to the "disk" group: bad idea, or terrible idea?

[Dave Collier-Brown]

A helpful addition would be to bracket the part of the program that does disk IO with setgid(), just like one brackets setuid code. --dave

On 2020-02-20 4:11 p.m., Chris Tyler via talk wrote:
Stewart, I'm having troubles understanding the author's reply to the SGID suggestion. What I was proposing was to set things up with a command like this (executed just once):

  BINARY=/path/to/binary ; sudo chmod 02711 $BINARY ; sudo chown root:disk $BINARY

...Which would mean that the user would have their effective group ID changed to 'disk' only while the binary was running. This means that, during program execution, it would be have the same level of access as if the user belonged to the 'disk' group; however, this would drop back to their previous group membership when the binary exited. As a bonus, you don't have to change the system group memberships. (The program in question should, of course, guard against writing to the wrong device while it's running, and prevent shell-outs).

-Chris


On Tue, Feb 18, 2020 at 8:27 AM Stewart C. Russell via talk <talk at gtalug.org<mailto:talk at gtalug.org>> wrote:
Hey, thanks to all who responded. I'm sticking with merely "bad idea",
as it would also give the user block-level access to everyone's secret
files. I know sudo already does this, but one has to manually clunk into
that mode. Though there are still plenty of Raspberry Pi users who 'sudo
ls', for some reason.

On 2020-02-17 7:23 p.m., Chris Tyler wrote:
> Wouldn't it be better to use SGID to enable access to disk devices only
> for the duration of that program's execution?

Author claims it's not possible. The program was written as a reaction
to finding out that balenaEtcher (https://www.balena.io/etcher/) "phones
home" with usage data. If true, this is unfortunate, because Etcher
fixes all the fiddliness of using dd as root, it's truly cross platform,
and avoids the Windows unzip bug. No-one's yet identified what exactly
Etcher phones home yet, but that hasn't stopped the heat, light and fury
of a Free Software Cause in the making …

The new program's quite usable for now (even if you have to run it sudo)
and appears to be cross-platform too. It's also a lot smaller than
Etcher, which is an Electron app.

Source repo is here: https://gitlab.com/bztsrc/usbimager
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--
David Collier-Brown,         | Always do right. This will gratify
System Programmer and Author | some people and astonish the rest
dave.collier-brown at indexexchange.com<mailto:dave.collier-brown at indexexchange.com> |              -- Mark Twain



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