linux-kernel - Re: [PATCH v6 3/5] test: add new driver

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Date:   Fri, 12 May 2017 17:59:23 +0200
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     AKASHI Takahiro <takahiro.akashi@...aro.org>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Daniel Wagner <wagi@...om.org>,
        David Woodhouse <dwmw2@...radead.org>, rafal@...ecki.pl,
        Arend Van Spriel <arend.vanspriel@...adcom.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        "Li, Yi" <yi1.li@...ux.intel.com>, atull@...nsource.altera.com,
        moritz.fischer@...us.com, Petr Mladek <pmladek@...e.com>,
        Johannes Berg <johannes.berg@...el.com>,
        Emmanuel Grumbach <emmanuel.grumbach@...el.com>,
        Luciano Coelho <luciano.coelho@...el.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        Andy Lutomirski <luto@...nel.org>,
        David Howells <dhowells@...hat.com>,
        Peter Jones <pjones@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 3/5] test: add new driver_data load tester

On Fri, May 12, 2017 at 09:28:47AM +0900, AKASHI Takahiro wrote:
> On Thu, May 11, 2017 at 11:32:30AM -0700, Luis R. Rodriguez wrote:
> > On Thu, May 11, 2017 at 11:26 AM, Luis R. Rodriguez <mcgrof@...e.com> wrote:
> > >
> > > It would seems to make sense to me to only need to verify files when read
> > > for the first time, once its cache I don't see why we would re-verify them ?
> > 
> > To be clear, the fw cache feature reads the files from the fs prior to
> > suspend, and then uses the in-memory cache on resume. So it would make
> > sense to me only to rely on fw verification on resume then when the fw
> > cache is used ?
> 
> Good point. I was thinking of need for verification on resume.

>From what we have discussed so far it would seem to me only necessary
for a sig_check_ok (if we accept a file can have only one signature
requirement) for a cache entry, and if its not set but a lookup needs
a sig check it can do a full fs lookup. If such a lookup succeeded
then it can fill the sig_check_ok in, provided the file contents
match of course, given the file could have changed under the hood
between the last file cache lookup (if the file did change that puts
us at odd with the first lookup, but since its an update and no sig
check is required, I guess it is fine to use its contents).

> As cache is not protected 

Cache should be protected, it should be const and if its not we should fix that.

> and visible to the kernel,

You mean it is visible to the kernel ?

>  some malware might want to rewrite it :)

Right, we want to be pedantic about that sort of stuff and signature
verification can help here but those benefits should carry their own
weight. We should do what we can without file signature verification to
protect the cache.

The cache is short lived though, it exists only during suspend/resume.

  Luis