linux-kernel - Re: [PATCH 01/11] media: tm6000: fix potential Spectre variant 1

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Message-ID: <20180424112129.GT4129@hirez.programming.kicks-ass.net>
Date:   Tue, 24 Apr 2018 13:21:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/11] media: tm6000: fix potential Spectre variant 1

On Tue, Apr 24, 2018 at 12:36:09PM +0200, Peter Zijlstra wrote:
> 
> Then usespace probes which part of the descr[] array is now in cache and
> from that it can infer the initial out-of-bound value.

Just had a better look at v4l_fill_fmtdesc() and actually read the
comment. The code cannot be compiled as a array because it is big and
sparse. But the log(n) condition tree is a prime candidate for the
branchscope side-channel, which would be able to reconstruct a
significant number of bits of the original value. A denser tree gives
more bits etc.