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([2601:646:c200:1ef2:9576:fc98:a516:2c9b]) by smtp.gmail.com with ESMTPSA id j32sm22964507pgi.73.2019.04.29.09.08.11 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 29 Apr 2019 09:08:11 -0700 (PDT) Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (1.0) Subject: Re: [PATCH] x86/entry/64: randomize kernel stack offset upon syscall From: Andy Lutomirski X-Mailer: iPhone Mail (16E227) In-Reply-To: <2236FBA76BA1254E88B949DDB74E612BA4C66A8A@IRSMSX102.ger.corp.intel.com> Date: Mon, 29 Apr 2019 09:08:10 -0700 Cc: Theodore Ts'o , Eric Biggers , "ebiggers@google.com" , "herbert@gondor.apana.org.au" , David Laight , Ingo Molnar , Peter Zijlstra , "keescook@chromium.org" , Daniel Borkmann , "luto@kernel.org" , "linux-kernel@vger.kernel.org" , "jpoimboe@redhat.com" , "jannh@google.com" , "Perla, Enrico" , "mingo@redhat.com" , "bp@alien8.de" , "tglx@linutronix.de" , "gregkh@linuxfoundation.org" , "Edgecombe, Rick P" Content-Transfer-Encoding: quoted-printable Message-Id: <6860856C-6A92-4569-9CD8-FF6C5C441F30@amacapital.net> 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" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > On Apr 29, 2019, at 12:46 AM, Reshetova, Elena = wrote: >=20 >=20 >>>> On Apr 26, 2019, at 7:01 AM, Theodore Ts'o 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.