Re: [PATCH v5 1/7] fs: introduce kernel_pread_file* support

[Scott Branden]

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! ÂYou'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. ÂThe file is read once into a buffer. ÂWith 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. ÂBasically
> 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