linux-kernel - Re: [PATCH v5 1/7] fs: introduce kernel_pread

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Message-ID: <202005131630.8B1ECE0@keescook>
Date:   Wed, 13 May 2020 16:34:14 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Mimi Zohar <zohar@...nel.org>
Cc:     Scott Branden <scott.branden@...adcom.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        David Brown <david.brown@...aro.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>, bjorn.andersson@...aro.org,
        Shuah Khan <skhan@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        BCM Kernel Feedback <bcm-kernel-feedback-list@...adcom.com>,
        Olof Johansson <olof@...om.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Colin Ian King <colin.king@...onical.com>,
        Takashi Iwai <tiwai@...e.de>, linux-kselftest@...r.kernel.org,
        Andy Gross <agross@...nel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        linux-integrity <linux-integrity@...r.kernel.org>
Subject: Re: [PATCH v5 1/7] fs: introduce kernel_pread_file* support

On Wed, May 13, 2020 at 07:00:43PM -0400, Mimi Zohar wrote:
> On Wed, 2020-05-13 at 15:48 -0700, Scott Branden wrote:
> > 
> > On 2020-05-13 3:12 p.m., Mimi Zohar wrote:
> > > On Wed, 2020-05-13 at 21:28 +0000, Luis Chamberlain wrote:
> > >> On Wed, May 13, 2020 at 05:20:14PM -0400, Mimi Zohar wrote:
> > >>> On Wed, 2020-05-13 at 12:41 -0700, Scott Branden wrote:
> > >>>> On 2020-05-13 12:39 p.m., Mimi Zohar wrote:
> > >>>>> On Wed, 2020-05-13 at 12:18 -0700, Scott Branden wrote:
> > >>>>>> On 2020-05-13 12:03 p.m., Mimi Zohar wrote:
> > >>>>>>> On Wed, 2020-05-13 at 11:53 -0700, Scott Branden wrote:
> > >>>>>> Even if the kernel successfully verified the firmware file signature it
> > >>>>>> would just be wasting its time.  The kernel in these use cases is not always
> > >>>>>> trusted.  The device needs to authenticate the firmware image itself.
> > >>>>> There are also environments where the kernel is trusted and limits the
> > >>>>> firmware being provided to the device to one which they signed.
> > >>>>>
> > >>>>>>> The device firmware is being downloaded piecemeal from somewhere and
> > >>>>>>> won't be measured?
> > >>>>>> It doesn't need to be measured for current driver needs.
> > >>>>> Sure the device doesn't need the kernel measuring the firmware, but
> > >>>>> hardened environments do measure firmware.
> > >>>>>
> > >>>>>> If someone has such need the infrastructure could be added to the kernel
> > >>>>>> at a later date.  Existing functionality is not broken in any way by
> > >>>>>> this patch series.
> > >>>>> Wow!  You're saying that your patch set takes precedence over the
> > >>>>> existing expectations and can break them.
> > >>>> Huh? I said existing functionality is NOT broken by this patch series.
> > >>> Assuming a system is configured to measure and appraise firmware
> > >>> (rules below), with this change the firmware file will not be properly
> > >>> measured and will fail signature verification.
> > So no existing functionality has been broken.
> > >>>
> > >>> Sample IMA policy rules:
> > >>> measure func=FIRMWARE_CHECK
> > >>> appraise func=FIRMWARE_CHECK appraise_type=imasig
> > >> Would a pre and post lsm hook for pread do it?
> > > IMA currently measures and verifies the firmware file signature on the
> > > post hook.  The file is read once into a buffer.  With this change,
> > > IMA would need to be on the pre hook, to read the entire file,
> > > calculating the file hash and verifying the file signature.  Basically
> > > the firmware would be read once for IMA and again for the device.
> > The entire file may not fit into available memory to measure and 
> > verify.  Hence the reason for a partial read.
> 
> Previously, IMA pre-read the file in page size chunks in order to
> calculate the file hash.  To avoid reading the file twice, the file is
> now read into a buffer.

Can the VFS be locked in some way and then using the partial reads would
trigger the "read twice" mode? I.e. something like:

open
first partial read:
	lock file contents (?)
	perform full page-at-a-time-read-and-measure
	rewind, read partial
other partial reads
final partial read
	unlock

-- 
Kees Cook