Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 15 Jun 2023 20:14:47 -0700
From:   Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
To:     Phillip Potter <phil@philpotter.co.uk>
Cc:     linux-kernel@vger.kernel.org, jordyzomer@google.com,
        linux-block@vger.kernel.org
Subject: Re: [PATCH v2 1/1] cdrom: Fix spectre-v1 gadget
Message-ID: <20230616031447.yslq6ep7lxe6sjv4@desk>
References: <20230612110040.849318-1-jordyzomer@google.com>
 <20230612110040.849318-2-jordyzomer@google.com>
 <20230615163125.td3aodpfwth5n4mc@desk>
 <ZIufZn+reW0rza1H@equinox>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <ZIufZn+reW0rza1H@equinox>
Precedence: bulk

On Fri, Jun 16, 2023 at 12:31:50AM +0100, Phillip Potter wrote:
> I've now looked at this. It is possible for cdi->capacity to be > 1, as
> it is set via get_capabilities() -> cdrom_number_of_slots(), if the
> device is an individual or cartridge changer.

Ohk. Is there an upper limit to cdi->capacity? If not, we are left with
barrier_nospec().

> Therefore, I think using CDI_MAX_CAPACITY of 1 is not the correct
> approach. Jordy's V2 patch is fine therefore, but perhaps using
> array_index_nospec() with cdi->capacity is still better than a
> do/while loop from a performance perspective, given it would be cached
> etc. at that point, so possibly quicker. Thoughts? (I'm no expert on
> spectre-v1 I'll admit).

array_index_nospec() can only clip the arg correctly if the upper bound
is correct. Problem with array_index_nospec(arg, cdi->capacity) is
cdi->capacity is not a constant, so it suffers from the same problem as
arg i.e. cdi->capacity could also be speculated. Although having to
control 2 loads makes the attack difficult, but does not rules out
completely.

barrier_nospec() makes the CPU wait for all previous loads to retire
before executing following instructions speculatively. This causes the
conditional branch to resolve correctly. I hope this does not fall into
a hotpath.