Re: setting up CDB filters in udev (was Re: [PATCH v2 0/3] block: add queue-private command filter, editable via sysfs)

[Alan Cox <alan@lxorguk.ukuu.org.uk>]

> > Not a good model. Any removal of filters and passing them to a task
> > should be explicit. The behaviour really ought to be to permit the
> > intentional setting of explicit filters then passing them, not touch the
> > default behaviour.
> 
> Yeah, well, then I guess it'll have to be a separate ioctl to switch
> SG_IO for !root users.


My first thought would be to have the basic behaviour as


allowed 	IFF	passes user filter
		&&	CAP_SYS_RAWIO || passes 'root' filter


that allows untrusted to also push unprivileged filters for their own
purposes (consider things like exokernel experiments or just trying to
ensure a raw disk emulation doesn't go wrong). The default user feature
would be 'allow anything'.

then add a way to 'set' the root filter only if you have CAP_SYS_RAWIO
with the default 'root' filter being the current hardcoded filter.

That also means that a normal app running as superuser for some reason
would set its user filter and any accidentally inherited descriptors will
be less dangerous as the are today. It also means a CAP_SYS_RAWIO capable
app can still use filters itself as good programming practise.

It effectively means you have to deliberately and intentionally set up an
'inherited' extra rights case.

Alan