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

[Mimi Zohar <zohar@kernel.org>]

On Wed, 2020-05-13 at 16:34 -0700, Kees Cook wrote:
> 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

The security_kernel_read_file(), the pre-hook, would need to be moved
after getting the file size, but yes that's exactly what would be done
in the pre-hook, when the current offset is 0 and the file size and
buffer size aren't the same.

Mimi