From: Andy Lutomirski <luto@amacapital.net>
To: Theodore Ts'o <tytso@mit.edu>
Cc: "Reshetova, Elena" <elena.reshetova@intel.com>,
Eric Biggers <ebiggers3@gmail.com>,
"ebiggers@google.com" <ebiggers@google.com>,
"herbert@gondor.apana.org.au" <herbert@gondor.apana.org.au>,
David Laight <David.Laight@aculab.com>,
Ingo Molnar <mingo@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
"keescook@chromium.org" <keescook@chromium.org>,
Daniel Borkmann <daniel@iogearbox.net>,
"luto@kernel.org" <luto@kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"jpoimboe@redhat.com" <jpoimboe@redhat.com>,
"jannh@google.com" <jannh@google.com>,
"Perla, Enrico" <enrico.perla@intel.com>,
"mingo@redhat.com" <mingo@redhat.com>,
"bp@alien8.de" <bp@alien8.de>,
"tglx@linutronix.de" <tglx@linutronix.de>,
"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
"Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
Subject: Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall
Date: Fri, 26 Apr 2019 11:34:29 -0700 [thread overview]
Message-ID: <57357E35-3D9B-4CA7-BAB9-0BE89E0094D2@amacapital.net> (raw)
In-Reply-To: <20190426140102.GA4922@mit.edu>
> On Apr 26, 2019, at 7:01 AM, Theodore Ts'o <tytso@mit.edu> wrote:
>
>> On Fri, Apr 26, 2019 at 11:33:09AM +0000, Reshetova, Elena wrote:
>> Adding Eric and Herbert to continue discussion for the chacha part.
>> So, as a short summary I am trying to find out a fast (fast enough to be used per syscall
>> invocation) source of random bits with good enough security properties.
>> I started to look into chacha kernel implementation and while it seems that it is designed to
>> work with any number of rounds, it does not expose less than 12 rounds primitive.
>> I guess this is done for security sake, since 12 is probably the lowest bound we want people
>> to use for the purpose of encryption/decryption, but if we are to build an efficient RNG,
>> chacha8 probably is a good tradeoff between security and speed.
>>
>> What are people's opinions/perceptions on this? Has it been considered before to create a
>> kernel RNG based on chacha?
>
> Well, sure. The get_random_bytes() kernel interface and the
> getrandom(2) system call uses a CRNG based on chacha20. See
> extract_crng() and crng_reseed() in drivers/char/random.c.
>
> It *is* possible to use an arbitrary number of rounds if you use the
> low level interface exposed as chacha_block(), which is an
> EXPORT_SYMBOL interface so even modules can use it. "Does not expose
> less than 12 rounds" applies only if you are using the high-level
> crypto interface.
>
> We have used cut down crypto algorithms for performance critical
> applications before; at one point, we were using a cut down MD4(!) for
> initial TCP sequence number generation. But that was getting rekeyed
> every five minutes, and the goal was to make it just hard enough that
> there were other easier ways of DOS attacking a server.
>
> I'm not a cryptographer, so I'd really us to hear from multiple
> experts about the security level of, say, ChaCha8 so we understand
> exactly kind of security we'd offering. And I'd want that interface
> to be named so that it's clear it's only intended for a very specific
> use case, since it will be tempting for other kernel developers to use
> it in other contexts, with undue consideration.
>
>
I don’t understand why we’re even considering weaker primitives. It seems to me that we should be using the “fast-erasure” construction for all get_random_bytes() invocations. Specifically, we should have a per cpu buffer that stores some random bytes and a count of how many random bytes there are. get_random_bytes() should take bytes from that buffer and *immediately* zero those bytes in memory. When the buffer is empty, it gets refilled with the full strength CRNG.
The obvious objection is “oh no, a side channel could leak the buffer,” to which I say so what? A side channel could just as easily leak the entire CRNG state.
For Elena’s specific use case, we would probably want a try_get_random_bytes_notrace() that *only* tries the percpu buffer, since this code runs so early in the syscall path that we can’t run real C code. Or it could be moved a bit later, I suppose — the really early part is not really an interesting attack surface.
next prev parent reply other threads:[~2019-04-26 18:34 UTC|newest]
Thread overview: 81+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-15 6:09 [PATCH] x86/entry/64: randomize kernel stack offset upon syscall Elena Reshetova
2019-04-15 7:25 ` Ingo Molnar
2019-04-15 8:44 ` Reshetova, Elena
2019-04-16 7:34 ` Ingo Molnar
2019-04-16 11:10 ` Reshetova, Elena
2019-04-16 12:08 ` Peter Zijlstra
2019-04-16 12:45 ` David Laight
2019-04-16 15:43 ` Theodore Ts'o
2019-04-16 16:07 ` Peter Zijlstra
2019-04-16 16:47 ` Reshetova, Elena
2019-04-17 9:28 ` David Laight
2019-04-17 15:15 ` Theodore Ts'o
2019-04-17 15:40 ` Kees Cook
2019-04-17 15:53 ` David Laight
2019-04-24 11:42 ` Reshetova, Elena
2019-04-24 13:33 ` David Laight
2019-04-25 11:23 ` Reshetova, Elena
2019-04-26 11:33 ` Reshetova, Elena
2019-04-26 14:01 ` Theodore Ts'o
2019-04-26 17:44 ` Eric Biggers
2019-04-26 18:02 ` Theodore Ts'o
2019-04-27 13:59 ` Andy Lutomirski
2019-04-29 8:04 ` Reshetova, Elena
2019-04-26 18:34 ` Andy Lutomirski [this message]
2019-04-29 7:46 ` Reshetova, Elena
2019-04-29 16:08 ` Andy Lutomirski
2019-04-30 17:51 ` Reshetova, Elena
2019-04-30 18:01 ` Kees Cook
2019-05-01 8:23 ` David Laight
2019-05-02 8:07 ` Reshetova, Elena
2019-05-01 8:41 ` David Laight
2019-05-01 23:33 ` Andy Lutomirski
2019-05-02 8:15 ` Reshetova, Elena
2019-05-02 9:23 ` David Laight
2019-05-02 14:47 ` Andy Lutomirski
2019-05-02 15:08 ` Ingo Molnar
2019-05-02 16:32 ` Andy Lutomirski
2019-05-02 16:43 ` Ingo Molnar
2019-05-03 16:40 ` Andy Lutomirski
2019-05-02 16:34 ` David Laight
2019-05-02 16:45 ` Ingo Molnar
2019-05-03 16:17 ` Reshetova, Elena
2019-05-03 16:40 ` David Laight
2019-05-03 19:10 ` Linus Torvalds
2019-05-06 6:47 ` Reshetova, Elena
2019-05-06 7:01 ` Reshetova, Elena
2019-05-08 11:18 ` Reshetova, Elena
2019-05-08 11:32 ` Ingo Molnar
2019-05-08 13:22 ` Reshetova, Elena
2019-05-09 5:59 ` Ingo Molnar
2019-05-09 7:01 ` Reshetova, Elena
2019-05-09 8:43 ` Ingo Molnar
2019-05-11 22:45 ` Andy Lutomirski
2019-05-12 0:12 ` Kees Cook
2019-05-12 8:02 ` Ingo Molnar
2019-05-12 14:33 ` Kees Cook
2019-05-28 12:28 ` Reshetova, Elena
2019-05-28 13:33 ` Theodore Ts'o
2019-05-29 10:13 ` Reshetova, Elena
2019-05-29 10:51 ` David Laight
2019-05-29 18:35 ` Kees Cook
2019-05-29 18:37 ` Kees Cook
2019-07-29 11:41 ` Reshetova, Elena
2019-07-30 18:07 ` Kees Cook
2019-08-01 6:35 ` Reshetova, Elena
2019-05-09 7:03 ` Reshetova, Elena
2019-05-06 7:32 ` Reshetova, Elena
2019-04-29 7:49 ` Reshetova, Elena
2019-04-26 17:37 ` Edgecombe, Rick P
2019-04-17 6:24 ` Ingo Molnar
2019-04-16 18:19 ` Reshetova, Elena
[not found] <20190408061358.21288-1-elena.reshetova@intel.com>
2019-04-08 12:49 ` Josh Poimboeuf
2019-04-08 13:30 ` Reshetova, Elena
2019-04-08 16:21 ` Kees Cook
2019-04-10 8:26 ` Ingo Molnar
2019-04-10 9:00 ` Reshetova, Elena
2019-04-10 10:17 ` Ingo Molnar
2019-04-10 10:24 ` Reshetova, Elena
2019-04-10 14:52 ` Andy Lutomirski
2019-04-12 5:36 ` Reshetova, Elena
2019-04-12 21:16 ` Andy Lutomirski
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