Re: [PATCH v25 10/12] LRNG - add TRNG support

[Greg Kroah-Hartman]

On Wed, Nov 20, 2019 at 08:51:11PM +0100, Stephan Müller wrote:
> Am Mittwoch, 20. November 2019, 14:29:18 CET schrieb Greg Kroah-Hartman:
> 
> Hi Greg,
> 
> > On Wed, Nov 20, 2019 at 09:58:35AM +0100, Stephan Müller wrote:
> > > Am Dienstag, 19. November 2019, 13:41:50 CET schrieb Greg Kroah-Hartman:
> > > 
> > > Hi Greg,
> > > 
> > > > On Tue, Nov 19, 2019 at 02:07:40AM -0800, Andy Lutomirski wrote:
> > > > > > As this would introduce a new device file now, is there a special
> > > > > > process that I need to follow or do I need to copy? Which
> > > > > > major/minor
> > > > > > number should I use?
> > > > > > 
> > > > > > Looking into static const struct memdev devlist[] I see
> > > > > > 
> > > > > >          [8] = { "random", 0666, &random_fops, 0 },
> > > > > >          [9] = { "urandom", 0666, &urandom_fops, 0 },
> > > > > > 
> > > > > > Shall a true_random be added here with [10]?
> > > > > 
> > > > > I am not at all an expert on chardevs, but this sounds generally
> > > > > reasonable.  gregkh is probably the real authority here.
> > > > 
> > > > [10] is the aio char device node, so you better not try to overlap it or
> > > > bad things will happen :(
> > > 
> > > Thanks for your insights.
> > > 
> > > Which device minor number could we use?
> > 
> > Get your own dynamic one by using a misc device if you _REALLY_ want to
> > add yet-another-char-node-for-random-data.
> > 
> > But I would have thought that we all realize that this is not the way to
> > do things.  Let's not have "random", "urandom", and "true_random" be
> > something we want to totally confuse userspace with, that way is insane.
> > 
> > Please just make the existing userspace api "just work", don't add to
> > the mess.
> 
> Thank you, I think we should follow that advise.
> 
> With that and considering Alexander's rightful remark we have a challenge. So, 
> changing the syscall may not be the right way unless we find a way to restrict 
> the permissions somehow (capability? LSM? None of that seems to be a good 
> fit).
> 
> What about providing a /sys file? I.e. adding a file that:
> 
> a) has permissions 440 per default and maybe the ownership of root:root
> 
> b) allow user space to perform a chown/chgrp
> 
> c) only supports reading of data from user space
> 
> But then, how could we provide a common interface for the existing random.c 
> and the LRNG?
> 
> Or should we use a proc file for that? If yes, I guess it should not be a 
> sysctl, but a "regular" proc file that should allow a chown(2) operation. On 
> the other hand, is proc the right place to provide a user space interface for 
> exporting data to user?

No, do not abuse sysfs or procfs for something like this.  Use a real
syscall please if you really need it.

greg k-h