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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Sun, 16 Apr 2023 19:26:08 +0200
Message-ID: <CAHmME9qP2jwSk7o6EYGK_=vRntfLq6O1BcnH46ibt=Ypv3v-vw@mail.gmail.com>
Subject: Re: [PATCH v6 3/3] arch/x86/mm/kaslr: use siphash instead of prandom_bytes_state
To:     david.keisarschm@mail.huji.ac.il
Cc:     linux-kernel@vger.kernel.org,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        keescook@chromium.org, ilay.bahat1@gmail.com, aksecurity@gmail.com
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On 4/16/23, david.keisarschm@mail.huji.ac.il
<david.keisarschm@mail.huji.ac.il> wrote:
> From: David Keisar Schmidt <david.keisarschm@mail.huji.ac.il>
>
> However, the seeding here is done by calling prandom_seed_state,
> which effectively uses only 32bits of the seed, which means that observing
> ONE
> region's offset (say 30 bits) can provide the attacker with 2 possible
> seeds
> (from which the attacker can calculate the remaining two regions)
>
> In order to fix it,  we have replaced the two invocations of
> prandom_bytes_state and prandom_seed_state
> with siphash, which is considered more secure.
> Besides, the original code used the same pseudo-random number in every
> iteration,
> so to add some additional randomization
> we call siphash every iteration, hashing the iteration index with the
> described key.
>
>

Nack. Please don't add bespoke new RNG constructions willy nilly. I
just spent a while cleaning this kind of thing up.