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

From: Peter Zijlstra <peterz@infradead.org>
To: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Dan Carpenter <dan.carpenter@oracle.com>,
	"Gustavo A. R. Silva" <gustavo@embeddedor.com>,
	linux-media@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 01/11] media: tm6000: fix potential Spectre variant 1
Date: Tue, 24 Apr 2018 20:48:43 +0200	[thread overview]
Message-ID: <20180424184843.GX4043@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20180424144755.1c2e2478@vento.lan>

On Tue, Apr 24, 2018 at 02:47:55PM -0300, Mauro Carvalho Chehab wrote:
> So, I'm wondering if are there any way to mitigate it inside the 
> core itself, instead of doing it on every driver, e. g. changing
> v4l_enum_fmt() implementation at v4l2-ioctl.
> 
> Ok, a "poor man" approach would be to pass the array directly to
> the core and let the implementation there to implement the array
> fetch logic, calling array_index_nospec() there, but I wonder if
> are there any other way that won't require too much code churn.

Sadly no; the whole crux is the array bound check itself. You could
maybe pass around the array size to the core code and then do something
like:

	if (f->index >= f->array_size)
		return -EINVAL;

	f->index = nospec_array_index(f->index, f->array_size);

in generic code, and have all the drivers use f->index as usual, but
even that would be quite a bit of code churn I guess.