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Subject: Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <2236FBA76BA1254E88B949DDB74E612BA4C66A8A@IRSMSX102.ger.corp.intel.com>
Date:   Mon, 29 Apr 2019 09:08:10 -0700
Cc:     Theodore Ts'o <tytso@mit.edu>, 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>
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References: <2236FBA76BA1254E88B949DDB74E612BA4C51962@IRSMSX102.ger.corp.intel.com> <20190416120822.GV11158@hirez.programming.kicks-ass.net> <01914abbfc1a4053897d8d87a63e3411@AcuMS.aculab.com> <20190416154348.GB3004@mit.edu> <2236FBA76BA1254E88B949DDB74E612BA4C52338@IRSMSX102.ger.corp.intel.com> <9cf586757eb44f2c8f167abf078da921@AcuMS.aculab.com> <20190417151555.GG4686@mit.edu> <99e045427125403ba2b90c2707d74e02@AcuMS.aculab.com> <2236FBA76BA1254E88B949DDB74E612BA4C5E473@IRSMSX102.ger.corp.intel.com> <2236FBA76BA1254E88B949DDB74E612BA4C63E24@IRSMSX102.ger.corp.intel.com> <20190426140102.GA4922@mit.edu> <57357E35-3D9B-4CA7-BAB9-0BE89E0094D2@amacapital.net> <2236FBA76BA1254E88B949DDB74E612BA4C66A8A@IRSMSX102.ger.corp.intel.com>
To:     "Reshetova, Elena" <elena.reshetova@intel.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Apr 29, 2019, at 12:46 AM, Reshetova, Elena <elena.reshetova@intel.com>=
 wrote:
>=20
>=20
>>>> On Apr 26, 2019, at 7:01 AM, Theodore Ts'o <tytso@mit.edu> wrote:
>>>=20
>=20
>> It seems to me
>> that we should be using the =E2=80=9Cfast-erasure=E2=80=9D construction f=
or all get_random_bytes()
>> invocations. Specifically, we should have a per cpu buffer that stores so=
me random
>> bytes and a count of how many random bytes there are. get_random_bytes() s=
hould
>> 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.
>=20
> Ideally it would be great to call smth fast and secure on each syscall wit=
hout a per-cpu
> buffer,

Why?  You only need a few bits, and any sensible crypto primitive is going t=
o be much better at producing lots of bits than producing just a few bits.  E=
ven ignoring that, avoiding the I-cache hit on every syscall has value.  And=
 I still don=E2=80=99t see what=E2=80=99s wrong with a percpu buffer.

> so that's why I was asking on chacha8. As Eric pointed it should not be us=
ed for
> cryptographic purpose, but I think it is reasonably secure for our purpose=
, especially if
> the generator is sometimes reseeded with fresh entropy.=20
> However, it very well might be that is too slow anyway.=20
>=20
> So, I think then we can do the per-cpu approach as you suggesting.=20
> Have a per-cpu buffer big enough as you suggested (couple of pages) from w=
here
> we regularly read 8 bits at the time and zero them as we go.=20
>=20
> I am just not sure on the right refill strategy in this case?
> Should we try to maintain this per-cpu buffer always with some random byte=
s by
> having a work queued that would refill it (or part of it, i.e. a page from=
 a set of 4 pages)=20
> regularly from CRNG source?=20
> I guess how often we need to refill will depend so much on the syscall rat=
e
> on that cpu, so it might be hard to find a reasonable period.
> In any case we need to prepare to do a refill straight from a syscall,
> if despite our best efforts to keep the buffer refilled we run out of bits=
.
> Is it ok to get a visible performance hit at this point? In worse case we w=
ill need to
> generate n pages worth of random numbers, which is going to take a
> while.=20

I think a small hit every few syscalls is okay. The real time people will tu=
rn this off regardless.

>=20
> I will try doing this PoC and measure implications (without the worker
> refill to start with). Let's see how bad (performance wise it looks).
>=20
> Best Regards,
> Elena.
>=20
>=20
>=20
>=20
>=20
>> The obvious objection is =E2=80=9Coh no, a side channel could leak the bu=
ffer,=E2=80=9D to which I say
>> so what?  A side channel could just as easily leak the entire CRNG state.=

>>=20
>> For Elena=E2=80=99s 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=E2=80=99t run real C code. =
 Or it could be moved a
>> bit later, I suppose =E2=80=94 the really early part is not really an int=
eresting attack surface.