Re: Add an option to dm-verity to validate hashes at most once

[Mike Snitzer <snitzer@redhat.com>]

On Wed, Mar 14 2018 at  3:09pm -0400,
Eric Biggers <ebiggers@google.com> wrote:

> Hi Patrik,
> 
> On Tue, Mar 06, 2018 at 03:14:56PM -0800, Patrik Torstensson wrote:
> > Add an option to dm-verity to validate hashes at most once
> > to allow platforms that is CPU/memory contraint to be
> > protected by dm-verity against offline attacks.
> > 
> > The option introduces a bitset that is used to check if
> > a block has been validated before or not. A block can
> > be validated more than once as there is no thread protection
> > for the bitset.
> > 
> > This patch has been developed and tested on entry-level
> > Android Go devices.
> > 
> > Signed-off-by: Patrik Torstensson <totte@google.com>
> > ---
> >  drivers/md/dm-verity-target.c | 58 +++++++++++++++++++++++++++++++++--
> >  drivers/md/dm-verity.h        |  1 +
> >  2 files changed, 56 insertions(+), 3 deletions(-)
> 
> The new option needs to be documented in Documentation/device-mapper/verity.txt,
> including a description of what the option does as well as how it affects the
> security properties of dm-verity.  There should also be a mention of why the
> option applies to data blocks but not hash blocks, assuming that's intentional.
> 
> verity_status() also needs to be updated to show the new option, otherwise it
> will not be visible via the DM_TABLE_STATUS ioctl ('dmsetup table' on the
> command line).
> 
> Also the minor version number in the struct target_type needs to be incremented,
> so that userspace can determine whether the option is supported.
> 
> >  
> >  	for (b = 0; b < io->n_blocks; b++) {
> >  		int r;
> > +		sector_t cur_block = io->block + b;
> >  		struct ahash_request *req = verity_io_hash_req(v, io);
> >  
> > +		if (v->validated_blocks &&
> > +		    likely(test_bit(cur_block, v->validated_blocks))) {
> > +			verity_bv_skip_block(v, io, &io->iter);
> > +			continue;
> > +		}
> > +
> >  		r = verity_hash_for_block(v, io, io->block + b,
> 
> Can you replace 'io->block + b' with 'cur_block' here as well?

Patrik, any chance you could act on Eric's review feedback and post v2
of this patch (assuming you still have a need for it)?

Thanks,
Mike