linux-kernel - RE: x86/random: Speculation to the rescue

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Tue, 8 Oct 2019 11:33:46 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Pavel Machek' <pavel@....cz>, "Theodore Y. Ts'o" <tytso@....edu>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Ahmed S. Darwish" <darwish.07@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Nicholas Mc Guire <hofrat@...ntech.at>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>
Subject: RE: x86/random: Speculation to the rescue

From: Pavel Machek
> Sent: 07 October 2019 23:18
..
> I have many systems including SoC here, but technology needed for NAND
> flash is different from technology for CPU, so these parts do _not_
> share a silicon die. They do not even share same package. (Also RTC
> tends to be on separate chip, connected using i2c).

NAND flash requires ECC so is likely to be async.
But I2C is clocked from the cpu end - so is fixed.

Also an embedded system could be booting off a large serial EEPROM.
These have fixed timings and are clocked from the cpu end.

So until you get any ethernet interface up and can look at receive packet
timings there isn't likely to be any randomness at all.
A high resolution voltage  (or temperature) monitor might generate noise
in its LSB - but they don't often have a higher resolution than the stability
of the signal.

I can't remember (from the start of this thread) why 'speculation' is expected to
generate randomness.
I'd have thought the loop was deterministic - but you don't know the initial state.
More iterations may just be amplifying the initial small randomness - rather than
generating extra randomness.
So while it gets the system to boot, it hasn't actually done its job.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)