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

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Date:   Wed, 13 May 2020 15:48:08 -0700
From:   Scott Branden <scott.branden@...adcom.com>
To:     Mimi Zohar <zohar@...nel.org>, Luis Chamberlain <mcgrof@...nel.org>
Cc:     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>,
        Kees Cook <keescook@...omium.org>,
        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 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.

Is there some way we could add a flag when calling the 
request_firmware_into_buf to indicate it is ok that the data requested 
does not need to be measured?
> Mimi