linux-hardening - Re: [PATCH] pstore: migrate to crypto acomp interface (take 2)

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Message-ID: <202210171227.35ED875219@keescook>
Date:   Mon, 17 Oct 2022 12:29:03 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     "Guilherme G. Piccoli" <gpiccoli@...lia.com>,
        Anton Vorontsov <anton@...msg.org>,
        Colin Cross <ccross@...roid.com>,
        Tony Luck <tony.luck@...el.com>, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] pstore: migrate to crypto acomp interface (take 2)

On Mon, Oct 17, 2022 at 08:14:14PM +0200, Ard Biesheuvel wrote:
> So once we use the same size for input and output, I was curious
> whether we could encrypt in place, and get rid of the big_oops_buf.
> And the answer is 'yes', precisely because we have this horrid per-CPU
> allocation which serves as a bounce buffer. And this is not specific
> to acomp, the old comp algorithms get wrapped in scomps which receive
> the same treatment.

Ah, in the sense that "in place" is actually happening in the per-cpu
allocation, and only if it succeeds does the input buffer get
overwritten?

> So at that point, I wondered what the point is of all this complexity.
> Do we really need 6 different algorithms to compress a couple of K of
> ASCII text on a code path that is ice cold by definition? Wouldn't it
> be better to drop the crypto API altogether here, and just use GZIP
> via the library interface?

Well, my goal was to make the algo "pstore doesn't care". If someone
picks deflate, do they still get all the per-cpu allocations?

-- 
Kees Cook