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Date:   Mon, 17 Oct 2022 12:40:13 -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 09:33:06PM +0200, Ard Biesheuvel wrote:
> On Mon, 17 Oct 2022 at 21:29, Kees Cook <keescook@...omium.org> wrote:
> >
> > 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?
> 
> Something like that IIRC.
> 
> > > 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?
> >
> 
> Not if you use the library interface directly.
> 
> The issue with the percpu buffers is that they are only kept if any
> scomp TFMs are active, but this is always the case for pstore, as you
> don't want to allocate it on the panic path.

Okay, so strictly speaking, eliminating the per-CPU allocation is an
improvement. Keeping scomp and doing in-place compression will let
pstore use "any" compressions method.

Is there a crypto API that does _not_ preallocate the per-CPU stuff?
Because, as you say, it's a huge amount of memory on the bigger
systems...

-- 
Kees Cook

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