linux-kernel - RE: [PATCH v2 0/2] crypto: removing various VLAs

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Date:   Mon, 9 Apr 2018 14:35:50 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Salvatore Mesoraca' <s.mesoraca16@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Kees Cook <keescook@...omium.org>,
        Eric Biggers <ebiggers3@...il.com>,
        Laura Abbott <labbott@...hat.com>
Subject: RE: [PATCH v2 0/2] crypto: removing various VLAs

From: Salvatore Mesoraca
> Sent: 09 April 2018 14:55
> 
> v2:
> 	As suggested by Herbert Xu, the blocksize and alignmask checks
> 	have been moved to crypto_check_alg.
> 	So, now, all the other separate checks are not necessary.
> 	Also, the defines have been moved to include/crypto/algapi.h.
> 
> v1:
> 	As suggested by Laura Abbott[1], I'm resending my patch with
> 	MAX_BLOCKSIZE and MAX_ALIGNMASK defined in an header, so they
> 	can be used in other places.
> 	I took this opportunity to deal with some other VLAs not
> 	handled in the old patch.

If the constants are visible they need better names.
Maybe CRYPTO_MAX_xxx.

You can also do much better than allocating MAX_BLOCKSIZE + MAX_ALIGNMASK
bytes by requesting 'long' aligned on-stack memory.
The easiest way is to define a union like:

union crypto_tmp {
	u8 buf[CRYPTO_MAX_TMP_BUF];
	long buf_align;
};

Then in each function:

	union tmp crypto_tmp;
	u8 *keystream = PTR_ALIGN(tmp.buf, alignmask + 1);

I think CRYPTO_MAX_TMP_BUF needs to be MAX_BLOCKSIZE + MAX_ALIGNMASK - sizeof (long).

	David