linux-kernel - Re: [PATCH v3] siphash: add cryptographically secure hashtable function

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Date:   Tue, 13 Dec 2016 00:39:48 -0800
From:   Eric Biggers <ebiggers3@...il.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        George Spelvin <linux@...izon.com>,
        Scott Bauer <sbauer@....utah.edu>,
        Andi Kleen <ak@...ux.intel.com>,
        Andy Lutomirski <luto@...capital.net>,
        Greg KH <gregkh@...uxfoundation.org>,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com>,
        "Daniel J . Bernstein" <djb@...yp.to>
Subject: Re: [PATCH v3] siphash: add cryptographically secure hashtable
 function

On Mon, Dec 12, 2016 at 11:18:32PM +0100, Jason A. Donenfeld wrote:
> +	for (; data != end; data += sizeof(u64)) {
> +		m = get_unaligned_le64(data);
> +		v3 ^= m;
> +		SIPROUND;
> +		SIPROUND;
> +		v0 ^= m;
> +	}
> +#if defined(CONFIG_DCACHE_WORD_ACCESS) && BITS_PER_LONG == 64
> +	b |= le64_to_cpu(load_unaligned_zeropad(data) & bytemask_from_count(left));
> +#else

Hmm, I don't think you can really do load_unaligned_zeropad() without first
checking for 'left != 0'.  The fixup section for load_unaligned_zeropad()
assumes that rounding the pointer down to a word boundary will produce an
address from which an 'unsigned long' can be loaded.  But if 'left = 0' and we
happen to be on a page boundary with the next page unmapped, then this will not
be true and the second load will still fault.

Eric