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Date:   Tue, 7 Jul 2020 15:04:18 -0500
From:   John Allen <john.allen@amd.com>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     linux-crypto@vger.kernel.org,
        Corentin Labbe <clabbe.montjoie@gmail.com>,
        Corentin Labbe <clabbe@baylibre.com>,
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        Maxime Ripard <mripard@kernel.org>,
        Chen-Yu Tsai <wens@csie.org>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Ayush Sawal <ayush.sawal@chelsio.com>,
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        Shawn Guo <shawnguo@kernel.org>,
        Sascha Hauer <s.hauer@pengutronix.de>,
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        linux-arm-kernel@lists.infradead.org, linux-omap@vger.kernel.org,
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Subject: Re: [PATCH v4 07/13] crypto: ccp - permit asynchronous skcipher as
 fallback
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 <20200707063203.5018-8-ardb@kernel.org>
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On Tue, Jul 07, 2020 at 09:31:57AM +0300, Ard Biesheuvel wrote:
> Even though the ccp driver implements an asynchronous version of xts(aes),
> the fallback it allocates is required to be synchronous. Given that SIMD
> based software implementations are usually asynchronous as well, even
> though they rarely complete asynchronously (this typically only happens
> in cases where the request was made from softirq context, while SIMD was
> already in use in the task context that it interrupted), these
> implementations are disregarded, and either the generic C version or
> another table based version implemented in assembler is selected instead.
> 
> Since falling back to synchronous AES is not only a performance issue, but
> potentially a security issue as well (due to the fact that table based AES
> is not time invariant), let's fix this, by allocating an ordinary skcipher
> as the fallback, and invoke it with the completion routine that was given
> to the outer request.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Acked-by: John Allen <john.allen@amd.com>