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

Stephan Müller
2019-11-21T13:07:23Z

Am Mittwoch, 20. November 2019, 21:32:32 CET schrieb Greg Kroah-Hartman:

Hi Greg,

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

You are right.

Ok, let us get back to the drawing board. What are our requirements? We need
to have an interface for the TRNG that should ensure other users of entropy
are not starved by unprivileged users.

What about the following: we use the getrandom(2) system call and add
GRND_TRUERANDOM as already indicated. However, there is one more caveat we
would add:

- if the caller of GRND_TRUERANDOM is !CAP_SYS_ADMIN the entropy pool can only
be depleted to the point where at least one or two full seeding operations
worth of entropy is left.

- if the caller of GRND_TRUERANDOM is CAP_SYS_ADMIN, the entropy can be
depleted completely

At runtime, the LRNG would then behave like the following:

- calling getrandom(..., 0), /dev/random or /dev/urandom would deplete the
entropy pool during reseeding operations but leaving an emergency level of 512
bits of entropy in the pool. If equal or less are in the pool, reseeding would
be skipped.

- calling getrandom(..., GRND_TRUERANDOM) with CAP_SYS_ADMIN allows the
entropy pool to be fully depleted.

- calling getrandom(..., GRND_TRUERANDOM) without CAP_SYS_ADMIN allows the
entropy pool to be depleted down to 1024 bits of entropy. If the pool has
equal or less, the caller is blocked. This allows the DRNG feeding /dev/
random, /dev/urandom or getrandom(..., 0) with 512 bits of entropy (i.e. two
reseed operations are possible). Only if the entropy pool has more than 1024
bits of entropy, the getrandom call would unblock and provide data.

With that approach, I think we can honor the request from Greg to not add any
new interface and yet honor the note from Alexander to not allow unprivileged
user space to deplete the entropy pool to the extent that other users of
entropy are too much affected.

If GRND_TRUERANDOM is not implemented, EOPNOTSUPP is returned.

Thank you.

Ciao
Stephan